Replaced leaking (would not hold vacuum) transmission modulator in '82 300D. Is it permissible to set the modulator pressure with the rear wheels off the ground, or is it essential that the car be on the road for the adjustment? I made the initial adjustment with the car on jack stands, at 30 mph. The 1-2 shift is pretty hard, 2-3 is good, 3-4 is firm. The clunk during downshift (that I was hoping to eliminate) is still there, at least some of the time. The downshift clunk is most noticable when accelerator pressure is reduced soon after an upshift has occured. The new modulator holds vacuum and the remainder of the vacuum system checks okay. Any ideas or insight will be appreciated.

Steve Brotherton is a moderator on this site... but on one of the other forums... he has a great site concerning setting these things... or you might email him direct if speed is needed...

Thanks. I found one of his articles earlier. He says that he adjusts overhauled transmissions with a gauge and "used" ones by feel. Others seem to think that it's important to set the pressure with a gauge.

Mine set with a home made gauge sitting still was pretty darn close to what it was at the rated speed. FWIW...

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One more Radar Lover gone...
1982 VW Caddy diesel 406K 1.9L AAZ
1994 E320 195K

That's good to know. I guess that I am a little disappointed that the new modulator really did not improve things much. The fact that the original modulator did not hold vacuum at all caused me to expect a lot of improvement when the new one was installed. (The new modulator does hold vacuum.) I will try backing off the pressure a bit to see if that helps.

The two valves on top of the valve cover are shot, that's your downshift clunk, the injection pump vac valve($170) is probibily shot as well, giveing eratic shifts out of spec. Sucks don't it?

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One more Radar Lover gone...
1982 VW Caddy diesel 406K 1.9L AAZ
1994 E320 195K

Just so I understand...to test vacuum on the modulator, one uses a "T" to keep the vacuum line connected to the modulator and get the vehicle up to 50 kph, then messure?

For WIW when I first tried to correct hard shifting on my 83 , I checked all the vaccum lines for leaks and found one that went no where. Thinking I had found the problem, I plugged it. The results were a hard downshift and the described clunk. Thought for sure the rear end was going out, but after researching old threads I learned that the line I plugged was a vaccum vent. Unplugged the line and no more clunk. This vent line comes off of the vaccum control valve on the IP. Check to make sure no one made the mistake I made.

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a few mb diesels

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NO MORE "CLUNK".

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'99 S420, 155k
'91 VW Vanagon GL, 150k
'85 VW Vanagon GL, 120k
'82 VW Westy, aircooled, 165k

Ok I read this thread....and I saw that Brian mentioned adjusting the vacuum valve on the injection pump to have higher idle vacuum...how does one do this?

Also, yesterday I put the mityvac on the line leading into that valve (the trans valve on the IP) and it leaks down really fast in the idle position, I pumped the mityvac as fast as I could and could only reach 12-13"...and as soon as I stopped it quickly leaked back down....does this mean the valve is faulty/leaky? It does "leak down faster" as you apply throttle...and it does it a linear fashion as you increase the throttle position....its just that at idle it leaks down too easily...is this normal? I read you should see 20" on that line at idle....

I checked my modulator for vacuum and it will hold 25" rock steady...no leaks. Could it still need replacement or is it ok?

I have the annoying downshift clunk when slowing down and my 3-4 shift slips/is soft unless I let off the throttle when it wants to shift...then it bumps right in...its also quick and firm when the trans is real warmed up.

Does my valve on the trans need rebuilding/re-sealing? I'll check the ones on our other cars today and see if they leak like mine. (both shift great and do not clunk when slowing down)

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X 2!!

There is a linkage going to said valve, take the linkage off and push the lever on the valve all the way down, all the way one way should give you more like 20" and the other way the valve will hold close to or no vacuum at all. If that checks out OK then the valve is good. What appears to be your problem is the vacuum valve on top of the valve cover, usually the little lever will wear off and not supply enough vacuum to the valve on the IP. You can make up for that valve by shortening the linkage going to the VCV on the injection pump so that it will supply 20 inches at idle.

Your modulator sounds fine.

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The first thing is to be sure you've connected the Mityvac properly. Above the VCV is a T............one end is supply vacuum............the opposite end goes to the modulator............the bull goes to the VCV.

You must remove the supply vacuum line and install the Mityvac in its place. Then, pump up the Mityvac and see what it reads. If it won't pump up and hold any vacuum...........and the modulator holds vacuum.............the VCV is NG. It should hold 10"-12" at idle.