87 TD shifting issue - advice needed - PeachParts Mercedes-Benz Forum

old goat · #1 01-23-2010, 11:07 AM

I now have an 87 300TD with about 8K on a rebuilt transmission, 265K on the car. Car has run quit well and shifted very well after replacing the sub-frame mounts. On a trip north after about 150 miles stopped at a rest stop. Started out of area and with significant traffic had to hit the accelerator to merge back in. The trans would not readily shift to 3rd without taking my foot off the accelerator and letting it slow quite a bit. Getting into 4th was just as troublesome. Made it to my destination....another 50 miles on two lane roads and just shut it off for the night.

Next day, the shifting from 1st to 2nd became an issue and it seemed to refuse to go to third. Under the hood the vacuum line from the intake manifold to the switch over valve was off....probably I thought because it was tight and the engine torque had just pulled it off. Put in new and longer hose but that made no difference whatsoever.

So, I am not sure if the issue is the kick-down switch which can make such issues and is probably original OR if there is some new vacuum issue. The vacuum pump just up and failing does not make much sense.

Any thoughts or suggestions will be greatly appreciated!!

Frank

Crazy_Nate · #2 01-23-2010, 11:20 AM

You should be able to unplug the kickdown switch (underneath the accelerator pedal), to allow you to test drive without it. It sounds like yours is stuck.

Here's what you're looking for:

Kickdown Switch (pictures)

kerry · #3 01-23-2010, 11:49 AM

Could also be a sticky bowden cable.

markg612 · #4 01-23-2010, 11:59 AM

I've had this same problem a few times with the same car.

The first was a loss of vacuum integrity, the second was corrected by adjusting the positioning cable that runs from the accelerator linkage to the xmission.

The vacuum issue was a bit tricky to sort, but one of the modulators behind the right headlight was leaking internally, enough so it messed up the vacuum module controller on the side of the injection pump.

The cable adjustment was an insidious decline, as opposed to what you're experiencing. The cable should be adjusted about 1-2MM short off the ball so when it's installed there is a slight tension.

old goat · #5 01-23-2010, 12:26 PM

Thanks for all the feedback so far. I do intend to unplug the kick-down switch....just as soon as I get back up there. I left the car there and got a ride home. I want to have everything I might need when I go back. The car is at a friends house so safe and sound.

I did check the cable best I could and appeared free and functional.

As for the vacuum problem being the cause, I can check but could the leak be so bad the trans does not shift at all?? I did not look at the area behind the right headlight to check vacuum....lines or parts....but obviously will have to do that as well.

Frank

markg612 · #6 01-23-2010, 12:34 PM

While it's good the cable is free, it must have the slight "preload" to ensure proper shift points.

But given your vacuum line or lines are off, all bets are off, the vacuum system has to be closed and functional, or all sorts of crap goes wrong, including no turbo and the climate control goes to defrost and sometimes the car won't turn off depending on where the breech is located.

old goat · #7 01-23-2010, 02:04 PM

While it is true the one vacuum line was off, reconnecting it did nothing. So even as I know I have to check for a vacuum issue, the "all of a sudden" shifting problems does seem to point toward the kick-down switch potentially sticking during that hard acceleration.

I will just have to go back with hoses and vacuum gauge to find out what is happening.....plus plenty of tools and patience. If I can get it home then I have a warm garage to work on it.

Thanks to all, especially Mark, for the comments and suggestions. Any other thoughts are most welcome.

Frank