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Anatomy of a Transmission Vacuum Control Valve

on the 1987 300D (OM603)

by sixto


anatomy of a 603 VCV

Replacing valve body springs and tweaking the transmission vacuum amp helped soften stiff shifts but not enough for the way I drive. In the backwards way I do things, I saved the easy fix for last. It wasn't until I had done all the difficult fixes that I attached a vacuum gauge to the modulator line, thus discovering the modulator never saw more than 5" Hg under even the lightest throttle. I further discovered that there was absolutely no vacuum supplied to the amplifier through the VCV. This is particularly stupid of me since I'd already sorted out an 87 SDL with this exact problem!

Long story short, where the SDL had a disfigured pad, this VCV had a big old hole (well not so big since the pad is only 5.5mm in diameter) through the pad. I fashioned a replacement pad out of a piece of bicycle tire inner tube which is thicker but not twice as thick as the original pad material... based on what's left of it, anyway.

So what pad is this? Keep reading...

The VCV is held to the left/US driver side of the IP by two bolts that take a 10mm wrench. There is an adjustment to consider when installing the VCV which I won't get into here

I used a 3/16" bit to grind off the rivet heads and countersink the holes for the screws I planned to use. The screws were planned, countersinking was an unexpected bonus FWIW, my great contribution to MB DIY-hood is discovering that common wallplate screws will self-thread perfectly into these drilled out rivets!

When you pry apart the aluminum cover from the plastic case of the VCV, the open face of the VCV has a rubber sheet over it. The hole in the lower left corner corresponds with the vent fitting. The nut at the center has a pin that slots into the aluminum case.

This is the negator spring and plugner of the VCV. VCV valve train already removed.

This is VCV valve train. The parts are out of order as I discovered when I put the VCV back together. The big spring goes between the black cap and the flower (doesn't look like a flower in this pciture), not between the valve train and the plunger as suggested. The outer edge of the diaphragm is rounded on one side and flat on the other. The flat side goes against the black cap. The plastic ring keeps the shape of the diaphragm.

This is the business end of the VCV. The tangs of the black cap are chewed up because it's a real PITA to release it from the plastic VCV body. You can see the vacuum supply nipple in the center, that's a metal tube, and directly to the left is a port leading to the amplifier fitting. The o-ring is what's left of what should be a solid rubber/foam pad that is pressed against the metal tube to cut vacuum to the amplifier. The pad goes into the holder (you're looking into the holder in this view) which has 4 tabs to hold the pad. The holder sits on the spring inside the cup below the spring. The cup retains a plastic ring and diaphragm (not shown in this picture) against the flower thing above. It doesn't look like a flower in this picture either.

This foam/rubber pad should be solid. A piston pushes this pad against the vacuum source nipple to cut vacuum to the chamber and thus to the port leading to the amplifier.

Pad thickness.

Technically, you don't have to open the VCV to replace the pad but it's really difficult to pry the tabs of the black cap through the slots on top. 2 or 3 of the tabs are shielded by the aluminum cover when the VCV is together.

I'll bet you wish I'd cleaned it before taking pictures image

Discuss this DIY here.


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