Transmission help Needed, th400 not shifting.

Still looking for some ideas.

 

i have a 67 grand prix with a 400 TH automatic transmission that's acting up. The symptons are if you put it in drive as soon as you turn the car on it will go into gear and move just fine. But when the car gets warm with in minutes it won't move in any gear and the transmission starts to whiney noise. i changed out the vacuum modulator, that didn't help. I did some searching and another thing to look at was the throttle body cable but my car doen;t seem to have one. The governor was another sujestion but i haven't done that yet. Has anyone expieriance this and if so what can i do to fix it other than taking it to a shop.

 

I am having the same problem and would appreciated any help. Running a heavily worked 60 over 69 Buick 400 with a Switch Pitch TH400 out of a Sportwagon that was rebuilt in the 80s by me with a B&M shift kit and KB SP converter. I have a "window rattler" cam so needed the converter. It has always worked great with firm shifts and zero problems. I have it set up for auto shift in drive with the ability to hold it in gear and manually shift. Car has a 12 bolt Moroso Brute Strength rear with 4:88 gears (yea they need to go). I have checked the governor, modulator, changed filter, checked lines, kickdown and SP control and all is good. It may be time to drop the valve body and/or flush the system but I wonder if its really time for a rebuild instead. Here is the story. The trans wouldnt shift out of first in drive or manually from first. Right before this happened the shift to second became very hard and long like a double shift with lots of wheel hop. At around that time, ONLY when the trans was HOT I also heard what sounded like a scraping noise (not a whine) at idle or slow speed in drive. I took it to a trans guy and he said the accumulator piston went and there were shavings in the valve body. Car ran great for 3 months and same thing exact thing happened, with the noise and super hard and long shift. I was driving the car to a near by shop in first gear to check it out and all of a sudden it started shifting again with no noise or problems. I changed the fluid and filter and all was well for another few weeks and then the same thing happened and once again it cleared up after driving it. Now the same thing happened again. Could it be that the crud from the bad part is still in the system and flushing it out would help? Or is there something going and when it gets hot its shedding particles and clogging up the 1-2 valve or VB? I have thought about getting a new TH400 but don't like the thought of a non-lockup converter and I don't want the hassle of switching over to one of the new GM lock up ones with all the changes needed. Only really driving the car on the street locally.

 

It would take you about two hours to pull out the trans and take it apart, and see what's up. With 30 year old gaskets and seals, it needs those parts replaced, at the very least.

 

A common problem on later 400's was the front servo piston would break. When you pull the pan you might find some plastic pieces, that would be the servo. I am not sure what year they went from aluminum to plastic but it will have 1st and reverse. You have to drop the valve body to rplace it but is pretty straight forward.

 

Here is what you are looking for. The servo is orange so the plastic is easy to see it is not the govenor or speedo gear. If that is the problem you need to find a used GM aluminum servo. The make China replacements but they are junk !

 

One more thing , when you have the valve body out you can do a couple air checks and hear if the clutch packs are moving.

 

thanks for all the suggestions, mine is aluminum from what I remember, the thing I don't understand is why would it go from not shifting to shifting and back to not shifting over weeks multiple times? if something is broken, it would seem it just wouldn't shift, is there something that is temperature sensitive because that seems to be when the problem starts, and does the blockage cause the noise (which goes away) or vice versa? Was hoping someone had the same problem

 

That is actually the accumulator piston that breaks. The spring under it pushes the outer part of it up against the servo for the second gear braking band, forcing it on, full time. Hope that engineer took the early retirement.

 

that's what the trans guy replaced, if he put a plastic one in as a replacement and that broke it would never allow shifting correct? this thing goes from not shifting to shifting ok

 

Correct.The direct drum would be locked on at all times....
See post 34

 

This sounds like the governor is stuck. Here is a trick that I learned years ago. Find a dirt road, drive slow, maybe 5 mph and throw the car in reverse. There is more internal pressure in reverse, and that will work to break the stuck governor loose. Because your in a dirt road you won’t damage drivetrain because the tires don’t grip well on the dirt.

 

I don't know if this has been mentioned, but I had a problem with a TH400 not shifting many years ago, and the fault was that the diaphragm failed in the modulator. Removing the vacuum hose and checking for oil at the opening will confirm if the modulator has failed.
It is also possible for the plunger in the modulator to stick. It only takes a few minutes to check for both these issues.
I can't remember whether it caused an inability to shift, or an erratic shift, but it is still worth checking.
Bob

 

Make sure the solenoid isn't leaking. Unscrew it and check the gasket. That gasket might of dried out..