Do I have a bad transmission modulator?
 

(I have been posting on another thread, but it is getting a little long).

Symptoms- moderately hard to neck breaking shifts at all speeds, at all temps, in a 300SDL 87 - 126 with OM603.

I have been monitoring the vacuum line pressure on the line that goes to the transmission modulator and have observed the following:
- idle parked is 14psi
- idle at highway speed (on a big downhill) is "usually" 14psi
- full throttle is around 5 or less psi
- pressure varies proportional to pedal pressure from 14 to +/- 5 psi

All that seems fine. Here is the problem. 50% of the time when on city streets, when I accelerate and get the vacuum below 10psi, the pressure will not return, and often leaks off and hovers around 4psi. I get terrible shifts at this point.

However, if I pump the mightly vac to get the vacuum back to 10psi or so, the needle will usually recover its freedom of motion and will "work like it should" - that is, varying between 14 and 4 psi. The shifts at this point are absolutely perfect.

There are no leaks in my vacuum lines or connectors.

The modulator will hold a 15psi vacuum applied directly to it.

Does it sound like my modulator is toast?

Any help appreciated.

thanks

I would suspect the proportioning valve on the IP or the vacuum transducer on the fender.

You may want to try using the MitiVac to control the vac on the modulator directly -- this will tell you if the modulator is working. Shifts should be smooth and sloppy at high vac and harsh at low vac, and you should be able to adjust them.

You can also bypass the vacuum transducer and connect the tranny directly to the proportioning valve temporarily to see what that does.

The rest of the vac circuit is for the 50C shift softening, and it shouldn't cause you low vac problems.

Peter

On the 300D it is not terribly hard to remove the modulator and test it. Remove two screws, unhook the vacuum line. With the modulator out you can see the part move in the modulator as you vary the vacuum applied to the vacuum line. Then just put everything back together.

my followup
 

So I attempted to remove the modulator to test it, and I promptly stripped one of the allen bolts that hold it in place.

So I followed Steve Brotherton's advice and adjusted the modulator for practice anyway.

Well, 3.5 turns ccw and the car shifts perfectly- hot or cold, at all speeds.

The previous modulator rubber cap was chewed up, which should have clued me into the idea that someone had been messing around with it. I think that they attempted to adjust the modulator to compesate for all the other vacuum leaks that they otherwise couldn't diagnose and repair.

So I believe that this is the end of the story on my hard slamming SDL transmission.

thanks everyone

Surfblau:

Cool! I really like to have things working correctly all the time!

Also goes to show, you need to properly diagnose and repair rather than just do quick fixes. Saves incredible amounts of time and money to do it right the first time.

Peter