TM 400 Troubleshooting

[Roger S.]

On my 73 Corvette L-82 w/ TM400 it does not shift out of first gear at WOT even up to 5800 rpm. The kickdown for 'passing acceleration' also does not respond.
I have check that 12V is going to the kickdown solenoid and that I have vaccum at the modulator. The engine has 14" Hg vaccum at idle. There appears to be no vaccum leaks: I have checked all hoses tubings, etc., had the carburetor rebuilt and the throttle plate rods openings in the base bushed, The engine idles smooth, a couple of old Chevy street rodders have found nothing wrong with the engine but believe the cam is hotter than a stock L-82 cam.
The modulator is a replacement, it has the vaccum connection pointing straight out of the housing not angled like in the pictures I have seen old old modulators.
Any suggestions on what else to check or do?
Roger

[Roy B.]

Re: TM 400 Troubleshooting

The governor plastic gear is stripped ,Pull cover , pull governor out and look! easy to do!

[Harmon C.]

Re: TM 400 Troubleshooting

Roger
I sent you a PM.

[Michael H.]

Re: TM 400 Troubleshooting

The governor plastic gear is stripped ,Pull cover , pull governor out and look! easy to do!

If the governor gear was stripped, the transmission would never shift out of 1st speed. I think he was refering only to full throttle upshifts which sounds more like a modulator or modulator related problem.

A low vacuum signal to the modulator (full throttle) is basically what controls the point in engine RPM that the transmission upshifts to the next gear. (among several other factors)

[Roger S.]

Re: TM 400 Troubleshooting

To clarify or make more murky whichever;
The kickdown solenoid wire has 12V only when the gas pedal is pushed all the way down and the switch is activated by the gas pedal rod. My '73 does not have the TCS and has an orange wire going to a single pin solenoid at the transmission.
I have checked the modulator and have found no signs of fluid inside it - used a pipe cleaner. What is considerd low vaccum for the modulator? I have about 14'Hg at idle and no idea of what it is at WOT. Are low vaccum mdulators available?
Roger

[Roger S.]

Re: TM 400 Troubleshooting

I have had the car for about 8 years and about 5000 miles. I have never know it to upshift at WOT. Had a recommended transmission shop check the transmission last summer and he said there was nothing wrong with the transmission. The car is driven for pleasure so WOT driving is not normally done. I did it last summer one day for fun and that is when I noticed the problem. At times it does seem to shift between 2nd and 3rd alot around 35 mph.

Roger

[Michael H.]

Re: TM 400 Troubleshooting

I have had the car for about 8 years and about 5000 miles. I have never know it to upshift at WOT. Had a recommended transmission shop check the transmission last summer and he said there was nothing wrong with the transmission. The car is driven for pleasure so WOT driving is not normally done. I did it last summer one day for fun and that is when I noticed the problem. At times it does seem to shift between 2nd and 3rd alot around 35 mph.

Roger

If the transmission shifts normally under normal acceleration, there's nothing wrong with the governor.
Governor pressure, and line pressure, are controlled by the vacuum modulator.
Light or part throttle/high vacuum results in low pressure and the opposite is true for full throttle. That means that if the vacuum line to the modulator is leaking or not connected at all, (duplicating heavy throttle conditions) pressure is high and that causes a pressure that opposes governor pressure, delaying the shift.

This is a somewhat common problem with aftermarket, or even many OEM/Delco modulators. The amount of vacuum needed to overcome the modulator spring pressure may be different from one to another.
Unfortunately, there's no published specification that I know of to check the actual amount of vacuum required to overcome spring pressure.
There is, however, a way to check a new replacement against a known original or properly functioning used one.
Make a short piece of 3/8" rod and 1" in length and scribe a line or mark in the middle of the rod. Place both the new and the original modulators over each end of the rod and slowly push both together while watching the movement of the rod. Continue compressing until the base of one modulator is even with the scribed line. The base of the opposite modulator should be within 1/16" of the line. If not, one is defective.

Old original Delco modulators have a screw adjustment at the end that was used to calibrate this pressure but I don't think newer ones have this adjustment. (calibrated by the folks at Delco and no service adjustment was recommended)

There are a lot of other things that can cause, or contribute to the condition that you mentioned. Engine tune up is one. If the engine is not in peak tune, there could be random spark plug misfire at high load/RPM which causes a slight drop in an already low vacuum signal.
Ignition timing is another.

If by chance the transmission has been overhauled in the past, there's a possibility that the governor or valve body are not the correct original and that can create a different shift point pressure.

As mentioned previously, if the unit functions normally under normal acceleration, that automatically means that almost everything inside the unit is functioning properly.

If you have power at the wire and terminal on the side of the transmission when the throttle is held open in the detent position, the switch is adjusted and working properly.
It's possible that the detent solenoid inside the transmission is not operating or the small orifice in the solenoid is plugged. If the solenoid is not functioning, it will cause a "no detent downshift" condition.

To dig deeper, I would need oil pressure readings from the transmission.

[Michael H.]

Re: TM 400 Troubleshooting

OK, NOW I think I understand the malady: It does up-shift at PART throttle, but won't upshift at WOT.

I have a 70 396 El Camino with a TH400 that I've owned since 1973. There was a period of time that it had a big cam, headers, etc. I modified the trans governor weights and springs to get a 6800 RPM WOT up-shift. I never bothered to change it back when I 'de-tuned' the car in the 80s at the last rebuild. It won't upshift at WOT either; the engine "runs out of breath" at about 5800, I have to lift slightly, and it shifts.

Sorry for the story. Sounds like your governer has been modified (likely). Or it's sticking (not as likely).

That may explain why there is no upshift at WOT but if the governor weights have been lightened, it would also cause higher than normal part throttle upshifts. Because there was no mention of abnormal upshift speed, I would guess that the governor is not modified.

Another possibility is that the governor is not the original from the car. There were at least 7 or 8 different TH-400 governors just for Corvette by 1974.

[Michael H.]

Re: TM 400 Troubleshooting

Here's something I vaguely remember: OEM 400's have a small 'helper' spring behind the modulator. If it is removed, the modulator valve will respond to smaller changes in vacuum.

I don't remember a spring behind the modulator. I thought that was PowerGlide that had the spring, not TH-400??

[Michael H.]

Re: TM 400 Troubleshooting

I know my 70 350hp/396 had it. Transmissions are not my main thing, but I've spent a lot of time with them , and have been mentored by a life-long friend that's been building transmissions professionally since 1972. This was a big thing in the 70's when a cam change resulted in decreased vacuum, and the shifts became late and harsh. Increasing modulator tension lowered the part throttle shift, but it [PT shift] then became 'mushy'. Removing the spring allowed the valve to respond as if there was less spring tension in the modulator, even though the modulator itself was 'tighter'. Overall, these two changes resulted in a 'firmer' early shift. Something like that. And my 400 wasn't the only one we did this to. This was a common 'fix' in his shop 'back in the day'.

As far as the subject transmission in this thread goes: I think the governor is wrong. I agree that part-throttle shifts are affected by governor changes, but this can only be detected in a back-to-back comparison by a novice. This guy bought the car this way. To you or I, the PT shift might be 'late'. To him, it shifts somewhere, so, OK, no real problem / issue.

The WOT up-shift is 'all' governor, as vacuum [modulator] is virtually zero at WOT. Gotta get the WOT right with the governor before working on PT 'quality' with the modulator.

Regards,
Dave

Maybe I'm just getting old and can't remember but I thought it was just the opposite. Light throttle/low line pressure. Heavy throttle/high line pressure.
Why would high line pressure be needed at light throttle? Why wouldn't it be needed at WOT?
Increasing the modulator spring tension would be the same as lowering vacuum, which should increase line pressure. That should result in a fermer shift.
Somewhere, I still have a lot of engineering stuff for TH-400's. It has all the line pressures for forward and rev and also detent.
It also has oil flow schematics.
I still don't remember a spring between the modulator valve and the modulator can. The parts breakdown sheets will show it if it was used. Covers all from 65 to 74.

[Jim T.]

Re: TM 400 Troubleshooting

Roger if you find the problem with no upshift from 1st to 2nd at WOT with your turbo 400 let is know. I have a turbo 400 in my 70 that has not given me any problems since the car was purchased new.
There was a small quick fix one day, after driving the car in Sep 1970 from Dallas to the east coast, with overnight stop, upon arriving on the east coast the turbo 400 would not shift normally at all from first gear. Quick fix, the rubber hose connecting vacuum to the vacuum modulator had come off.
If you select first gear and apply WOT and move the shifter to 2nd gear at redline for your L82 will your turbo 400 shift to second with WOT still applied?

[Roger S.]

Re: TM 400 Troubleshooting

I have not tried shifting to 2nd, I am too worried about throwing the belts again or worse. BTW just lefting off the gas pedal a little does not allow the transmission to shift into 2nd. I had to slow to about 35 mph before it shifted.

The vaccum lines, rubber hose and steel tubing, to the modulator are in place and when disconnected at the modulator and the intake manifold fitting holds a 28" Hg vaccum. I could not find a part number on the modulator or governor. I am certain that the modulator is a non-GM replacement and the governor's external appearance is new so it probably is also not a GM replacement. It looks like the modulator is held in place with a bracket secured to the transmission housing with a 3/8" or so bolt. I have not gotten close enough to the governor to check it out.

My Chevrolet friend is going through his 60's/70's manuals to come up with possible fixes and in which order. I'll post what we due to fix the problem.

Roger