Vacuum control valve vs. vacuum modulator question. 1992 300D 2.5 Turbo - Page 2 - PeachParts Mercedes-Benz Forum

whunter · #16 02-17-2014, 07:45 PM

Quote:

Originally Posted by airfoill

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!

In scanning this thread, I don't see where you list any issue?
What is the issue?

The vacuum modulator is a common failure item.

Transmission shifting is effected by almost any vehicle vacuum leak, excluding the central lock system.

.

Mxfrank · #17 02-17-2014, 08:19 PM

Quote:

Originally Posted by sixto

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

+1 The VCV, the Vacuum amplifier (blue UFO), and the Vacuum modulator are all part of the same system. As the name implies, the purpose is to modulate shifts. When out of adjustment or malfunctioning, shifts will be harsh.

The shift points are primarily controlled by the valves in the transmission's valve body and the torque converter. Changing the spring rates will change the shift points and spacing. A problem in the valve body can cause stacked shifts or missed shifts. If the transmission is otherwise operating normally, the entire shift pattern can be moved higher or lower with the Bowden cable.

Neither the Bowden cable nor the vacuum modulator are especially problem prone. Most adjustments get done because if you have a knob, you turn it. Sometimes you even get lucky and the issue goes away. There are exceptions, but vacuum leaks are the root cause of most modulator issues.

compress ignite · #18 02-17-2014, 09:37 PM

reply # 15
READ Brotherton's article until you're (Impersonal you) Bored to Tears,then stand on your head (Altered Perception A.K.A. POV) and read it some more!!!
Put it under your pillow for a week and mayhaps more info will "Osmosisize" into
your cranium.
LOTS to comprehend!