Need Trans Help, Turbo-400

Apologies for some duplication, if one of my posts has to be deleted, I'd rather it be this one, but this is a truck trans, but we just installed it in a 1970 Chevelle. So I did post in Classics, but I have updated that thread more than this one. Was hoping to reach someone in either forum that can offer advice.

It is a Rebuilt trans, but it sat for 5 years in dry storage. Stock valve body. Will not shift into 2nd or 3rd, stays in low even up to 4500 RPM. Replaced Vacuum Modulator, still same. With shifter in low gear, will stay in low, even on deceleration. In 2nd or Drive, will freewheel when you let off the gas.
Has manifold vacuum at modulator.
I have to leave for the airport in 2 hours, really would appreciate any help or advice.
Guessing I will go check the governor first, then drain fluid and drop the pan, and look around at the valve body.
Hoping for some help on what to look for.
Thanks a bunch, -Jim.

Apologies for some duplication, if one of my posts has to be deleted, I'd rather it be this one, but this is a truck trans, but we just installed it in a 1970 Chevelle. So I did post in Classics, but I have updated that thread more than this one. Was hoping to reach someone in either forum that can offer advice.

It is a Rebuilt trans, but it sat for 5 years in dry storage. Stock valve body. Will not shift into 2nd or 3rd, stays in low even up to 4500 RPM. Replaced Vacuum Modulator, still same. With shifter in low gear, will stay in low, even on deceleration. In 2nd or Drive, will freewheel when you let off the gas.
Has manifold vacuum at modulator.
I have to leave for the airport in 2 hours, really would appreciate any help or advice.
Guessing I will go check the governor first, then drain fluid and drop the pan, and look around at the valve body.
Hoping for some help on what to look for.
Thanks a bunch, -Jim.

check that the detent cable is not stuck

I agree with your indirect suggestion that rust from 5 years of storage has managed to leave something stuck, such as the governor or the valve body.

But there is the chance the trans was incorrectly assembled when it was rebuilt.

I suggest you get a transmission service manual, if you do not have one already. Haynes sells a good one for older GM auto transmissions:http://www.haynes.com/products/productID/39

My copy of this book outlines the following possibilities for this problem (in the following order)

1. Governor valve sticking, or the governor drive gear broken, worn or missing the retaining pin.
2. Governor incorrectly assembled and missing balls, spring.
3. Exhaust check -balls missing, stuck or damaged.
4. Valve Body: 1-2 shift valve sticking.
5. Vlave body spacer plate or gaskets incorrectly assembled or damaged.
6. Valve body face warped or damaged.
7. Governor filter plugged
8. Check the 2-4 servo assembly for:
. a. Apply-passages in case plugged or restricted.
. b. Servo pin or pin bore nicked or burred (or rusty :) )
. c. the 4th servo piston is installed backwards.
9. Check the 2-4 band assembly for a band that's worn, damaged or not engaged on the anchor pin.
10. check the intermediate or 2nd clutch for
. a. piston seals leaking
. b. intermediate roller clutch damaged.
. c. Housing check ball assembly damaged ... (rusty. )
11 Check reverse reaction drum for damaged splines or missing drum plate.

Will not shift into 2nd or 3rd, stays in low even up to 4500 RPM. Replaced Vacuum Modulator, still same. With shifter in low gear, will stay in low, even on deceleration. In 2nd or Drive, will freewheel when you let off the gas.

Has manifold vacuum at modulator.
Last time I had a TH 400 freewheel on deceleration, the owner had the vacuum modulator connected to ported vacuum instead of manifold vacuum.


check that the detent cable is not stuck
There is no detent cable on a TH 400. The 400 uses an electric switch and wire harness, with a solenoid inside the transmission.

Magic Rat, does your manual show where the Gov. Filter is located? Is it accessible from inside the pan, with Valve Body removed?

Vacuum line is still connected to unported manifold source.
I installed the known-good governor from the old trans, and pulled the vacuum modulator, made sure the mod valve was free & clean. No difference, same problem. Draining the fluid now, and will replace the valve body with the good one from the old trans. I dropped the pan before installing this trans, and saw no signs of rust at all. Anything else I should look at while I am in there? Thanks again for advice.

Last time I had a TH 400 freewheel on deceleration, the owner had the vacuum modulator connected to ported vacuum instead of manifold vacuum.



There is no detent cable on a TH 400. The 400 uses an electric switch and wire harness, with a solenoid inside the transmission.

your right I was thinking of my 350 tranny that had this occur. my 1967 400 tranny did not have a cable.

you could be on to the vacuum line being wrong.

UPDATE - Problem Solved! :smile:
I took the valve body from the previous trans, installed a Trans-Go Shift Kit in it that I had sitting around, (just to add the enhancements, improved shift feed passages, and updated parts the kit includes), and swapped that in place of the existing VB. Also added the stronger Low/Reverse accumulator spring, and used all of the "Stage 3" settings from the kit, (firm, street-strip performance setup). Located and cleaned a small amount of debris from the governor filter, (very small conical filter that slips into one of the governor tubes where they meet the case).
When I removed the existing VB from the "new" trans in the car, a servo piston (2-3, I believe), with it's spring & pin, dropped right out of the case. It had a gapless teflon ring which was essentially flush with the O.D. of the servo, and obviously did not seal well or fit tightly. I have never had one drop out like that. So I took the one from the old trans, (which had a steel ring on it, and was fitted quite snug in the old case), and put that into the new trans. Had to compress the ring to install it, and it stayed up in there nicely. Installed the reworked old valve body, put everything back together, and the trans now shifts just fine. Didn't have time to inspect the failed VB to look for stuck valves, etc. But I am thinking that loose-fitting servo may have been the culprit, or at least a contributor to the problem. Funny that the original, 1970-vintage 43 year old version of that part still works better than the newer design...
Anyway, just wanted to say that having low & reverse gears only, does not necessarily mean trans must come out or get rebuilt! I am sure that in some cases it would, but it is worth trying other options first!
Thanks to everyone for their advice & information!
-Dan.

Thank you for the update!