Transmission shifting problems – a basic checklist

[RunningTooHot]

I have been ‘dialing in’ my transmission utilizing the previous postings here by Stevebfl as well as others. Thanks to those postings, I have a pretty good conceptual understanding of how the controls interact with each other – this has allowed me to get the car to where it is now, which is a HUGE improvement from when I bought the car. I hope that this can serve as a sort of basic starting point & checklist for others, as transmission shifting problems seem to be an endless subject of postings here. Some of the steps here apply only to diesels, as the car I am working on is a 1984 300D. However, the basics still apply to all.

This is what I have done so far:

Change the fluid & filter. Don’t skimp here people – drain the torque converter as well. If your really fanatical, as a preventative measure the cooler lines going to the radiator should be changed if they are very old. This will also allow you to drain a little extra old fluid from the system. If you find lots of crud in the pan, don’t bury your head in the sand – you know what that means. Fixing it now will likely be far less expensive than fixing it after a catastrophic failure.

Check the various linkages for proper lengths according to the manual, and also clean & lubricate them properly

Check ALL the vacuum lines & rubber fittings for leakage with a vacuum tester. (An invaluable and inexpensive tool for diesel owners)

(Diesels only) Cleane the restrictive vacuum orifices with brake cleaner. Use brake cleaner to avoid melting the plastic and thereby changing the calibrated bore size. Carburetor cleaner sometimes contains acetone, which can damage plastics.

(Diesels only) checke the function of the vacuum valve (variable leak point) on the injection pump for proper range & smooth transition. Also check it for specified vacuum with the lever set at 10mm away from the stop.

I replaced the (leaking) vacuum modulator and set the pressure to spec using a proper pressure gauge on the transmission test port, so no guesswork was involved there.

Check the Bowden cable for proper adjustment.

An otherwise overlooked fact is that the engine needs to be putting out the proper amount of power (torque) for the transmission to respond properly – this is a whole other Pandora’s box that I will not open here. Remember that the transmission was engineered for each specific application with differing valve bodies and shift points / strategies. If your engine isn’t up to par, then your transmission’s behavior is not going to be “perfect” no matter what you do.

The difference in my car has been absolutely amazing. It went from a beast that slammed every shift with enough force that I am still concerned about the rear end having been damaged, to a drivable car with firm yet smooth shifts (except for some minor fine tuning which I will ask about in a separate posting today.)

I hope that this will be of some help to others in the future.