Adjusting Automatic Transmissions

[brettj]

Hi again,

I had the seals, fluid and filter replaced in my 75 450SE automatic transmission. Now it shifts hard especially when you accelerate after stopping at a light. Can I adjust the transmission to soften the shift a bit or is something wrong with my transmission? I thought I read somewhere that you can adjust the kick down lever in the motor to do the adjustment but I'm not sure where to go on that.

[koan]

Three things control shifting, manifold vacuum, throttle position rod and kickdown switch.

Throttle position rod controls how long a gear is held for, kickdown forces a downshift if possible and manifold vacuum to modulator determines how firm the shifts are.

If you are not happy after the rebuild take it back and talk about it.

koan

[brettj]

Hi Koan,

I did take the car back and was told that because the new vac modulator installed was the old version which you can't adjust they had no idea how to adjust the trans and that I should leave it as is.

[s class]

well, I believe you can adjust them.  On the old version, AFAIK one changes the length of its internal pushrod slightly to alter the pressure.

Are these the same geniuses that couldn't use a CO meter to set the air/fuel ratio?

[WGB]

Hi Brett,

Your problem is presumably the modulating pressure.

Here is the page from the transmission manual which tells how to adjust the two types of modulator.



If you are a keen DIY with time on your hands you can fiddle about with an allen key as shown or if you want to go back with to a professional with the pressures I can provide a chart of the correct pressures if you require.

Bill

[brettj]

Thanks Bill,

What manual did you find the adjustment information? I've been looking through all of the shop manuals I have and none of them cover the transmission. If you would, please send me the chart which lists the proper transmission pressures. I'm currently looking for a new mechanic to correct this issue and to do a valve job on my car. So that information will be helpful.

Brett

[WGB]

Hi Brett,

There is a specific M-B transmission manual that I purchased some time back from www.schiffeuropean.com and have scanned it into digital format.

Schiff European seem to have gone off the air.

I will post the pressure chart when I have time over the weekend.

Bill

[oscar]

FWIW Brett, when I replaced my modulator in 06/07 sometime, I just reused the original pin in the new modulator and shifting was as smooth as it was before.  There was nothing really wrong with the old modulator, just thought I'd replace it for the sake of it.   The one time when I did have harsh shifting, particularly during downshifting when the trans would send a thud throughout the car, it turned out to be a cracked vacuum line to the modulator. 

When you went back to these guys did they look at the car and check the vacuum line, or did they just tell you the modulator can't be adjusted ?  At the very least they should've checked your vacuum line is intact based on the symptoms you describe.

You can do this yourself, tracking your vacuum line from the modulator to the engine to make sure it's connected, but you need car stands or similar to get under there to check the modulator side of the line.

Similarly, I hope they had the presence of mind to reuse the old pin.  A modulator will do nothing if that pin's not installed.  If you're without a way to get under the car I'd go back to the workshop and ask about the pin and to check that the vac line is intact and connected. 10 mins up on a hoist would confirm the pin is present and vacuum line functional.

An old photo, but it shows a relatively new modulator A0002700179 with vacuum line with a big split where it's kinked.  Though the hole is obscured from view in this photo it was the sole cause of this harsh shifting for me, hopefully that's all it is for you too.

[brettj]

Hi Oscar,

After discovering that the car shifted harshly, I took the car back to the shop and was told that they could adjust the modulator. When they put the car on the lift the mechanics discovered that my car has the old style modulator that doesn't have an adjustment lever attached to it. I was then told by the mechanics that they would have to research how to adjust the modulator. After that I gave up on that shop. They have no idea how to work on my car. So I'm looking for someone who knows what they are doing so the car runs correctly and I stop wasting time and money on this car. I do a lot of the repair work myself on the car but I'm very limited in what I can do because I don't have many of the tools, space or time to take on the big jobs. Therefore I'm at the mercy of the very dishonest and inept mechanics in my area. Most likely I'll have to travel to the Boston area where there are a few classic Mercedes mechanics who seem to know what they are doing. The only problem with that is Boston is a 2 hour drive from my home.

This is the second vac modulator I've installed in the last 3 years. When I replaced the original unit I asked the mechanic to check the vac lines and he said they were fine. After that the trans worked fine. The only reason I changed the modulator once more was due to the fact that it was leaking trans fluid. I'm pretty sure that when the current modulator was installed, the pin was put back in as I collected the replaced modulator an it didn't have the cap or pin left in it.  You may be right in thinking that the vac lines are cracked and as soon as I find a new mechanic I'll have the lines checked.

[WGB]

Here is the approved method of measuring all teh pressures but the modulating pressure is the only adjustable one.



Here is a table of pressures for different 116 transmissions.



Bill

[oscar]

With WGB's posts there and a couple of Carl888's photos I just found I'm completely stumped now.  These old modulators must be able to be adjusted....
From this post on http://forum.w116.org/mechanicals/280-se-big-service/msg68853/#msg68853
It's for a d-jet 280 but still it's the same modulator.