C3cadet

My 75's trans went bad last night and wont shift out of 1st. Checked fluid, it's OK, any idea as to what is wrong? It seems to work OK otherwise, it just wont up-shift. Any help is appreciated.

Wrencher

Might not be that bad. Often the governor (mounted on the output shaft of the trans in the tailshaft housing) will hang up, causing the problem you describe. If you were to wind it up well past the normal shift point, it would probably go ahead and shift into second.

The governor can be replaced without removing the trans on most applications, although I have never actually done it on a Corvette.
Good luck.

Hans

Wendell Allen

I had a the same problem on a TH350 a few years ago, except that it also had reverse. The problem was the nylon governor gear that's on the side of the transmission. You might check that first. It's a cheap and easy fix.

And while you're at it, you might think about one of the B&M governor recalibration kits to raise your shift point if your car, like most, is shifting too early. It's cheap and you have to get into it anyway to replace the gear.

BlueL36

First thing to check are the two short rubber hoses at each end of the vacuum modulator pipe. One is between pipe and modulator at the transmission, the other at the intake manifold. A simple vacuum leak may be the problem.

Freepop

JME

Three years ago - my first post asked the same question. It nice to be able to turn it around and pass it on... It was probably BlueL36 who responded to me back then (but I can't find the old thread to give proper credit).

The answer was the vacuum hose on the modulator. It had simply dried out and split just ever so slightly.

I hope your's is as easy to fix as mine was.

Good luck

joe73vette

joe

C3cadet

I replaced the 2 rubber hose connections on the modulator line; nothing changed. I can not locate the gov. I also cannot find one listed at Corvette Central. I also can not find it illustrated in my build manual for the 75 Vet. I presume it is a part of the trans. So, should I buy a new modulator and see if that works? Buy a new gov., if so where?

vincevette78

Hy hose was bad on the trans . Replaced it and all worked fine after.

Big Fish

I see you live in Sandwich. There is an excellent transmision shop in Elburn called PROFORMANCE TRANSMISSION. Try calling those guys, they would probably have that governer, but might have other ideas for you before you buy things you really don't need. I had always thought the modulator was the only thing that will cause upshift

gerry72

The modulator has a diaphram in it that can rupture. Just because the hoses are good doesn't mean the modulator works. You can hook up a hand vacuum pump to the modulator, pump it down and see if it holds vacuum. Approaching the problem from the modulator end is the easiest since you don't have to take anything apart so you want to rule that out first. Going after the governor is a far more difficult proposition. And it's also almost always going to be the modulator in a no-upshift situation.

6318

might be some gunk in there...might check the filter...I've seen guys have this problem with a clogged filter...

Garrett1972

Two things cause an automatic transmission to up shift. First is the modulator, the higher the vacuum, the sooner it will shift. That’s why if you take off from a stop very slowly the transmission will shift all the way to third before you are going 20 miles an hour (high vacuum). The second is the governor, it works very similar to the way your mechanical advance works. It won't cause an up shift until a very high RPM. An easy way to figure out if the governor is working, take a drive and accelerate briskly like you are pulling onto the hi-way. If the transmission shifts to second when you hit 4000 or 5000 (the same RPM that it would normally shift when you are hammering it) then the Governor is working, and the modulator isn't. Now you know where to start trouble shooting. Governors don't go bad very often.

C3cadet

A mechanic is going to put my car up on a lift tomorrow and look at the gov. If it is OK I may just buy a modulator and put that on. I will let you all know what I find out Thur. Let's hope.

Big Fish

My bad, I just remembered one more thing for you to check. Your TH400 uses an electric downshift solenoid valve. If the solenoid is getting power it's telling the trans to downshift and it will prevent upshifting. The solenoid gets power VIA a switch on the gas pedal linkage. Locate the wires going into the trans on the driver's side, I think they are orange. Try disconnecting the wires and see if it will upshift. If so, problem's in the wiring or switch. If not, could be the valve stuck open, or it could be the modulator as well as a host of other things. Modulator is fairly inexpensive and is probably worth replacing before it goes to a trans shop. Any trans fluid in the vacuum portion of the modulator would indicate a bad diaphragm.

C3cadet

The solinoid solutiuon sounds promising, cause the problem occurred for the 1st time when I pushed hard on the gas to pass a slow moving truck and it downshifted and never up-shifted. I will try to disconnect the wires, but if it is stuck, then what? Hammer the little bugger?

It did try to upshift once, but sputtered and went back down to 1st.

Big Fish

If removing the wires does the trick I would start with the switch. It might just be stuck. Beyond that, it's probably time to visit a transmission shop that is known to have a good reputation.

C3cadet

Gotcha, if all else fails.

saudivette

I hate to steal your thunder C3cadet, and using your post for a question of my own, but I posted a gearbox question which has now slipped to page 2 with no replies.

Big Fish, it sounds like you've had some experience with gearboxes, I thought you might know the answer to my problem.

Whether I drive nice and easy or sink the boot right into the throttle, my gearbox changes from 1st to 2nd almost straight away. I even tried putting the shifter into 1st but it does the same and changes almost straight into 2nd?!??! Any ideas?

Big Fish

I don't think I can help on that problem. My experience is somewhat limited. What happens from 2nd to 3rd? If there are no other replies, try starting a new thread, someone here knows for sure.