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Automatic Gearbox

This component is part of Transmission and Clutch.

Definition

The Automatic Gearbox (German: Automatik Getriebe) allows the engine to operate in its narrow range of speeds while providing a wide range of output speeds. The Mercedes-Benz Automatic Transmission of the first generation is used in a wide series of models produced in the same era. Mercedes-Benz uses a Fluid Coupling between the Engine & Transmission, instead of a Torque Converter that is currently standard. The operational differences between these two is significant. Fluid couplings have low, low speed torque and are most efficient at higher speeds, where they can approach 1:1 ratio.

The Mercedes-Benz Automatic Transmission provides a reverse gear and four forward gears. When in normal driving position "4", and starting off, the Pagoda starts in second gear. First gear is only selected when the gear lever is put in "2", and then drive off from standstill.

General backgound on how automatic transmissions work can be found here: http://auto.howstuffworks.com/automatic-transmission.htm

Part numbers

W113 series, 16 bolt pan, K4A025, automatic transmissions

The early version of the K4A025 ( 16 bolt oil pan) and the later version also type K4A025 (16 bolt also) are physically the same but have a slightly different valve body in the transmission. This later version of the K4A025 is identified by "GA 280 SE" label instead of "230 SL-E". Both these units are basically the same and are interchangable.

The early K40A025 (230 SL-E) was used in models 230SL, 250S (up to chasis #3651), 250SE, 250SL, 250S/8.

The latter version K4A025 "GA 280 SE" was used in models 280S/8 (early models only), 280SE/8 early models only), 280SL/8 (all), 300SEL/8 (early six cylinders only with).

A new K4C025 (4 bolt pan) replaced the K4A025 transmission in all the later models except for the W113 which kept the (16 bolt) K4A025 until the end of production.

Function

Cutaway pictures of the Automatic Gearbox




Maintenance

Oil and filter replacement

Consumables

- Oil: approx. 4 litres ATF Type A

- Filter: E.g. Mann 1810/1 kit or A 112 270 00 98

Procedure

1. Drain the oil from the pan, remove the cap nut for the hollow line of the oil dipstick very carefully (hold up both bolts) because line can bend, remove hollow screw for oil cooler line to have more space, remove the pan carfully because ther is still oil in it. Now you can remove the filter, notice the direction of the rubber lip, open end to the rear of the car.

2. Remove the slotted plate underneath the torque converter, turn the engine or torque converter by hand to find the small drain plug of it, drain oil.

3. Replace gaskets and put everything together, don't overtight the cork gasket.

4. Refill the oil via the dipstick line, don't overfill it, start engine and shift gears through all positions for some seconds.

5. Control oil level: warm with running engine, look for leaks.

Installation and Adjustment Instructions

This text and diagram is courtesy of Sun Valley Mercedes Dismantlers, http://www.mercedesdismantlers.com/, 11203 Tuxford St., Sun Valley, Ca. 91352, Phone: 818 7680704 FAX: 818 7680526 E Mail: SVMBZ@aol.com. It is reproduced here to safeguard the material.

Apply a small amount of light grease to the snout of the fluid coupler. This will prevent scouring of the front pump bushing. Pour one quart of ATF into the fluid coupler before installation into the transmission. This will prevent an air lockout of the ATF. Flush out and blow out the transmission oil cooler lines. Continue until all of the debris is removed from the lines and oil cooler.

The first set of these instructions will pertain to the double acting solenoid that is located on the top of the transmission just behind the bellhousing. This solenoid controls normal driving modulator pressure, kickdown modulator pressure and deceleration modulator pressure. The wires to this solenoid must be installed properly. The control switches to this solenoid must be in good working order. The idle and deceleration switch is located on the front carburetor or on the throttle valve of a fuel injected engine. Operational problems arising from this solenoid are as follows: No kickdown, early shift, early slipshift, slipping in all gears, late shift, clunking during deceleration downshift when the car is slowing to a stop. Any or all of these problems could also be intermittent.

  1. Move the solenoid's rod up and down manually to assure free movement. There should be more spring tension pushing the rod down than lifting it up. If it is binding, there may have been some damage caused during shipment or installation. If this has happened, call us.
  2. Look at the wires that are connected to the terminal block on the left side of the bellhousing. The size of the eyelet on the different colored wires will dictate which wires should be connected to each other. Large hole with large hole, etc. It is easy to mix up the Green/Red wires. The Green/Red wires attach to the servo pressure switch located on the side of the transmission. One wire will connect to the left pressure switch and go up and over the top of the transmission to the other pressure switch (if your trans has that switch). These switches cause the fast idle solenoid to activate when the car is put into D or R. if the configuration is correct, you should have 6 wires connected to the terminal block, 3 wires connected to the left pressure switch and one wire connected to the right pressure switch.
  3. Have someone sit in the car with the key on, but the engine off. The fuel pump should be running. Make sure the emergency brake is set. With the throttle in idle position, the double acting solenoid should be in the down position. Touching the throttle lightly moving the throttle into off idle position up to approximately 3/4 throttle, the solenoid should be in the center position. If this action does not occur, check the fuses in the fusebox. If they are OK, check the switch located on the front carburetor of the fuel injection throttle body. If the switch is bad, replace it.
  4. Push the throttle to the floor, depressing the kickdown switch below the footpetal. The double action solenoid should lift. If this does not occur, bypass the kickdown switch to see if it is working properly. If not, replace the kick down switch.

Apparently missing from the above diagram: the positive + feed for the idling switch on the throttle body bringing power from fuse #3.

Vacuum Modulator

These instructions concern the Vacuum Modulator. Adjust the modulator only after you have verified the proper function of the double acting solenoid. The modulator tells the transmission when to shift and it determines the quality of shift. The modulator pressure should be set at around 4 Bar at idle with no vacuum line attached. This setting will give you a place to start, however, the modulator pressure should be set according to the operation of the car. Remember: Early and/or slip shift is low modulator pressure. Late and/or hard shift is high modulator pressure.

  1. Remove the vacuum line from the modulator assembly on the transmission with the engine running. There should be at least 15 inches of vacuum at the line.
  2. Check the modulator pressure at the test port to make sure you have around 4 bar at idle with no vacuum.
  3. Install the vacuum line and drive the car. If it shifts too early, you need to raise the modulator pressure. You do this by loosening the lock nut on the small shaft in the center of the vacuum modulator. Turn this small shaft in (clockwise) 1/2 turn. Tighten the lock nut while holding the adjuster shaft in place. Repeat until the shift is correct. If the complaint is late or hard shift, follow the same procedure except turn the adjustment shaft out (counter clockwise) to lower the modulator pressure.

Some operational problems may be caused by valves in the valve body reacting slowly due to the bores being freshly honed. These symptoms will go away after a few days of driving.


K4A 025 Transmission

How to adjust your transmission to shift smoothly

The K4A 025 transmission can be adjusted to shift smooth: There is a 3-position solenoid on the top of the transmission which actuates a push rod that goes to the modulating valve. It is accessible by removing an oval cover on the right side of the transmission tunnel forward of the passenger seat. First, disable running of the fuel pump by removing fuse no.4. This is only to reduce the annoyance of a running pump. Observe the push rod and turn on the ignition. The push rod should shift aft. Now, depress the gas pedal about half way: The push rod should travel to the mid position. (Same position as with ignition off). With the toe of your left foot reach under the gas pedal and depress the kick down switch: The push rod should move forward. That forward move is what triggers the downshift of the transmission. If the rod is adjusted too long, it won’t shift down by actuating the kick down switch. If the solenoid does not move the push rod, you must fix that first. You may need to run the engine to get the system voltage up, for the solenoid to shift. Next, connect a 60-PSI scale pressure gage to the modulator pressure port. You will need a banjo fitting with a M8 bolt. This is a good time to check your modulator pressure: It should be about 40 PSI with the vacuum line disconnected and the engine running at fast idle. No vacuum to the transmission simulates max. Engine load. With the engine idling observe the pressure gage. Shorten the push rod until the pressure rises, then lengthen the push rod until the pressure drops. This sets the push rod to the pressure transition point. As a result, when you start driving and the transmission is in 2nd gear, and you accelerate to about 2800 rpm and then take your foot off the gas, the transmission will shift up smoothly.

This information came from Vintage Reprint No. 1 of The Star.

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Rough Shifting

The usual problem with rough downshifting automatics in these 113's is non-functioning or mal-adjusted three position solenoids or linkages. Oftentimes the intake butterfly stop screw is mistakenly used as an idle adjustment scew. If this happens the transmission solenoid switch on the intake is thrown out of adjustment causing the three position solenoid on the transmission to not function properly or not function at all. This solenoid should soften that last downshift when working properly.

I'm wondering about a switch type device on the right side of the throttle venturi area. There are no wires attached to it nor is there a wire harness near it. Should this be here on a July 64 230SL? Doesnt seem to affect starting or running. Could it be a newer version ?

Tom Sargeant says: sounds like you may have a newer air intake manifold that has a idle throttle switch. This switch affects transmission pressure (among other items) when working properly, at least on an automatic. Mine went bad and when I replaced that switch, my severe "clunk" when coming to a stop was eliminated-it activated the transmission pressure switch.

No switch required on earlier cars with Manual transmission. There should be a cover over the area where the switch would normally go.

It sounds like your on the right track for the hard downshifting. Make sure the transmission three position solenoid is actually moving the linkage on the transmission all the way to position one when the accelerator is closed. If not then re-check the workings of the accelerator linkage and venturi switch. Take a passenger for a test drive to observe the system working through the opening in the tunnel. As last resort the control pressure linkage on the transmission (between the three position solenoid and the modulator can be adjusted. (make sure everything else is correct first). Since your kickdown is now working, your probably on the right track. Just doublecheck yourself. Some additional and more unusual things to check: Tom found that his linkage had been bent. Also there is a modulator diaphram that can fail and cause shifting problems also. If this happens the car usually puts out some white smoke and spark plug cylinder #6 will foul out from transmission fluid getting sucked up the vacume line going from the modulator to the intake manifold. This diaphram can be changed from under the car without removal of the transmission.

I am still testing my auto transmission to see what is causing it to clunk when shifting. There have been posts on here in the past that have told how to test if the 3 position solenoid is working. I think the pressure switch is in the activating circuit for the 3 position solenoid on the auto trannsmission. My 3 position solenoid looks to work properly. I looked at it through the hole in the tunnel and the arm moved to all 3 positions at the right position of the accelerator pedal. I didn't take it for a drive to observe it, I just did it with the ingition switched on. Could it mean my pressure switch (on the left hand side of the transmission) is working properly or does it mean that the pressure switch is stuck with the contacts MADE closed all the time? Looking for a further test procedure, I was reading the Hanynes manual and it says (PAGE 82 FUEL INJECTION WITH AUTOMATIC TRANSMISSION reference 32) to "test the pressure switches in the auto transmission" to take the plug off the relay box on the left hand side engine bay and to connect a lamp across terminals 1 and 8 of the plug and then take the car for a drive to see when the light is on and off". Question..... Do I perform the test without the plug back on the relay box ? OR do i connect the test lamp across the terminals of the plug and put the plug back on the relay box, then do the test? It doesn't say to put the plug back on the relay. Will the car run with the relay plug disconnected?

My 1967 230 SL has a jerky automatic, and my 1965 220 SE Heckflosse has the same problem. The guy I bought the Fintail from told me you can adjust from the inside of the car, on the tunnel from the passengers side. Is this correct, and is there somebody out there who can get me some information on the MB auto gearboxes , or knows an internet address ??

Joe Alexander: You can remove the access cover on the passenger side tunnel area and observe the movement of the transmission linkage and three position solenoid, while test driving the car. You will need a second person and a flashlight. The kickdown or passing gear switch on the floor under the accelerator pedal must be engaged and working to activate the solenoid. When fully depressed this switch is engaged and it energizes the three position solenoid which will move the control pressure linkage on the side of the transmission. This linkage changes the control pressure at the modulator assembly on the transmission and causes the downshift. Make sure you have power going in and out of the accelerator kickdown switch when it is depressed. Next try observing the operation of the three position solenoid. Position one at idle in gear, position two after driving off, position three at full throttle (accelerator fully depressed and switch engaged). If all seems to be working ok then more complicated adjustments can be made at the modulator or the linkage on the transmission (do this only as last resort and after everything else is checked out). Be sure to check the switch on the venturi. It is a brown bakelite switch with two wires going to it. It is mounted on the intake venturi,a little further up from the air cleaner. With the ignition on, car in gear, the switch should have power at both wires (closed switch). After slightly moving the throttle open the switch has power at only one wire (open switch). Your transmission will never shift right unless this factory setting is restored.

Bob Smith: I read in the MB service manual for my 250sl, 1967 Auto that the gear box was designed to change when there is a negative pressure in the gear box. I drive it like a "clutchless manual". If I accelerate hard the gearbox hangs onto the lower gear and then "CLUNKS" as it changes. SO, instead I listen to the revs of the engine and when they are about 2500 RPM I lift my foot off the accelerator (like you do when you change gears in a manuual car) and then the gear box changes very smoothly. Audibly the car sounds like a fast changing manual.

Cees: last Sunday I met with another Pagoda enthusiast with an auto 250 SL, mine is an auto 280 SL. He and I swapped cars for a drive in the country. HIS CAR shifts always around the same RPM level, pretty much irrespective of how deep you press the accelerator. When you want the car to shift back (i.e. to accelerate faster) you press the gas pedal deeeeep - and nothing happens! You have to manually select the lower gear (which means, ususally, to put it from 4 into 3). If you really floor it (i.e. press the kickdown switch) it cloncks into lower gear, finally. MY CAR shifts at up to very low RPM's when I accelerate very lightly, and goes up to HIGH RPM's when I accelerate hard - and not with the kickdown switch engaged. In other words, shifting is very responsive to intended acceleration. WHEN I want the transmission to shift back, relatively little gas pedal action is needed to make it do so. If I am driving fast in relatively high RPM's (say, 3500), and I press the kickdown switch, it will shift back and pull all the way to redline. The way I would expect an automatic to shift. QUESTION IS - I assume my car behaves as intended, and "his" does not. Is this the case? If so, how many others recognize the poor behavior and, what should be done about it? I assume some linkage or pressure switch adjustment? Can this be done by shade-tree mechanics?

Joe Alexander: hello Cees, it sounds like your transmission is acting correctly. The factory transmission manual along with the technical data booklets both confirm that the transmissions shifts differently depending on the position of the accelerator pedal. There are charts in these manuals showing the shift points at different pedal positions for each drive gear selection. In actuality this input is regulated by intake manifold vacuume which is altered by moving the accelerator pedal. Any change in engine vacuum is transmitted via the metal vacuume line attached to the intake and ending at the modulator diaphram on the transmission. Inputs from the three position solenoid also change shiftting characteristics. The mechanical linkage on the engine however, has no direct input to the transmission on this model. If you experience shift changes after adjusting engine linkages this is a result of a change in engine vacuume (rpm's, mixture etc.) or change in operation of the three position solenoid. Later Mercedes transmissions used manifold vacuume and engine linkage to control shifting. The three position solenoid vanished!

Cees: one comment I would add - on my venturi, there is no switch. Instead, a microswitch is mounted on the firewall, triggered by a bracket for the gas linkage, so that it operates the same way the venturi microswitch would. My engine is a 280SE (sedan) replacement one, and that may be why my intake venturi lacks the switch. Many others have the same engine, so may also have to look for the switch somewhere else - or install one if one is not there!

Joe Alexander: Cees you are absolutely correct. I recall seeing this firewall microswitch. I am not sure if it was just used on earlier cars or just sedans? I am sure I have seen both styles on sedans. So this switch was used instead of the venturi microswitch . We need to research this! These automatics tend to shift firmly. However if you are getting a seat jerking downshift just before you come to a stop and or unusually hard upshifts at moderate acceleration, then some type of adjustment is usually needed. In most cases it is the venturi switch or possibly Cees' dash microswitch!

Rodd: Cees, my '66 230SL Auto shifts like yours. The accelerator position does effect the RPM shift points like a modern transmission.

Bob Smith: Hi Cees and Joe - my Auto changes at different revs too depending on the accelerator pedal position and sometimes it is as smooth as silk. However when I push it a bit then I need to drive it like a CLUTCHLESS MANUAL so it does not CLUNK. Should I expect it to be smooth as silk when it changes with no clunks every time? My car is an EARLY 250SL 1967. In the past I have tested the 3 position solenoid, and the switch on the venturi unit. Both were working properly. The Kickdown switch needed to be cleaned and then that function worked too. Joe, 1. Are you saying that the linkage to the 3 position solenoid does NOT DIRECTLY affect the modulating pressure of these auto transmissions but that it is only the manifold vacuum pressure that affects the modulating pressure? AND the Manifold Vacuum pressure is affected when the linkage to the 3 position solenoid is adjusted? 2. If by adjusting the linkage to the 3 position solenoid I alter the manifold vacuum pressure and hence the AT modulating pressure, can I (as a shade tree mechanic) do damage to the auto trannsmission? 3. what do you mean by "change in operation of the three position solenoid"?

Cees: Bob - all I can say is that I really never feel the inclination to drive the car like a clutchless manual to prevent clunking - and I am fairly sensitive to this kind of mechanical "roughness". So my car shifts pretty smooth most of the time, although I too have the occasional clunky downshift (3 to 2, when coming to a fairly abrupt stop), and sometimes when upshifting, but that is fairly rare and does not seem to follow any pattern. My mechanic when he got to know the car commented on how smooth he thought it shifts, compared to other period auto trans Mercedes. Bottom line I suppose - it would be nice of we could all drive at least a few other cars, to be able to tell how we rate on the "smooth shift index" and THEN have someone, or some clear instructions, to turn to in order to improve things when necessary...

My U.S. '69 automatic has two microswitches. One is located at the throttle plate just downstream of the air filter. It is part of the control for the transmission 3-position solenoid. The other switch is at the firewall. It is part of the control circuit for the fuel cut-off solenoid. My U.S. '69 manual car has just the microswitch on the firewall for the fuel cut-off solenoid...So far so good. Now this is strange. I had a U.S. '70 manual car with just the one microswitch for the fuel cut-off solenoid as expected. But it was the one on the throttle plate that was present. There was none on the firewall! And this setup is consistant with Haynes!

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I'd like to point out that the M-B Automatic transmission uses a Fluid Coupling between the Engine & Transmission, instead of a Torque Converter that is currently used. The operational differences between these two is significant. Fluid couplings have low, low speed torque and are most efficient at higher speeds, where they can approach 1:1 ratio. This can be corrolated as two fans facing one annother. Many models were simply air-cooled. Some of you may recall some of the early GM transmisions that had fluid couplings with 4 or 5 speeds. Torque converters, on the other hand, develop much higher torque when starting, due to a set of stationary vanes (stator) between the two which redirects the oil to the output member, creating this higher torque. However the early models were not very efficient at full speed, requiring much more cooling. The next development was a one-way clutch on the stator, allowing it to spin free at higher speeds (similar to a Fluid Coupling). Finally about 6-8 years ago, transmissions began to use a lockup clutch connecting the two together for maximum efficiency. What I'm trying to point out that with a lower ratio, you'll not only get poorer starting torque, but may in fact lose efficiency.

Any recommendations on any kind of service for my auto transmission? The car has finally come back to life with the rebuilt engine as the restoration comes to a close...While everything has been replaced-restored as necessary, we have not done anything to the transmission because we don't know it needs anything yet. However, the car has essentially been sitting for 15 years, and the auto trans obviously unused in that time. Short of new filter and fluid (and perhaps some good running), anybody have any recommendations on what if anything to do to the trans after sitting for so long?

Pete Lesler: when you drop the pan to install the new filter, check out the bottom for residue. If the pan is clear are relatively clean, just install your filter and fluid and drive it for a while then get it good and hot and change again, making certain to drain the torque converter each time as well. You will need to do this a few times to make certian you get all of the residual gunk, varnish and whatever out of the trans. If it behaves well, consider your self lucky. If the trans is out of the car, replace front and rear seals, as they will be dry from sitting so long. Might be wise to see if any trans shop can flush out your torrque converter while its out of the car. That's my two cents.

Joe Alexander: It's always a good idea to replace the front and rear seals. The rear driveshaft flange needs to be remowed (special slotted spanner socket needed) available from Mercedes and not too expensive. You can sell it to a nother list member when your done with it. Use a seal remover, available at your local auto parts store(cheap). No other disassembly is needed. Make sure that the three position electrical solenoid and linkage works freely. Replace that little plastic (nylon like) bushing on the shift linkage arm on the trans. It can be ordered from Mercedes and is very inexpensive. Submerge this bushing in hot water for a few minutes before installing and it will be soft and limber and will install easily. Don't forget to drain the old dirty fluid out of the torque converter also.

Trying to help my mechanic here with any of your great ideas. Presently we seem to have all sorted out with the transmission, but we can't get the kickdown to work. We discovered a broken wire which prevented the relay from letting the kickdown solenoid on top of the tranny to work; now that is all working, but still no kickdown. His next task is to remove the solenoid (remember it is working) and try to engage the kickdown manually from the top of the transmission while the car is in motion. Anyone have any ideas on this? Can the transmission seemingly work fine (shifts pretty nicely), but no kickdown working internally? As I mentioned we do have the solenoid and kickdown circuitry working, and we think there is enough travel on the solenoid as well. Any assistance would be appreciated.

Joe Alexander: you can remove the access cover on the passenger side tunnel area and observe the movement of the transmission linkage and three position solenoid, while test driving the car. You will need a second person and a flashlight. The kickdown or passing gear switch on the floor under the accelerator pedal must be engaged and working to activate the solenoid. When fully depressed this switch is engaged and it energizes the three position solenoid which will move the control pressure linkage on the side of the transmission. This linkage changes the control pressure at the modulator assembly on the transmission and causes the downshift. Make sure you have power going in and out of the accelerator kickdown switch when it is depressed. Next try observing the operation of the three position solenoid. Position one at idle in gear, position two after driving off, position three at full throttle (accelerator fully depressed and switch engaged). If all seems to be working ok then more complicated adjustments can be made at the modulator or the linkage on the transmission (do this only as last resort and after everything else is checked out). Let me know what you find. Good Luck, sounds like you are getting close.

I finally got a call yesterday from the mechanic, and my car is done. Turns out that the transmission passing gear issue was, and many of the "clean up" and "sorting out" issues he discovered were matters of adjustments as I thought. In the initial case of the kickdown issue, 1st there were wiring problems, wires on the tranny were in the wrong place. 2nd, there was a broken wire on the harness at the relay. When those items were repaired, then the solenoid was able to work. Manually, they were able to get the kickdown to work; however the solenoid did not give enough travel in order to activate automatically. They tell me that the final resolution was linkage adjustment AND a bent "rod". When they straightened the bend rod (I think this is that the solenoid pushes on in the tranny) and adjusted the linkage, voila, it all works.

I have a 1969/70 280SL. I had a problem with the transmission about 2 months ago, the trans would upshift/ downshift/ upshift etc from 3 to 4 below 40 MPH. Above that, it was o.k. Had the "module” changed or rebuilt from a MB rebuild package. Now it won't shift from 1 to 2 until about 18 MPH, from 2-3 at 25 MPH, and to 4 at 65MPH. All this at a low throttle pressure. Is there some adjustment that I missed? I looked in the MBZ shop manual (the 2 1/2" one) and it doesn’t seem to mention how to adjust the pressure.

Tom Sargeant: you can have the transmission adjusted based on various pressure measurements so that it shifts to factory spec (assuming all else works). If you are mechanically inclined and don't have a pressure gauge, you can adjust the trany yourself by adjusting one of the linkages that regulates transmission pressure. To describe where the linkage is, think of the linkages relative to the road. The main regulating rod goes across the valve cover. This is *horizontal* to the ground, as is the linkage that connects to this rod going to the venturi throttle. Another horizontal linkage runs from the main regulating rod toward the front of the car and connects to a bracket. This bracket then has a *vertical* rod that goes down throught the intake and exhaust manifold toward the ground, all on the passenger side of the car. This rod regulates transmission pressure. I am not sure which way to rotate the rod to achieve a downshift at a lower speed. I *think* that if you are oriented as if looking down at the ground, rotate the rod *counter clockwise* one rotation and then test drive. This should smooth out the shifting. Disclaimer-if you don't check the pressure in the transmission with a pressure gauge prior to adjusting-you could face a trany rebuild.

I just topped off the tranny fluid again and had to use over 500ml to bring it up to level. I regret not checking it each time. Now I have a few leaks under the car, nothing major. My question: should it be burning that much fluid in 1000 miles of summer driving?

Rodd: Is your motor consuming lots of oil? That is, one quart per 500 miles? You should check your #6 spark plug for fouling. You could have a bad diaphram (can't recall the name) on the auto trans that is bad. It is supposed to be vacuum activated and it can suck auto trans fluid up the vacuum line to the intake manifold and foul plug #6. It would also create white smoke out the exhaust. Just one thing to look at. You could have a simple external leak.

You have a leak (probably a seal).

Tom Sargeant: was with a friend today lending moral support (and holding the light) as he re-installed an engine in a w107. We got into a discussion of the high RPMs at highway speed in the 113 and how to get them lower. He mentioned the rear end change, and I noted that I did not want to impact my off-the-line performance. He noted that I could change the transmission modulator and have the car start in first gear all the time. Has anyone done this and does it make any sense? I am not sure I would change anything-rear gear ratio or transmission starting seqence, but this seems to be an option that, if available, might be of interest to others.

Rodd: Do you really want a first gear start? Try driving away from a stop with your gear selector in "2" every time for a whole day. This forces the first gear start on the automatic and it's how we can drive our cars like the new "manu-matics", by keeping the selector where we want it. After a day of this, you may decide you don't like it. First gear is really low and short. I almost always chirp my tires between gears 1 and 2 when doing this, so I wouldn't want it all the time. But, if I were to do this, I would really want the 3.69:1 ratio rear end. That way it makes that first gear slightly more "mellow" and it will slightly reduce RPM on the highway.

Cees: I did change to the 3.69 on my automatic 280SL a couple of months ago. This weekend, I drove to the Veterama German car jumble in Mannheim, Germany (600 miles in all) and mostly at speeds between 80 and 90 MPH - at 90 MPH the road and wind noise totally block out the noise of 4,500 RPM's. Then consider the readline: above 6,000 RPM's - these engines are meant to rev high. In town, when I really want to accelerate, I simply press the accelerator pedal briskly - the car will start in 1st automatically and pull away quite fast. So no need to modify the tranmission modulator, really. If I keep my foot down far enough, the tires will even spin shifting from second to third. The change over to the 3.69 (from 4.08) makes about a 10% difference on the RPM's. This sounds like a lot but I cannot really tell the difference, mostly due to all the extra wind/road noise that comes in at higher speeds.

Joe: altering your modulator pressure possibly could make your automatic start off in first. However as Rodd mentioned, it would cause all the shift points to be very high and very uncomfortable to drive drive under normal conditions. The transmission "valve body" is the very complex hydraulic computer which regulates the shifting along with other inputs from the engine, gear selector, and the driver. In order to acheive a usable first gear start-off, this valve body would have to be modified. It is probably not practical. If the seemingly high reving final drive is uncomfortable, you may want to consider some sound insulation, maybe under the rear decking behind the seats. This helped my 450-SLC a lot. You are aware that changing the rear end ratio is the other option. In this case acceleration would suffer slightly. The late Frank King wrote an excellent article in May/June "The Star" magazine on Mercedes automatic transmissions, the variations, characteristics, descriptions, designations, etc. The article has a lot of good information condensed. It is not a repair guide. Good factory transmission repair manuals are scarce. The 113 cars along with the 250-S,SE and possibly the early 280-S,SE used the exact same K4A-025 automatic. Mercedes sedans stopped using this unit sometime in 1968, however the 113 cars used it until the end of their production. The latter Mercedes automatic #K4C-025 1968-73 was never used in the 113. This transmission was a first gear start off unit. It was a more modern design, smoother shifting. Engine input was via engine vacuum and engine linkage (no three position solenoid). This could be a more practical conversion, if you must. A donor 280- S,SE sedan (late 69-71 6cyl), could provide almost 100% of parts needed to do the conversion to the later transmission. Check to see if it will physically fit first! My advice, be happy that your original is working well and leave it alone.

Joe Alexander: Before removing the transmission, always unhook the torque converter from the flywheel and remove it with the transmission. As mentioned the bronze bushing in the pump can be damaged and cause problems. It can be ordered separately and save the high expence of replacing the front pump. A even more elusive leak may be the rivited connection between the torque converter hub and the converter body. This often is the problem. It appears to be the front seal or bushing but turns out to be a leak at this riveted connection. Some large transmission shops can rebuild these converters. Mercedes eventually went to an all steel welded seam on this connection to prevent this failure. Unfortunately I believe all the 113 converters were aluminum with a riveted steel hub and cannot be welded. If your front transmission seals seem fine and the bushing is ok, this is most certainly the problem. It is hard to identify since it will show up only by pressure testing the converter. Sometimes it can only be detected under pressure when it is hot! Three feet of 1/2" wobble extensions are best for reaching the top transmission bolts from behind and under the transmisision.

A leak in my torque converter appears only under pressure. The only thing is, that I do seem to remember that the converter was steel (cast) with a coat of surface rust as opposed to aluminum with a riveted part. I guess that leaves the bronze bushing. Can I do much damage to an otherwise very good transmission if I don’t do the job right away? On my last AFT change only 2000-3000 mls ago I didn't find anything unusual in the pan or the filter. Maybe I did damage the seal although I'm pretty sure I separated the converter from the tranny before I pulled it out.

Dan Caron: Nah, It's none of those things. Sometimes the main seal will leak but what it usually ends up being is the primary pump seal. Big O ring that tends to leak. No way to stop it except to pull the trans and remove the bell housing. Seal goes around the primary pump and stops all the leaks. Cheap part too.

Joe Alexander: the early steel converters also had riveted hubs. My advice is; the seals are inexpensive. Replace the front seal and possibly the o-ring as Dan mentioned, also the bushing if damaged. Now let me tell you about the transmission I had in and out four times, replacing seals, before finding that the riveted connection was leaking. Have run across others since then.

Thanks for the seal tip. Of course, now I'll just replace all these seals and check the riveted joint as I don't even feel like taking the trans out once, let alone four times :-) I'm assuming I can't do any damage if I don't get to that job very soon and just top off the ATF. I guess once I pull out the tranny, it's "might as well time" though, am I right? (Clutch packs, brake bands, seals?) Would I upgrade to a newer style torque converter at that point and would I still get the same great torque and shift characteristics as with the w113 unit?

Dan Caron: kind of depends on how well you're set up. A hoist sure helps. I'd say somewhere between 4 to 6 hours - maybe a whole day. The actual front bell housing has to be removed from the front of the trans. and then the pump is removed from the housing. There's a gasket between the housing and the main body of the trans. and you may want to replace that too but it usually comes apart without damage. Joe is right though, check everything carefully. It's a lot of work to keep going over.
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