Tranny Modulator - PeachParts Mercedes-Benz Forum

#1 08-26-2002, 08:33 AM

Is the Transmission Modulator (where the vac. line attaches to transmission) supposed to hold vacuum? Mine leaks down slowly. The shifting is smooth, but the engine shut off is slow and the only vac. leak I can detect is the line to the transmission.

surfblau · #2 08-26-2002, 02:43 PM

And the answer was....

Yes it should hold vac.

There is a rubber cap on the modulator about the size of a nickel and about 1/4 inch thick. $2 at the most expensive MB dealer in the US.

I replaced and it no longer leaks. There are other threads that describe using RTV to seal. It may be worth a try.

There is also the rubber connector and the line itself that should be tested to make sure that the leak is not there.

best of luck

diesel don · #3 08-29-2002, 02:41 PM

mccan:

how did you test the leak down on the tranny modulator. i've tried about everything else and still get engine flare. perhaps its as easy as a leak. did you do the mighty vac deal from the top?

don

surfblau · #4 08-29-2002, 04:36 PM

Do it from the line that comes out of the TRANS output on the blue vacuum amp/flying saucer, or if you have an older 300, do it on the line (black) that goes to the transmodulator from where ever it originates (probably near the top of the IP).

#5 08-29-2002, 04:52 PM

don,

I did the mighty vac. test. As my car is a 79, the modulator is of a diiferent type and in a different location than in the lewer models. I'm not exactly sure how to fix (yet), but it's for certain that the leak is occuring within the modulator as everything from there to the pump checks out.

engatwork · #6 08-29-2002, 04:55 PM

Don't forget the line/hose from where you are measuring to the vacuum modulator. Check for cracks in the rubber hose end.

#7 08-29-2002, 05:16 PM

Thanks, Jim. The line and rubber coupling are both sound.

engatwork · #8 08-29-2002, 05:31 PM

mccan - the replacement of the modulator is pretty straightforward.

gsxr · #9 08-29-2002, 06:39 PM

Don, if by "engine flare" you mean the tranny flares when shifting, that's the opposite problem of a vac leak. My 1984 300D flared annoyingly when I bought it, but a quick adjustment of the modulator cured that. It should shift quickly and crisply at full throttle, if not, turn the modulator adjustment (behind the rubber cap) in 1-turn increments to make it shift firmer. I can't remember if you turn it CW or CCW to make firmer... CW, I think. When full throttle shifts are firm, then see if part throttle shifts are smooth - if not, then you may have a vacuum leak.

Full vac to tranny = soft shift
Zero vac to tranny = full firm shift

The VCV on the IP, and the "blue flying saucer" (vac amp) on 1985+ models are what control the signal between full (15") and zero vac at varying throttle.

HTH,

diesel don · #10 08-30-2002, 09:29 AM

Gsxr:

Thanks for the info. I've already adjusted the modulator to fine the best option. In addition, I've adjusted the cable up top and checked on variable vacuum related to idle. Everything checks out fine.

Under full throttle, the car performs well. The problem resides in a moderate or light takeoff. This is where the flaring occurs. So one wonders if the tranny is slowly going. I hate to jump on the acceleration as soon as she leaves the garage, but its the only way to avoid the troublesome flare.

I am still hoping to find a leak in the modulator cap or the hose.

Don

#11 08-30-2002, 10:28 AM

Can anyone give me the rundown on the procedures for replacing the madulator cap on a 79?

gsxr · #12 08-30-2002, 11:43 AM

Don,

It sounds like you have a problem with the VCV on the IP or something related to that. If the tranny was really going out, it would flare at full throttle shifts too. I bet you have too MUCH vacuum at light throttle. Try connecting a vacuum gauge with a "tee" in the line that goes to the tranny (after the VCV). Route the hose into the engine compartment (through the window or something) so you can drive the car and watch the vac gauge. It should be near zero at full throttle, at 15 inches at idle (no throttle), and be pretty linear anywhere in between. If not, you need to adjust or replace the VCV.

Note: You didn't specify which of your cars has this problem. The 1985 model has a more complex system which improves transmission shifting compared to older models. If it's this car, it could be one of several other things too.

Here's an EXCELLENT article on transmission adjustments:

http://www.mbz.org/info/articles/trans/service/

Regards,

surfblau · #13 08-30-2002, 11:51 AM

If it is like the 87 (it may not be), this is a carbon rubber cap the size of a nickel about 5mm thick that caps the adjustment T on the modulator.

Pry off cap with finger nail or small standard screwdriver. Replace cap by pushing on it with thumb.

All this part does it keep the vac inside the modulator.

It is so easy, that I am wondering if that was not your real question...

#14 08-30-2002, 05:11 PM

This one is on a 79 and is a couple of generations previous to the 87. It's not even the same as the 81 I had a look at earlier this week. The modulator is a different design and on the opposite side of the tranny, even though they are both 617 engines.

The procedure may be the same. It does have a plastic inserted cap. I've ordered a new one. I'll give it a try when it arrives. As the cap was $1.50, I thought I would give it a try before spending $50 on a new modulator.

gsxr · #15 08-30-2002, 06:06 PM

The original modulator on a 1987 is the same as 1983/84, at least on my cars - they all have the modulator on the driver's side with a rubber cap. However, newer modulators (if replaced) and the modulators on newer cars are quite different. They have a plastic cap that must be turned 90 degrees and then pried off to reveal the metal "T" underneath. My other '87 has a factory rebuilt tranny, installed in Nov-00, with this type of modulator. I have no idea what the really old ones (pre-1980?) are like!

HTH,