300D Problems with my vacuum control valve - PeachParts Mercedes-Benz Forum

orthotomeo · #1 02-15-2006, 04:25 PM

I hate to repost, but I've been reading many an article and writeups but can't find an answer.

I just bought a 1980 300D with automatic transmission and am trying to fix some flairing issues in shifting.

I've taken the cap off the vacuum control valve to adjust the vacuum levels, but I don't see anyway to adjust what is there. It looks like it's just a small brass circle. If anyone has a close up picture of their's or could tell me what exactly I need to adjust it with I'd greatly appreciate it.

orthotomeo · #2 02-16-2006, 11:09 AM

Well I've isolated the problem to the vacuum. I disconnected the Transmission vacuum hose and it shifts fine, albeit a bit late (3rd at 60, 4th at 100).

I'm planning to pick up a vacuum gauge and make my own vacuum tester, but I'm thinking I may have to adjust the vacuum control valve. If anyone has an answer or a diagram for my original post i'd appreciate it.

orthotomeo · #3 03-02-2006, 11:44 AM

Well I just wanted to update on my isolation of problems.

I found the vacuum leak. I was having problems where the car wouldn't shut off when I took out the key. I was getting and holding pressure in the engine compartment, so when it was overunning after beeing turned off I found the damper that connected the vacuum to the door locks (door locks are yellow hoses with green/red stripes).

I put my finger over the damper and the engine shut off. There's a walk through for diagnosing vacuum leaks in your locking/unlocking system somewhere on the board. Anyway, it turned out to be leaks to my trunk unlocking and my fuel tank door lock. I set the fuel tank for unlock and the trunk for lock and then plugged the problem leaks.

This made the shifting a lot more definite and hard. Since then I ordered a new lever for the 3/2 valve and replaced a section of hard black plastic tubing going to the vacuum pump.

So far so good!

MattBelliveau · #4 03-02-2006, 12:42 PM

That little "brass circle" on the vac control valve IS a small nut - look closer. You will not be able to fit a socket on to it. It has two flat parallel sides, so you will need to use plyers or a small creasant wrench. Honestly, depending on where you live (even better, depending on weather or not they do emissions testing), you may want to get rid of the 3/2 valves, or at least cut off the vac supply and return from it, if you are looking for softer, slightly earlier shifts. On the note of the shut-off valve, it is on the same supply as with the door locks. If you disconnect your door locks, and cap them off at the supply, the valve should work fine. Basically, under the hood, there is the main vac line (runs between the pump and the brake booster). This line will have two junctions (one goes to trans shifting and EGR, the other goes to locks and shutoff). It sounds like you may be working on the wrong junction if you are troubleshooting shutoff problems. PM me if you need more help, I just finished this nightmare about a month ago myself.

orthotomeo · #5 03-02-2006, 03:49 PM

Thanks for the support but I already figured out the problem (my last post may not have been clear).
As a note:
On my model the VCV is not adjustable.
There is no EGR.