Vacuum - what should I expect?

[roadbuzz]

The car: 84 300D, ~210,000 miles, brown , runs good, but transmission won't pull in 3rd so never gets to 4th. (See this thread: 84 300D Transmission Woes ).
Me: cheap, know little about benz's, less about diesels, nothing about transmissions.

So I looked at the DIY page about transmission vacuum control tuning (http://www.peachparts.com/Wikka/TransVacTune). The first test recommends testing vacuum coming out of the pump. To test it, I pulled the line that behind the engine block out of the first wye connector past the green damper thingy. Using my Mity Vac, I got 14 inches at idle, that dropped to about 9 at around 2000 RPM, and didn't decrease at 3000 RPM, which was all I could get my assistant/wife to rev it to ("somethings burning!" "It'll blow up!").

I figured that was reasonable, since 3000 RPM is about what it revs going 65-ish, so maybe that's enough. The DIY page says that vacuum should drop to zero at full throttle, and it did drop near zero during the hard accelerations. So, after all that preamble, my question. Is that pretty much what I should expect, or does my pump suck too much?

TIY for any and all advice, comments.

[Brian Carlton]

Firstly, I would suggest to you that the vacuum to the transmission is the least of your concerns right now. It affects the quality of the shift but does not affect the timing of the shift or anything else regarding the performance of the transmission. If you must check it, you must do it with a T in the line and route the guage into the cockpit to read it while driving. You can't simulate a situation with maximum power in the driveway.

The greater concern is why the vehicle won't develop enough power to get out of third gear. If I'm reading the post correctly, it won't shift to fourth due to insufficient power?

This needs to be addressed by looking at the basics of valve timing, chain stretch, and injection timing. Something is seriously amiss and it's not likely transmission related.

Brian, the way I read his original post the tranny was slipping in third, so he never made it to forth. I don't think he said it was low engine power, but the power wasn't getting to the wheels due to the slipping.

[roadbuzz]

Post wasn't clear. Power isn't the problem, the engine runs fine. When the car shifts into third, the tranny just lets go and the engine spins up. Under those circumstances, 4th ain't an option. Tho' I expect your advice for checking the vacuum is right on. Where is the best place to tie into the vacuum system for checking?

[Brian Carlton]

Is what you are experiencing a "flare".

This is common on the 2-3 shift on older transmissions. The engine runs away, momentarily, and then the shift occurs. Usually, the driver lets go of the pedal and the shift happens because of this.

If this is the situation, then the vacuum adjustment may have a benefit.

The best place to put a T is right in the line that goes down to the transmission. Just interrupt this line, near the top, and put a T in there.

Also, to see if you can affect the flare, disconnect the line to the transmission and plug the supply at the top. Drive the vehicle without any transmission vacuum and report back as to whether the flare is eliminated. The shifts will be very firm, but, this is just a temporary test.

[roadbuzz]

Alas, tis not a flare. The tranny goes into third and pulls momentarily, but then lets go. That's the best description I can give, since I'm afraid to drive it much in this condition... don't want to risk doing any damage.

Anyhow, thanx for the tips. Gotta get a bunch of that little plastic hose for the on-the-road vacuum test! Meanwhile, I'll try the no-vacuum test.

[Brian Carlton]

Originally Posted by roadbuzz

Alas, tis not a flare. The tranny goes into third and pulls momentarily, but then lets go.

OK, that's a more serious problem. I'd be doubtful if the vacuum adjustments can provide a solution.

But, driving it with no vacuum will provide the maximum info.