Anyone Know GM transmissions Mainly turbo 400?

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johnparts

johnparts

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I have a 72 Chevelle here that will not shift into 3rd and I cant seem to figure out why. The car was originally a 350/350thm combo but now has a 454/400thm combo that was done by the original owner and I don't know if he did it right. I have a vaccum line to the valve on the passenger side of the trans But there is a little electrical connector on the drivers side that does not have a wire going to it. Is that the reverse light switch aka multi function switch?
 
How is reverse? any chance the valve body is installed with bad gaskets or has dirt in it? Is the vacuum modulator working properly?
 
I believe 400's have a electric kickdown for passing gear. The wires for that may be for that. If not hooked up properly the trans wouldn't shift into 3rd.
Just a guess. Call a transmission shop. Mike
 
I haven't played with GM slushboxes in a while, so it's a little fuzzy. Sticky modulator, sticky governor, busted accumulator spring and sticky shift valve jump into my head as possibilities.
 
Reverse is fine and I'm thinking it's vacuum related since the power brakes work for poo too.
 
I was a mech, gm, in 1969. Those turbo's gave us a fit, until gm figured out the problems. We would rebuild, test drive, not good (man I loved those Belairs, plain jane, that the ATF used, 427) I was young, so road tested. Have to be full throttle, 90 MPH and it shifts into 3rd then. If not, look at the vacuum.
 
green1 said:
I was a mech, gm, in 1969. Those turbo's gave us a fit, until gm figured out the problems. We would rebuild, test drive, not good (man I loved those Belairs, plain jane, that the ATF used, 427) I was young, so road tested. Have to be full throttle, 90 MPH and it shifts into 3rd then. If not, look at the vacuum.
Click to expand...
My personal favorite quirk is the "it often bangs second at ~5000 RPM despite the shifter being in first and you may very well scatter that expensive big block if you back out of the throttle without bumping the shifter to second" thing. Good times.
 
johnparts said:
Reverse is fine and I'm thinking it's vacuum related since the power brakes work for poo too.
Click to expand...

Bingo! You got a vacuum problem. The switch on the drivers side is for the trans kickdown. GM's used a switch on the accelerator pedal that sent power to the switch on the trans when you dropped the hammer. T-350's use a cable operated kickdown usually pulled off the bottom of the carb. This explains why you don't have the wiring for the kickdown. Both GM turbo trannys are vacuum referenced. Make sure you have a solid vacuum source and that you run a 1/4 hard line down to the trans.
 
The single wire, halfway down the left side of the transmission is for the detent solenoid (kickdown). However it is energized open, spring closed. So, if you have no power to the detent solenoid, you will have no kickdown, but 3rd should apply.

If your 1-2 shift is normal (10-15 mph and smooth) it's not the modulator. If the 1-2 shift is late (35-40 mph and sharp) the modulator or governor may be at fault. I just rebuilt the Th400 in my truck and they are a straightforward, robust unit. Sadly, I am on the road for work and don't have an ATSG manual handy. But pretty much any GM factory service manual should have extensive troubleshooting tables. See if you can score a book, especially if you decide to rebuild yourself.

The TH400 is my 2nd favorite automatic (behind the A727) and is capable of handling a lot of power while delivering glass smooth shifts. No wonder Jeep, Jaguar, Rolls Royce, Ferrari and others used them for years.
 
C130 Chief said:
The single wire, halfway down the left side of the transmission is for the detent solenoid (kickdown). However it is energized open, spring closed. So, if you have no power to the detent solenoid, you will have no kickdown, but 3rd should apply.

If your 1-2 shift is normal (10-15 mph and smooth) it's not the modulator. If the 1-2 shift is late (35-40 mph and sharp) the modulator or governor may be at fault. I just rebuilt the Th400 in my truck and they are a straightforward, robust unit. Sadly, I am on the road for work and don't have an ATSG manual handy. But pretty much any GM factory service manual should have extensive troubleshooting tables. See if you can score a book, especially if you decide to rebuild yourself.

The TH400 is my 2nd favorite automatic (behind the A727) and is capable of handling a lot of power while delivering glass smooth shifts. No wonder Jeep, Jaguar, Rolls Royce, Ferrari and others used them for years.
Click to expand...

My truck has a 400, but, my Jeep has a 727....:cheers:
 
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