Chevrolet THM 400 transmission?

Question

Does anyone know anything about troubleshooting the chevrolet THM 400 3-speed tranny?

okay. sometimes its shifts at low rpms through all three gears and work perfect, normally when its cold. Then sometimes, i have to start it in second let it shift to second then place it in drive and if i let off the gas it "slips" or so it seems. I have changed the vacuum modulator and the fluid/filter. The fluid was dark, but there were no visible metal chunks or shavings(im sure there were shavings) However, I just bought the truck a week and a half ago, and am just confused as what to do? maybe something about the tranny getting too hot?> well thanks for any and all of your help!!!

Answer 1

First look at the wires connecting to the trans and the sensor on the trans. If they are clean and connected ; check the throttle kick down cable going from the trans to the throttlebody. If this is fine not loose broken or in need of adjustment check the shifter linkage. then check for fluid leaks in the hoses/ tubes to the cooler. If these are fine then it is likly an internal problem.

Internaly it could have debree blocking the valve body / worn or burnt clutches , a leaking or worn torque convertor etc. Automatic transmissions are one of the hardest systems to diagnos properly without expensive tools and machines to check them. Most modern auto transmissions are computer controlled as well making it even more complex and yet additional high dollar tools to check everything. Even then the good old fashion tear down with a visual inspection is still nessesary often .

Automatic transmission are costly and tempermental with more parts than most standerd transmissions . I prefer a standerd trans anyday .They are generally tougher, cheaper , get better gas miledge and give more control over the car in driving and racing.
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Likly a visit to the transmission repair shop to check it out will be nessasary.

Answer 2

THE THM 400 IS A NON CPU CONTROLLED SYSTEM UNLESS IT IS IN ONE OF THE LATEST MODELS. LACKING THE YEAR, I CAN NOT GO INTO IT VERY FAR. CHECK THE FLUID LEVEL, HOT AND IDLING IN PARK. THE BAD NEWS IS THAT IF THE FLUID IS THAT DARK, THE CLUTCH PACKS MAY HAVE A ;LAYER OF VARNISH ON THEM THAT WOULD BE WASHED AWAY WITH FRESH FLUID. THIS CAN CREATE A NASTY SLIP ONLY RESOLVED BY A REBUILD. A TRICKY THING TO DO MIGHT BE TO DRAIN THE PAN INTO SOMETHING SO THAT YOU CAN REUSE THAT FLUID AND THEN DROP THE PAN. THE FILTER MAY HAVE A RESTRICTION THAT IS LOWERING THE FLOW OF FLUID. THIS WOULD BE A ONE TIME THING THOUGH.

Answer 3

The Turbo 400 is a great tranny. Was used in a wide variety of trucks and cars from '67 to about '90. There are pressure test ports on the side, where you can hook a special gauge. The gauge can help you troubleshoot the hydraulic circuits in the tranny.

The best thing to do is normal checks- Got fluid? Is it pink, or black/burned? Does the fluid smell burnt? Is the vacuum line hooked up to the modulator? Is the gearshift actually moving the shift lever on the tranny? How are the test port pressures? Beyond that, its time to drop the pan. You will always find some metal and clutch fiber in a tranny pan. Large piles of ground metal, or ribbons of torn metal are bad sign.

The TH-400 will last 100K-150K in normal service. Abuse, racing, or towing will shorten its lifespan. It is fairly easy and cheap to rebuild, and parts are readily available.

Answer 4

If you would give us an idea about what the problem is we could probably help. If your trans fluid smells burnt or looks black instead of red then you need to take off the pan and see if there are any metal shavings/shrapnel in the pan. If there is then you need to take out your transmission and take it to a shop and have them tear it down to see what happened. If something major happened inside your transmission there is no easy fix you have to take it out and tear it down. Don't worry though a Turbo 400 is one of the easiest tranny's to remove and install.

Answer 5

yea if I can have some details of what it is doing? This tranny is usually bullet proof. I have owned a few of them put them threw hell and back and they never gave me a bit of trouble. THe last one I had I put a shift kit in. Broke the back glass on my truck more than once when it would shift Passenger would hit there head.

Answer 6

Yes,Is it kicking into neutral when you accelerate
try unplugging the connector pass side,if it won't change gears pull governor cover off the side near tail housing remove governor hold with plastic gear up move centrifugal weights in and let them spring back their spring loaded look at slots
in side is there movement?Tiny aluminum particles get in there and makes stick.