Auto Trans Adjustment

[jedwards]

Gentlemen,
I have an early RHD 250SL Auto and I am trying to understand how to interpret the behaviour of my auto transmission, so I can adjust the modular linkage to give the best changes. 
 
I have all the basics working. The 3 position solenoid is working well and modulator linkage moves smoothly into all three positions in response to accelerator position. ATF is new and to correct levels.  Change downs are perfect and barely perceptible and kick down works well.
Upshifts are generally OK and will change smoothly if I back-off at around 2,400rpm, but are sometimes a little harsh.   Changes under acceleration are too harsh and moving from reverse to drive can be a bit clunky. I believe this is all down to adjustment.

From what I understand,  I think I have the modular pressure  set a bit too high and should bring it back a bit.  But I don’t know if lengthening the modular rod increase or decreases pressure.  How sensitive is the system to length?  Are adjustments down by flats or by rotations of the turnbuckle?  I don’t have a pressure gauge and must therefore rely on trial and error.
Any advise?

[dario]

Hi jedwards,
Changing the length of the linkage rod you only fine adjust the pressure, when your kick down is activated (so I understood from the BBB).
The comfort of shifting is adjusted by changing pressure on the modulator (rotative pin with 1mm hole to seal, which is blocked with M10 control nut - FYI only).
I'm such a wise guy because yesterday during linkage adjusting, the characterisctic of the auto transmission changed dramatically. In my case the auto trans started to slip. Having also no gauge (especially having no modulator pressure PORT-connection - M8x1mm untypical thread, very small place 2-3 cm to connect) I simply changed the pressure "by eye"(?). I started from increasing pressure, rotating this pin 1 full rev CW, then after a test ride only a half of additional rev was needed to reach smooth work of the transmission.
As I compare your transmission's behavior to my, you could start from decreasing pressure but a very little (marking start position to possible return, turn the pin CCW only 1/4 rev first, test ride and next 1/4 rev if needed.
The solution should be very close to your actual adjustments (on my nose only very fine adj is needed).
The good idea would be also check, if the sensor at the butterfly disconnects at max. 1600 rev (or less). If not, you should adjust this shut off-position first by rotating in small narrow (right or left) the sensor at about 1200 rev of engine. The 2 housing bolts should be only loose a little to allow for moving the sensor.

Sorry for the lengthy post, and if you already know it all, I apologize for writing the obvious.
Please keep us informed about the results of your work.
Dariusz

BTW, if it the rough and late shiftings occure only 3 to 4-th gear change, the possible issue could also be too high RPM at idle.

[jedwards]

Thanks Dario
Great advice and really helpful.
I'll give it a go next weekend.

Jeff

[jedwards]

Hi Dario,
I am back from my travels and I am keen to try your recommendation, but I am not sure what I am dealing with.
I took a look under the car but the modular seems to be tucked behind the exhaust shield, so will have to hammer that out of the way first.

in your message, you state "rotative pin with 1mm hole to seal, which is blocked with M10 control nut".  The only image I have of the modulator is the one from the Tech manual (attached) which is too low res to show me what I need to look for.

From your description, you indicate there is an control nut (M10). Does this act as a lock nut? Do I remove this, or loosen it or is the the adjustment itself?
Then you said there is Pin with a 1mm hole. Does this take an Alan key or is it turned some other way?

Any advice, pictures or instructions  would be greatly appreciated.

Thanks in advance
Jeff

[jedwards]

After reshaping the exhaust heat shield (RHD car) and cleaning the area up, it became clear how the Modulator adjustments are made.
My adjuster still has the factory lead and wire seal on it, so I assume it is not far off the original factory settings.
Before i start playing with it, can anyone tell me how tension on the Modular diaphragm alters shifting behaviour and what effect screwing the adjuster in or out makes?


many thanks in advance.

[stickandrudderman]

It's much the same as an adjustable fuel pressure regulator. Screw in to increase pressure, screw out to decrease. Only 1/4 turns will make a difference.

[jedwards]

Many thanks stickandrudderman.

I assume then that screwing in equates to increasing pressure which equates to "firmer" or harsher changes?
And therefore to soften harsh changes, I back the adjuster out 1/4 or a turn at a time?

Does this effect the entire engine load range evenly or only the lower RPM range when vacuum levels are higher?

And finally, once I the change characteristics as good as I can using the modulator, do I then "fine time" by adjusting the linkage  rod length?  If so, is longer equal to higher pressure and shorter equal to lower pressure?

Sorry for the continuous questions, its just that there is not much info on the Tech Manual on this area or elsewhere on the web that i can find.

[stickandrudderman]

Adjusting modulator is the LAST thing you should do AFTER you have checked and adjusted all other parameters.

[ctaylor738]

I (strongly) recommend that you either adapt a gauge and understand and follow the procedures in the manual, or leave this to a transmission shop.  Especially since your trans is almost working properly.  I did an adjustment on a 250SL that had an shift to third at 2200 rpm regardless of throttle, and no kick down.  I learned that you really need to get the pressures right with the rod adjustment before you fine-tune the shifting with the modulator.

You can adapt a gauge by disconnecting one of the pressure switches and using the banjo bolt to connect to the test port.  You have to find a bolt and a sealing ring to plug the connection for the pressure switch.

One of the biggest pains was to go out for road test, and then jack the car up and support it to tweak the modulator.  The job will be a lot easier if you can get access to a lift for an hour or so.

My $.02

[ja17]

Yes, I agree with Chuck.  If a basic minor adjustment will not work you should try to get the modulator pressure reading. I use an old AC gauge and hose with a adapter fitting (banjo fitting) at the modulator test port. I jack the car up and at idle you can activate the different inputs of the solenoid (kickdwon, idle and mid-range) to get it right.

Here is some good information. These guys have been doing it for many many years.........
http://www.mercedesdismantlers.com/16BoltInstallationInstructions.html

However, you will need to refer to the BBB or the MB automatic transmission manual for the pressure set-up procedure.

[ja17]

Also since engine timing effects engine vacuum pressure, make sure your ignition timing is correct. It can drastically effect transmission shifting.

[jedwards]

Thanks you for all the help and advice.  Much appreciated.

From what I have learned, this is what I need to do and ask of the transmission guy to do.

1>   Check timing to ensure good vacuum
2>   Get the transmission guy to adjust the Modulator pressure, via the Modulator Pressure Adjustment screw until we have 40 PSI showing from the port.
3>   Then shorten the Modulator push rod until the pressure starts to rise above the 40 PSI and then  lengthen it until it starts to fall,. 
4>   Set the rod length to that transition point
5>   Reset the modular pressure to 40 PSI and go for a drive.
6>   If this results in harsh changes, then reduce the modulator pressure ½ tune at a time until it improves.
7>   If this results i  slipping or “soft changes, then increase pressure until it improves.

Is that the correct procedure?

My aim should be almost faultless changes not far off that of a modern cars.
Is that right?