Vacuum control valve vs. vacuum modulator question. 1992 300D 2.5 Turbo
 

Hello to forum. The car is a 92 300D 2.5 Turbo. On these cars the shifting of the transmission is controlled by two vacuum units. One of these valves is up at the injection pump and according to the manual it is called the vacuum control valve. The other unit is attached to the transmission and it is called the modulator.

I've read somewhere that the vacuum control valve at the injection pump controls the timing of the shift and the modulator controls the quality of the shift. Does this sound correct?

Thanks!

The following is for a 1984 300 D (123.133).

The VCV helps control the harshness, or quality I guess, of the shift. Not sure of the modulator.

The timing of the shift is controlled by a Bowden (pressure control) cable. On a 123, it is attached to the linkage on the cam cover and leads down to the transmission.

Mine doesnt have a bowden cable.

I see, will you provide the six digit chassis number?

124.128

Thanks, is this a California or Euro model? The epc lists a Bowden cable for Federal only.

Is the transmission a 722.418?

Yes transmission is 418. I don't know if its a california model.

Okay, I'm not familiar with that transmission, perhaps whunter will chime in.

The epc is helpful, but only to a limited extent.

I believe it should have a bowden cable. Also, you should test your modulator to see if it holds vacuum. You can also test the VCV. Both tests are done with a vacuum tester (Mityvac is the popular brand). If you are experiencing harsh shifts, that normally indicates a loss of vacuum control from the VCV to modulator (or a bad modulator). The vacuum signal from the VCV regulates the vacuum to the modulator to adjust the pressure. No vacuum= more pressure, higher vacuum= lower pressure= softer shifts. You can adjust the pressure as well by removing the rubber cap on the modulator, and turning the "T" (clockwise for higher pressure). Pressure adjustments should only be made if all the vacuum components are working properly. It is recommended to use a test gauge fitted to the test port on the side of the trans to test the working pressure. However, I have made adjustments on several transmissions without a gauge with satisfactory results. It takes more time this way (adjusting, test driving, adjusting.....etc), but I think the "seat of the pants feel" is better. This is particularly effective with vacuum only transmissions (no control rod or bowden).

Not quite. The VCV and modulator are components of the same system. The VCV simulates gasser manifold vacuum signal based on throttle position. The modulator converts the vacuum signal to transmission pressure signal.

Sixto
87 300D

Sixto,

So as the throttle is wide open, less vacuum is being applied to the transmission modulator. So is it the modulator that determines when to upshift?

Vacuum should be about zero at wide open throttle. The control pressure cable/Bowden cable determines shift point. When the transmission has one. You say your transmission doesn't have one. I don't know what else determines when to upshift. Are you sure you don't have a socket ended cable with red rubber bellows coming up alongside the engine from the transmission dipstick (ignore the red arrow)?

http://www.peachparts.com/shopforum/...urbo-cable.jpg

Sixto
87 300D

Sixto,

Boy do I feel like an idiot. :o

I didn't think of looking on the other side of the engine as you indicated. I kept looking on the drivers side and only saw the vacuum line.

Yes, now I see the Bowden cable now. I noticed that the sliding part sticks a little bit. That may be why my transmission is shifting late!

Should have posted the attachment earlier, mine has a black bellows and a white plastic nut. I believe I unscrewed (looks pretty far out) it to have later shifts.

Do yourself a big favor and read this:

Mercedes-Benz Transmission, Steve Brotherton, ImportCar, February 2002

And also this:

Diesel Vacuum system adjustments