71 TH400 Transmission Problems

[Mark E.]

Re: 71 TH400 Transmission Problems

Let us know what you find.

Again, it's possible a new, unrelated transmission issue began coincident with the radiator work, but unlikely. So I still suggest you advise the transmission shop to first look at things that the radiator shop may have buggered up: wrong fluid added (if they replaced the radiator, they added fluid regardless of what they're telling you), contaminated fluid, pinched cooling lines, bad/clogged cooling tank in radiator,...

[Seth G.]

Re: 71 TH400 Transmission Problems

Since the last thread, I was able to drop the pan and check the filter and pick-up tube. They both are properly in place. We found a few tiny particles in fluid (had to look very closely as it was minimal). The transmission no longer slips, but it still does not upshift. I have to upshift manually and then it drives fine. The transmission shop did not suspect the modulator but thinks the Governor may be hanging up. However, you have to drop the crossmember and exhaust to drop the transmission a bit to remove the Governor. I did not want to do this without further research. Any added ideas? How would you know if Radiator tank where transmission lines connect is clogged. They were boiled out prior to installation of the new core. The transmission fluid is not contaminated so I doubt its leaking that way. Thanks

[Ray G.]

Re: 71 TH400 Transmission Problems

Hello Seth;
Considering your original post one would certainly suspect the work on the Tranmission cooler or lines. If the lines were plugged the pressure
will build up to line pressure and something breaks. So the lines probably are not plugged.
Your following posts suggest contamination.
As long as the transmission doesn't slips, works properly while shifting manually, but will not shift automatically, the problem appears to be foreign material in the valve body.
If the filter is seated properly and not sucking air I would drive the car on an extended run while shifting many times to try and move the foreign material through the system into the pan to achieve automatic shift points.
Failing this process the valve body will need to be removed and cleaned. I suggest a professional transmission tech. for that procedure.
Knowing the history of this transmission problem I would install a filter in the return cooler line. I don't remember which line is the pressure out
line to the cooler. Someone who works on these 400s everyday can probably answer the question.
Ray

[Steven B.]

Re: 71 TH400 Transmission Problems

After I rebuilt the 400 in my 67 Camaro the first time, I installed a Transgo stage 3 shift kit. The only difference between the stage 2 and stage 3 is a set of springs to hold the governor weights closed (or open, I don't remember which. I'm pretty sure it's open). The reason for keeping the weights from working was to convert the transmission to full time manual shift. I decided I didn't like it, so I removed the springs (could have saved some money by using the stage 2 kit, but o well). The most recent time I rebuilt the transmission, I made some additional modifications to the center support to accommodate a snap ring in the case. Well, after cleaning and cleaning some more, I re-bushed, resealed and put the trans back together. It shifted the way I expected it to but not automatically. I traced the problem down to the governor valve. It was hanging up due to some very small particle (of something). The spool valves in these transmissions are extremely close in tolerance and the very smallest foreign object can keep them from traveling as they are supposed to. As Ray mentioned, a small particle can cause a big problem. Since your transmission shifts properly but not on its own, I would suspect what the transmission shop mentioned will turn out to be the ultimate culprit. As much of a pain it would be, I would imagine that you will ultimately wind up having to drop your cross-member and go through the process of removing the governor and pulling the valve out to clear any foreign objects. If the valve hangs up even slightly, the transmission won't shift correctly. Sometimes a tiny burr on the edge of the valve will cause it to hang up also, but I don't believe you mentioned whether it has been out of the transmission through all of this. I would suspect that since there was nothing done to the transmission other than the radiator work that something loose traveled to the valve. I don't remember if you can get to the valve to work it back and forth by removing the valve body or not. The spool is in the case if I remember correctly. I also don't know how close the cover is to the side of the transmission tunnel, but you might not necessarily have to remove the valve. If you can take the cover off and work the valve back and forth, you may be able to dislodge whatever is in there and get it to work. The valve just has to travel freely and the allow the weights to work the valve through centrifugal force.
Hope you can get a resolution to your problem.
Steve

[Seth G.]

Re: 71 TH400 Transmission Problems

Thanks Ray, good advice. I will try it and let you.

[Seth G.]

Re: 71 TH400 Transmission Problems

Steve - You may be right. In thinking this through, the Radiator had been re-cored. Perhaps in that process of sweating the new core to the existing tanks, a bit of foreign material may have found its way into the tank and contaminated the spool. In any event, I will drive the car a bit more to see if I can work it free. If not, the governor spool will be checked after a hearty does of pain medication. Thanks again for your help which is appreciated.

[Stephen B.]

Re: 71 TH400 Transmission Problems

it may be a sticking governor or possible the gear on the governor may have apple cored then it does not turn

[Seth G.]

Re: 71 TH400 Transmission Problems

I tried changing transmission fluid and filter. There was a very tiny piece of rubber in the pan. Put in new fluid and filter and changed modulator. The car shifter the same (upshifted at around 45-50 mph) until I drove it again and now it is upshifting at around 35mph. I think something is still hanging it up. To check the Governor valve, I need to drop crossmember and exhaust. Is there any other way to access the Governor valve? My car has only 2400 actual miles on it and I don't want to disturb things if possible. Thanks again.

[Steven B.]

Re: 71 TH400 Transmission Problems

If your transmission is shifting but at high RPM's, it is likely a vacuum leak or poor vacuum signal. I would try replacing the vacuum hose or at least checking it throughout its length for a split or small hole. Hoses can get brittle over time. It sounds like your transmission is shifting but at high RPM after developing enough vacuum and RPM to get to the shift point. There may be an adjusting screw that you can play with to adjust the shift points on the modulator (aftermarket modulators often do, but I don't know about stock modulators). I would not do that until I proved the vacuum line and vacuum source. I would thoroughly check the vacuum signal at the transmission end of the vacuum line before I when through the hassle of dropping the trans to get to the governor. The governor may be contributing to the problem but not likely.
Steve

[Jim T.]

Re: 71 TH400 Transmission Problems

Seth hope your turbo 400 is working again. My 1970 Corvette with it's original turbo 400 and over 160,000 miles is operating very good with only multiple fluid and filter changes for the past 45 years.

[Mark E.]

Re: 71 TH400 Transmission Problems

Again, a likely cause is fluid contamination from the radiator work. The debris found in the pan is possible evidence of that.

Have you had the entire system flushed and filled with fresh fluid? Dropping the pan only drains a small amount of the fluid.

If flushing doesn't fix it, then you may want to disassemble and clean the valve body. It's very vulnerable to dirt, contamination, and sticking from lack of use.