300D Trans. Woes
 

Hello,
I have been enjoying reading this forum and have finally decided to join.
So, like everyone else, I have been trying to get my '83 300D to shift better. This has been going on for about a year without much luck.
So, after reading the various threads I decided to disconnect the egr (for testing purposes) and disconnect the black 2/3 box.
Problem is, now I have no vacuum reading at all where the vacuum line comes out of the vacuum control valve to the modulator. I am not sure if this is a result of my recent changes, or somthing else, but I do know I had vacuum at this point last year.
The only possible problem I can see it that now the vacuum line on the side of the VCV goes into the cabin and attaches to nothing. Is this a vent line? Should it attach to somthing? Any suggestions on what I should do next?
The strange thing is that it shifts better than ever-- Nice even shifts, not too harsh.
All help/comments appreciated.

The operation of the VCV is quite confusing. On the surface, it appears as though vacuum enters the top of the VCV and comes out the bottom hose. However, this is totally incorrect.

The top line feeds the VCV and also sends vacuum to the transmission. There is a critical orifice in the line to the VCV and only a small amount of flow can pass across the orifice. Then, the VCV starts its "leaking" of vacuum. As the VCV "leaks" more vacuum, the vacuum level to the transmission drops proportionally to the leak. Eventually, at maximum throttle, the leak is so huge, that the vacuum to the transmission is zero.

So, if you wish to check vacuum to the transmission, put a Mityvac or a vacuum gauge in the line to the trans and read what you have. On the older vehicles, you can disconnect the supply vacuum and substitute the Mityvac for the vacuum pump. Pump up the system and then open the rack and see if the reading falls proportionally. This won't work on any '85 or newer vehicle, however.

The line on the side of the VCV is a vent line........it goes into the cabin to obtain "clean" air for the VCV. Remember, the VCV is always drawing air from the outside to "kill" the vacuum level.

Still confused
 

Brian,
Thanks so much for your reply.
As I understand it, when I attach the vacuum guage to the Y in place of the line to the modulator, I should read about 10" of vacuum at idle. Right now, I have no vacuum at this point at all. The line supplying the VCV has plenty of vacuum until I hook up the VCV, and then the gauge reads zero. What happens to the vacuum? I cannot hear any vacuum bleeding off, and just cannot understand what's happening. Any Ideas?
Thanks for your help.

If there is no vaccum to the modulator and you've definitely got supply vacuum.........then the VCV has failed.

But, under this condition, your shifts should be very hard and the downshifts should bang. Since this is not occurring, I'm questioning your testing procedure.

Brian,
Thanks for your reply. I discovered a small vacuum leak in the connecteor between the surge damper and the VCV. When I corrected that I was able to read about 5" of vacuum available to the modulator. However, the vacuum does not diminish at all when I apply the throttle ( either while parked or driving). The only time that the vacuum diminishes is while shifting into park or reverse. Also, Adjusting the VCV has no effect at all on the amount of vacuum available to the modulator. Seems to me like the VCV is the culprit here. Any ideas how to test it, or if it is repairable.
Thanks again,
Ray

If the VCV only provides 5" of vaccum and doesn't modulate the vacuum as the rack is opened, then it's failed.

But, what confused me is the fact that you don't have excessively harsh shifts, which would be a characteristic of low vacuum.

It's possible that the combination of modulator pressure (relatively low) and vacuum (very low) results in an acceptable shift pattern, but, this would be quite unique and the long term ramifications of such a setup are unknown.

Progress...
 

I adjusted the throttle linkages on my engine, and was able to get vacuum headed toward the modulator. In fact, I had 15" of vacuum measured at the tee. It would go to zero when I throttled. So far so good, except that the 2-3 shift was VERY soft. I attemped to adjust the vcv to bleed off more vacuum, but when doing so the vcv starting making a noise like when you pinch the end of a balloon while air is rushing out. The vacuum decreased, but the needle on the guage was shaking massively. On a test drive the shifts were better, especially after tweeking the bowden cable (which deifitely affects shift quality) But as before the shifts were unpredictable.
So, I guess I'm in the market for a better VCV. A new surge damper is on the way.
Thanks for all the help.