Transmission modulator adjustment - PeachParts Mercedes-Benz Forum

tangofox007 · #1 01-08-2005, 03:22 PM

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.

leathermang · #2 01-08-2005, 03:37 PM

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...

tangofox007 · #3 01-08-2005, 04:17 PM

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.

wolf_walker · #4 01-08-2005, 05:51 PM

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

tangofox007 · #5 01-08-2005, 10:01 PM

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.

wolf_walker · #6 01-08-2005, 10:50 PM

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?

braverichard · #7 01-09-2005, 12:57 AM

Quote:

Originally Posted by wolf_walker

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?

Is that what causes the downshift clunk - bad valves on the top of the valve cover? My 300SD also has that downshift clunk, in fact it has always had it since I purchased it. I'd like to get rid of it someday, but for now I have enough things to fix on that car to keep me busy for a while.

wolf_walker · #8 01-09-2005, 12:57 AM

You need to check each port individualy of the 3/2 valves to insure they hold/release vac as they should, especialy holding.

On the OP valve, pull vac on it with a gauge and watch to see if it bleeds down past where it's suposed to be.

If all really does check out(it's a minor miricle), then your vac sugnature needs adjusting. Closed throttle you should have way high vacuum, I forget the figure(this is what the 3/2 valves do) to keep downshifts from being felt, just the slightest throttle and it should let the vac valve on the IP take over, and go by it's setting.

On a trans with lots of miles, it's like that article said, OEM new specs are not always going to cut it. Adjust, fiddle, play, learn. If it's not flairing of slipping, it can be made to shift well usually.

pawoSD · #9 01-09-2005, 01:10 AM

I had the dealer take out my transmission and replace all the seals about 1 year ago, and they said the rest of the tranny was fine and had quite a ways to go, and they reinstalled it, all the leaks were gone, but it still does a clunk downshift from 4th gear when coming to a stop. Our 83 is far smoother, it engages drive and reverse with no feeling at all, and does not make the clunk. I don't really mind it, but even with the dealer servicing it and putting it back in/adjusting it, the clunk didn't go away.

wolf_walker · #10 01-09-2005, 10:34 AM

Quote:

Originally Posted by pawoSD

I had the dealer take out my transmission and replace all the seals about 1 year ago, and they said the rest of the tranny was fine and had quite a ways to go, and they reinstalled it, all the leaks were gone, but it still does a clunk downshift from 4th gear when coming to a stop. Our 83 is far smoother, it engages drive and reverse with no feeling at all, and does not make the clunk. I don't really mind it, but even with the dealer servicing it and putting it back in/adjusting it, the clunk didn't go away.

Play in the driveline? Rear end maybe?

Cateaux · #11 01-09-2005, 01:12 PM

The downshift "clunk" is generally caused by not enough vacuum being sent to the modulator valve via the vacuum control valve on the injection pump. When new, the valve should have 19 or 20"Hg going to the transmission when at idle (accelerator not depressed). If you have less than 15"Hg or so, the transmission thinks that you are trying to accelerate, so it downshifts into first gear, hence the clunk. When operating properly the transmission will stay in second gear when you slow to a stop, and will only shift back down into first when you hit the accelerator (and the vacuum signal to the trans drops).

I had the same clunk in my transmission. It turned out that I only had 13"Hg at idle, which was caused by leaks in the vacuum system. Since this car is only used for off-road racing

, I plugged the lines going to the 2/3 switches on the valve cover that control the EGR. I also disconnected and plugged the vacuum lines for the ACC (I have at least 3 leaking vacuum solenoids). This bumped my idle vacuum to the trans up to 17"Hg, and the downshift clunk is no more

.

braverichard · #12 01-09-2005, 01:16 PM

Could that unnecessary shift into 1st gear result in lower fuel efficiency by any chance? Just a thought.

Cateaux · #13 01-09-2005, 01:20 PM

Quote:

Originally Posted by braverichard

Could that unnecessary shift into 1st gear result in lower fuel efficiency by any chance? Just a thought.

I wouldn't think so. The downshift only happens when slowing to a stop and doesn't increase engine RPM's much. And the engine isn't under load, either.

tangofox007 · #14 01-09-2005, 01:21 PM

My "clunk" occurs at the 3-2 downshift, which appears to occur at the proper time. The 2-3 upshift is very smooth.

Cateaux · #15 01-09-2005, 01:31 PM

Quote:

Originally Posted by tangofox007

My "clunk" occurs at the 3-2 downshift, which appears to occur at the proper time. The 2-3 upshift is very smooth.

And it doesn't clunk on the 2-1 downshift? How strange! I guess you could try loosening the bowden cable a little, which might ease the clunk, but would also make the transmission shift sooner. In any case, it seems that the transmission is under the impression that you are pressing the accelerator when you aren't. Maybe this is an internal problem? Unfortunately, my understanding of the inner workings of the transmission are vague, at best. I've come to know the vacuum controls rather well, though

.