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Manual Gearbox

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Definition


Manual Transmission

A device for providing different gear or drive ratios between the engine and drive wheels of an automotive vehicle, a principal function being to enable the vehicle to accelerate from rest through a wide speed range while the engine operates within its most effective range.

German Name: Getriebe
English Name: Manual Transmission or Gearbox

Function

An automobile equipped with a manual transmission requires the driver to shift from gear to gear. A manual transmission bolts to a clutch housing (or bell housing) that, in turn, bolts to the back of the engine.

The function of any transmission is transferring engine power to the driveshaft and rear. Gears inside the transmission change the vehicle's drive-wheel speed and torque in relation to engine speed and torque. Lower (numerically higher) gear ratios serve as torque multipliers and help the engine to develop enough power to accelerate from a standstill.


G72 Manual Transmission 280 SL

Initially, power and torque from the engine comes into the front of the transmission and rotates the main drive gear (or input shaft), which meshes with the cluster or counter shaft gear -- a series of gears forged into one piece that resembles a cluster of gears. The cluster-gear assembly rotates any time the clutch is engaged to a running engine, whether or not the transmission is in gear or in neutral.

In order to mesh the gears and apply engine power to move the vehicle, the driver presses the clutch pedal and moves the shifter handle, which in turn moves the shift linkage and forks to slide a gear along the mainshaft, which is mounted directly above the cluster. Once the gears are meshed, the clutch pedal is released and the engine's power is sent to the drive wheels. There are several gears on the mainshaft of different diameters and tooth counts, and the transmission shift linkage is designed so the driver has to unmesh one gear before being able to mesh another.

All mainshaft gears are in constant mesh with the cluster gears. This is possible because the gears on the mainshaft are not splined to the shaft, but are free to rotate on it. With a constant-mesh gearbox, the main drive gear, cluster gear and all the mainshaft gears are always turning, even when the transmission is in neutral.

Alongside each gear on the mainshaft is a dog clutch, with a hub that's positively splined to the shaft and an outer ring that can slide over against each gear. Both the mainshaft gear and the ring of the dog clutch have a row of teeth. Moving the shift linkage moves the dog clutch against the adjacent mainshaft gear, causing the teeth to interlock and solidly lock the gear to the mainshaft.

To prevent gears from grinding or clashing during engagement, a constant-mesh, fully "synchronized" manual transmission is equipped with synchronizers. A synchronizer typically consists of an inner-splined hub, an outer sleeve, shifter plates, lock rings (or springs) and blocking rings. The hub is splined onto the mainshaft between a pair of main drive gears. Held in place by the lock rings, the shifter plates position the sleeve over the hub while also holding the floating blocking rings in proper alignment.

A synchro's inner hub and sleeve are made of steel, but the blocking ring -- the part of the synchro that rubs on the gear to change its speed is usually made of a brass. The blocking ring has teeth that match the teeth on the dog clutch. Most synchros perform double duty -- they push the synchro in one direction and lock one gear to the mainshaft. Push the synchro the other way and it disengages from the first gear, passes through a neutral position, and engages a gear on the other side.

Maintenance

Describe common maintenance procedures, and common faults that may occur. Describe how these may be diagnosed and resolved. Again, include diagrams, photographs and explanations. Where possible, include measures, tolerances, weights etc.

Symptoms of problems

The standard transmissions are very tough. Major failures are rare, some minor glitches like nipping going into some gears, some resistance in shifting up or down, or popping out of 4th gear etc. can occur over time. Strange noises are usually a result of defective main roller bearings, misalignment of gears often due to loose gear clusters caused by a loose spanner nut on the transmissionís output flange, or possible problems with the clutch or throwout bearing assemblies. If your speedometer needle bounces more than it should when you accelerate or de-accelerate and the transmission is a little hard shifting, check for a loose spanner nut on the transmission output flange at the driveshaft. Synchromesh rings are seldome the problem. Most problems are brought on by lack of maintenance at some time in the unitís life, low fluid level, wrong fluid, leaky seals, no maintenance, sometimes just bad luck.

One thing a lot of owners do not know is that the standard Mercedes 4-spd of that era uses automatic transmission fluid, not engine or gear oil. The transmission itself according to my experience and my literature is basically the same unit as used in the 230-S, 250-S, 250-SE, 280-S, 280-SE sedans. Some other models are close but gearing is a little different in the diesel and 4 cylinder sedans of the era.

Anyway the one unique part is the top transmission cover. If your transmission goes bad save this, it is unique to the 113's and possibly 111 and 112 coupes and convertibles. You can use a transmission out of many of these other models by simply changing the top shift cover. The 113 top cover has a single shift rod which goosenecks down unlike other models which are straight. A good experienced Mercedes mechanic can repair or rebuild one of these units in a long afternoon. Replace with new: gaskets and seals, all four main shaft roller bearings, replace damaged gears, shift sleeves, and synchros (usually are not bad but are cheap), and loose shaft keys. Gears are expensive, a good sedan transmission is a goldmine of parts. These units are much less difficult to repair than the automatics, but like anything else there are some features and details that require some insight and expertise.

  • How to test if it is faulty - what tools to use
  • How to fix / change

Link to related components where appropriate.


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Pete Lesler: The shift knob is forced onto the shift lever which is splined. It is a tight interference fit. I usually tap the shift lever off from the underside. Don't tap too hard when removing as you may pull the lever out of the nylon bushings that it rotates on.

I have to double clutch my 230SL to drive it, I guess because the syncros have gone out on the transmission. Does anyone know if I am looking at a huge expense to fix, or is there a simple solution?

The transmission is not a complex one, but the job does require knowledge of reassembly of the box. There are several loaded shims in there that need to be right. There is no simple fix for bad synchro rings. I would suggest a very good independent, or your local MB dealer, unless you are interested in taking it on yourself. I would not think that to be out of the realm of an advanced DIY job. Or just find one on Ebay for $150, or contact one of several scrappers that have them. Maximum price for a used one should be no more than $250.

Will: Replacing the syncros is just part of the problem. First, make sure you are using Automatic Transmission Fluid in your manual trans, not 30wt or gear oil. This is because there are needle bearings in the trans. The synchros work in conjunction with 2 synchronizer bodies. These are the things that you move when you shift gears. They mesh with the synchros. These bodies wear as well. This is most noticeable when the car jumps out of gear. The bodies run about $1000 each, add parts and labor for the synchros, bearings, seals, and you can get a $2000-2500 trans real quick. As stated before, if you can get a good used transmission, that is a cheaper way to go. The key is how do you know it is good and how does the person selling to you know it is good.

You should also check the linkage bushings between the shifter knob and the transmission itself. The plastic pieces wear out and can cause poor shifting. It is a fairly easy job to do and well worth a try before doing major surgery.

When my 230 was running better than ever and nicer than ever with my brand-new painted Ivory wheels, an hour ago, as I was going out of parking slot, and as I did hit the road, I heard a distant sound from under the car, as if a non metallic part had fallen to the floor. Immediately after as I pressed the accelerator the car did not move, while engine effectively revving. Cheched gear in, more gas, no movement. No oil on the floor, no pieces on the floor, no nothing.Could engange first, second, rear, all gears, but no movement. Clutch seems to be ok. Once under the car no oil, no missing pieces, not a single clue of problems. Pushed the car for the first time in public, with the help of a friendly neighbour, back to the parking spot (luckyly ground floor). Once back in place, I did try again to select all gears, and I could hear that in all of them, when engrained, somewhere in the transmission mechanism a noise was produced that ressembled boiling water as the engine was revving up. Has anydoby seen/suffered this before? Is there any easy attempt that I can make to avoid towing the car to the repair shop for a "ridiculous" thing that I could have checked or done myself?

I think you need to look if the propshaft is turning or not. Just be careful that you don't get run over by the car if anything should suddenly start working again. It could be the slip and universal joint inside the rear axle.

If your flex disk fell apart it could create the symptoms you described and it would be a cheap repair job. This is just a guess.

Joe Alexander says: I would check to see if the failure is in the differential. It sounds like a classic case of differential slip joint failure.

Walter Klatt says: your smptom is the rear end input shaft is snapped. I had this happen on a 71 280SE and I was at a light and it turned green and I pushed on the gas. The car moved nicely and then a snap, and the engine revved up to max rpm. I did a quick check to see that was wrong, looked under the car while my friend was sitting in the car, in gear and engine idling. I looked underneath to see the axle and it was turning fine as it enters the rear end. So you need to either rebuild your rear end or replace it.

Cees Klumper: for your information, I bought a good used European rear end, complete with axles and everything, for 450 euros - I traded my American rear end, which is not worth much here, to get that price. Complete with installation and new rear wheel bearings it cost me something around 800 euros.

A few questions for informed members re manual transmission:

  1. where is the fluid fill opening for (230sl) manual trans? Found the drain plug on the lower right side and a vent cap on the top right behind the plate on the right side of the transmission tunnel.
  2. how to determine fluid level?
  3. if fluid level is low, better to drain & replace vs. top off?
  4. proper ATF?
  5. could low fluid level cause slight binding or resistance when shifting? The nylon bushing at the base of the shifter is intact and looks clean but the shift linkage does have some resistance.
  6. other suspects for 'sticky' shift linkage?

Naj:

  1. The filler plug is on the right hand side of the box about 1/2 way up. Uses 14mm allen key to undo.
  2. Level should come up to plug hole
  3. There is only 1.4 liters in the box. If you don't know its history, I would drain and refill. Replace the aluminum washer on drain plug.
  4. ATF is the right oil. If there are no leaks you might consider fully synthetic.
  5. I have resistance/binding when the car has been standing for days. Soon changes when the oil warms up and is splashed around.
  6. Check the plastic bushes under the stick and also at the front end where the rod connects to gearbox link. Accessible after removing cover on top of transmission tunnel (under carpet).

I've got sort of a whining noise coming from the base of the shifter assembly (4 speed manual). Anybody know what the problem could possible be?

David Seidman: yes - or at least a guess: there is a little collar (plastic) that holds the shifter in place and prevents it from dropping down on a spinning shaft. It is about to break and will be even less fun to drive. Mine died last spring and I had it replaced - though you might do it yourself. The parts are @ $10-15 and it should take you @ 1.5-2 hours (unless you are quite good at this sort of thing and thus do it in half of that time). Fortunately Ė even if it breaks and sounds like a costly problem - it is not an expensive fix. How often can we say that about these cars?

It might be a bearing or the shifter assembly rubbing against the driveline. Try to lift the shifter while driving to see if the noise changes pitch or goes away. Perhaps it is what David says it is- the shifter collar.

Joe Alexander: jumping out of fourth gear is usually caused by a worn upper fourth gear. More uncommon, a worn gear selector sleeve , a loose gear cluster due to improper assembly, or shifter linkage problems, can cause these symptoms also. This is the part of the gear that the gear selector sleeve engages. Look closely at the small secondary teeth on the output shaft ( upper cluster part of fourth gear) of your tranny. This part in my edition "C" parts book is #111-260-13-20 and is reffered to as a "drive shaft with sychronizing cone" more commonly known as the output shaft. When this wears it will kick the gear selector sleeve off when put under load, (acceleration) and the transmission will go into nuetral. Once this problem starts it will only become worse. Replacing the worn part will solve the problem. Fortunately all fourth gears on Mercedes 4spd. Standard transmissions during that era are the same. The final drive is 1:1. So you can salvage a fourth gear out almost any W110,W111, W108, W109,W113, series Mercedes. In short almost any top shift cover transmission of the era sedan, coupe or convertible even diesels! The part number indicates that it was probably first used in the fin series sedans. Transmission gears are expensive new. If you salvage a used one you will have to make sure it is not worn badly also. If you plan on using a whole transmission, the gear ratios on some of the other three gears may vary. Some external parts may have to be switched, (front or rear cover, bell housing, top cover etc). Otherwise they are interchangable and will work fine. the earliest transmissions had a four bolt top cover and the later had a six bolt top cover. The six bolt top cover will work on the four bolt transmission. (probably not recomended), but it will work and I have never seen it cause a problem. This info will probably make more sense to your mechanic. The Mercedes "technical data" booklets list the ratios of each gear for all the models.

5-speed

Pete Lesler: this is a rare option - some 800 odd cars from 1965 to 1971 were equipped, The 5 speed, a ZF transmission type ZFS-5-20, is long out of production with a paucity of spare parts available. There is a 5 speed for sale in Hemmings, and I believe it has been reconditioned. The asking price is $5000. Unless the seller has the driveshaft, which is shorter, the linkage and the shift knob, you will need to procure these as well. I'd guess an outlay of $6000 to $6500 would cover it, and that doesn't cover the cost of replacement labor.

Has anybody tried to put a 5 speed trans from another make of car in to a 113, I'm thinking a termac 5 speed from a late model Mustang might work?

Walter: In my case it was using the 5 speed manual tranny that came form the same car the engine came from-a 1985 190D 2.2L diesel with the 5 speed manual and clutch attached to it. I don't think it would be any better or worse if you used a different make tranny. You need to think about the changes that the engine mounting-bellhousing and driveline will require. You'd be better off by changing the motor too if changing the tranny, both from the same car-as a mustang engine and tranny.

Achim: there were, I think, a few options to put a non ZF 5-speed into our cars. However, most people refer to the original Mercedes-Benz GETRAG 5-speed with side linkage rods if they mean another 5-speed for the 113. This was the gearbox Mercedes used _after_ the ZF for instance for the 280 SE/W108 and all following cars, 114, 107, etc. My 2 cents: Don't do it. Either original GETRAG 4-speed (which our manual cars have), optional ZF 5-speed (hard to find and very $$$) or original automatic transmission. Nothing else. But of course, this is to everybody him-/herself.

Overdrive

Has anyone fitted an overdrive unit to a 280SL? I have seen overdrives marketed for RV's or pickups. Seems like a natural for a 280SL.
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