Strange Shifting After Minor Collision - 1985 300TD
 

Hi All! I've got a 1985 300TD that I'm doing a light restoration on.

Mechanically the car is good; I have put a few thousand miles on it in my short ownership. Up until recently it ran and shifted beautifully.

Unfortunately, I was sandwiched between a Kia and a Nissan in a 4 car rear-ending pile up. It was relatively low speed, and my beloved 300TD only received minor damage (Front grill & aux fan destroyed, and rear bumper shocks collapsed).

The only worrisome symptom that I've noticed since the collision is that the shifting timing and feel has become strange. Here's what it's doing:

• 1st to 2nd: Early and extremely firm
• 2nd to 3rd: Early and firm
• 3rd to 4th: Hangs/late and smooth

After scouring the forums I see that the VCV/modulator use vacuum to adjust shift firmness, and the bowden cable adjustment can adjust the timing. Since my problem involves both feel and timing I'm a bit lost.

So far I've lightly searched for vacuum leaks in the engine compartment using a mityvac and haven't found anything obvious. The line from the blue "UFO" down to the transmission holds vacuum and does not leak, which I think rules out the modulator getting damaged (right?).

Are there any other vacuum lines that are known to be in a "pinch point" that may have gotten crushed from the engine/trans shifting during the impact?

Can a vacuum leak at the rear of the car (aka a line for the hatch lock, or the fuel door lock) effect the transmission?

Yes, other vacuum leaks will affect the overall available vacuum. Look around the hinges on the lift gate for vacuum hose problems. Did hoses at the vacuum pump get bumped by the fan or shroud?

As far as I'm aware those lines were unaffected. Really the only damage to the front is the grill, aux fan and the AC condenser was broken from it's brackets. The rad/fan are all visibly unscathed...


As a quick check/data point I just tested my central locks. All 4 doors do still auto lock (the hatch and fuel door never did and still don't) but the system is apparently still holding vacuum enough for the doors to work.

Sounds like a vacuum issue. Have you visually inspected all the vacuum lines under hood? I'd try to get a gauge and test it at vac modulator.

Door locks, use vacuum from electric vacuum pump in rear of car.
A/c heater system vacuum door control leaks could effect shifting.
To ck use hand vacuum pump tie in to supply line, key on engine off, with a/c -heater
in the mode that you use, system should hold vacuum, very slow leak down is ok.
How much vacuum is vacuum pump producing?
How much vacuum at vcv? How much falls off when opening throttle?

Hi All, I dug a bit deeper on this issue.

I have gone over all the vacuum lines in the engine bay and found nothing glaringly obvious. I took off a few fittings and spot checked for leaks with my mityvac...found nothing terribly concerning. Also, all of the vacuum functions on the car work normally; locks, climate control, engine shutoff.

Then I did a driving test with and without the VCV input connected. No difference in shifting was observed with or without the VCV...interesting. So there is definitely an issue with the vacuum modulation.

I removed the vacuum line from the "TRA" nipple on the blue UFO that feeds the vacuum line that goes down to the transmission modulator. I pulled vac on this line with my mityvac and it held 20inHg and very slowly leaked down.

I then hooked the mityvac directly to to VCV and pulled vacuum on it with the engine off. With the throttle at the idle position I could pump the VCV up to 20inHg and it leaked down pretty steadily. I then pulled the throttle to WOT position, and I could no longer pull any vacuum on the VCV.

I then hooked the mityvac onto the "TRA" nipple of the blue UFO that feeds the vacuum line that goes to the transmission modulator. In neutral I ran the engine up to a few thousand RPMs and saw the the vacuum pressure decrease with applied throttle. You can see a video of my test here: https://youtu.be/oPvN9Qdqi_A

Does this seem like healthy VCV behavior? As far as I can tell this seems pretty reasonable.

There is a adjust disc 20mm in size, that is placed on a pin on the vcv so the throttle
hits the disc ,engine running, briefly open throttle till hits disc ,take vacuum reading.
Adjust to about 9.5 inches of vacuum.
Next test, remove disc, BRIEFLY open throttle to full, take vacuum reading, should have
about 3 inches of vacuum or less. Good luck!

The system is trying to create a similar vac source as it would get from a gasser.

Hey folks, after a bit more fiddling my car is shifting better than it was before the collision.

The root of my problem was the Bowden cable adjustment. I’m thinking the engine/trans shifted slightly from the impact and put some slack in the cable. Or the cable was jammed and I released it by adjusting it.

First I cranked the cable adjuster 4 rotations loose to ensure the adjustment was working. The shifts were immediately worse. I then returned the adjuster to its initial position, and then added one more rotation tighter. My shifts are now properly timed and crisp but not jerky. May need a bit more fine tuning, but it’s already better than when I bought it. Maybe I also fixed some vacuum leaks along the way?

This also implies a more complex relationship between the cable control and vacuum control of the transmission. I think vacuum = shift feel and cable = shift timing is a bit misleading. Both systems need to be in harmony with each other it seems, aside from very very minor adjustments.

Thanks all for the help! I also took a video of the (apparently healthy) vacuum pressure at the modulator while driving if anyone would be interested for posterity.