Recently I was taking my car to the racetrack.
It was bracket racing with the local corvette club here.

No problem at the all for first two race.
But on my third race the TH400 shifted from the first gear at 4500 rpm to the second for a half second and then it went back to the first gear again.
Of course it did happen again under one of the competition race, and I got eliminated



The same thing happend a couple of times that day.
This only happens when the engine pulls really hard.

Is it time for a rebuild?
Is this something I can do myself, or is this a work for an TH400 expert?

 

I've run nothing-but THM400s in my 12-second '79 Z28, and will soon put one in my '82 Corvette, but have never had that problem, so I can't offer help:
sorry it cost you a round of competition.

 

Thank you for your sympathy!

 

I would try and adjust the vacuum modulator, If it is adjustable,some are black with a lock nut screw system, mine has a red stripe and you pull the vacuum hose off and adjust with a screw driver. I would try 1 turn counter clock wise, nothing to loose and you can put it back to original . I would raise your shifts points for compettion unless 4500 is your horsepower peak. You can use a B&M governor spring kit. Start with the weights 3-4 and midrange mid range springs, if i remember correctly 1-2 was 5500 and 2-3 was 4800. With the lightest spring and weight combo mine shifts at about 6100 1-2 and 5500 2-3. If you pull the governor on a lift, make sure the front is lower than the rear (I drop the air out of the front tires), this way you will not lose very much fluid. Hope this helps you. Ron B.

 

Thanks Ron!

So you don't think there is anything wrong with the TH400?

I don't think I can tweek my modulator, looks like it's pure stock.
Is it the govenor that controls the upshifts at WOT?
Forgot to tell that I was shifting the TH400 manually when the upshifts happend.
Does a govenor change affect the manually upshifts, or is it only the automatic upshifts that will change?

With the cam I use now my hp peak is 5500-6000 rpm.

 

This is just my swag (scientific wild assed guess). The governor controls wot shifts. The vacuum modulator controls part throttle up and down shifts. The electric kickdown switch (by the gas pedal) and kick down solenoid(internal), connection above trans pan driver side, control the shifting. With the gear selector you can make the shifts occur later by overriding the governor. When you shifted the car at 4500 into second the rpm MIGHT have dropped low enough after the shift that either the modulator or the governor weight spring combo caused it to down shift. Kind of like when you do a burnout, and shift to second then the tires recover, rpms drop and transmission down shifts back to first. If this was my car, I would make sure the fluid is is nice and clean,or change it and the filter so I know how everything looks and the level is correct (full line after fluid is operating temp). Then go from there with testing. I don't think your transmission is hurt, but I don't know how hot it got at the track. I always try to let it have a 20-30 minute cool down between runs. Get a transmission temp guage like the B&M one and put it in the output line ( hottest point for coolant) to the radiator, or just get a good inline cooler. The 400 is a good transmission and heat is it's biggest enemy. More Important Lets keep these cars at the track where they were ment to live, not under a cover in the garage. Ron B.

 

not a TH400 expert here, but things do happen differently inside the tranny when manual shifting vs auto shifting. better to adjust to shift auto; need parts as suggested above

 

I didn't shift!
I started manually in first gear and plan was to shift to second gear at 6000 rpm.
It was the TH400 that did the upshift and rpm was suddenly 3000 rpm and a half second later it down shifted to 1 gear and 4500 rpm.
After that it went up to 6000 rpm as nothing had happend.

Pedal was at the floor the whole time.

 

I would pull the pan inspect and replace the filter, fix the governor. Ron B.

 

The max. shift points in the THM400 valve body are set with selected springs on the shift valves. The governor mechanism provides a regulated pressure (regulated by governor RPM) to those shift valves. Vacuum sent to the vacuum modulator provides another source of modulation which is a measure of throttle opening. The main part of the transmission doesn't need adjustment...it could need repair, if there is internal damage.

First, check the vacuum modulator. Pull off the vacuum hose and stick a Q-tip in the hole, swab it around, and then remove it. If it has Dexron on it, the diaphragm is probably bad and the vacuum modulator needs to be replaced. If not, go to the next step. Step #2 is to remove the governor (under the metal cover with 4-bolts on the right-rear of the unit) and check for damage on the drive gear teeth and look for any debris. Also swing the flyweights up and down and look to see if the regulating valve (in the shaft area) is freely opening and closing. If no gear damage and/or no debris, the governor should be OK. If problems are visible, change the governor. Get the transmission P/N on the side plate so that you can order the correct governor. If you see no problems with the V.M. or governor, drop the tranny pan to look for any debris; this would indicate that an immediate rebuild should be done so that you would prevent trashing the whole unit by running it more.

THM400's aren't bullet-proof...but almost. I'd bet on the vacuum modulator or the governor being the problem.

 

Thanks 7T1vette!
I have to check the modulator and govenor.
Anyway, it's time to garage the vette for this year now, winter will be here in a couple of month.

Lucky you in southern US, we will have rain, snow and cold weather for 5-6 month now .
With a little bit of luck I can open up my garage door april 2007.

 

The modulator test is very easy and is most likely your problem. With your weather 'situation', maybe now is the best time to do the checking and repair so that your car will be ready to go when you can get out next spring. Also, with the shifting situation you described, you run the risk of some engine damage if your tranny downshifts at when the engine is near max. RPM. Get it fixed before you put much power to the engine. Stay warm.