1981 Chevy C20 Trans issue

1981chevyc20 · #1 03-03-2007, 11:29 PM

I own a 1981 Chevy C20. The issue is my Trans will not shift normal. I have to push the gas pedal all the way down for it to change gears. But when it shifts it kicks hard into gear. I recently bought this from my Father-in-law to restore. And when I looked under the truck I had to replace the trans mount. It was warn and 1 bolt missing and the other bolt was loose (on the cross member). But it still kicks into gear. Can any one help please? Thanks.

silicon212 · #2 03-04-2007, 02:40 PM

First, what transmission is it? If it's a TH400 or TH350 (both 3-speed, non OD), check to make sure that the vacuum line is attached both to the source on the intake behind the carb, and to the vacuum modulator. The modulator on the TH350 is on the passenger side, at the rear of the transmission (pointing rearward) over the pan 'cutout'. On the TH400, the modulator is also on the passenger side, but oriented to the side just behind the bellhousing and above the pan.

If the truck has a TH700R4 in it, make sure the TV cable is properly adjusted. The TV cable replaces the function of the vacuum modulator on this transmission. Just behind where this cable attaches to the carb bracket, there is a "D" shaped tab (on the top). Press this in hard while pulling back on the cable as far as it goes toward the firewall, release the tab and then open the throttle butterfly with a screwdriver as far as it will open - you should hear some clicking (might sound like stripping the teeth off a plastic gear), at this point the cable is properly adjusted.

If your truck has a TH350 or TH400, and you've verified the vacuum line to the modulator to be in good shape, and it still shifts hard, perform a tuneup on the engine. If the engine in this case has a large cam in it, there might be insufficient vacuum to operate the modulator, resulting in your condition. Likewise if the engine is misfiring on one or more cylinders.

1981chevyc20 · #3 03-06-2007, 12:08 AM

Thanks for the info. Buy the way the truck has TH400.

silicon212 · #4 03-06-2007, 12:42 AM

Quote:

Originally Posted by 1981chevyc20

Thanks for the info. Buy the way the truck has TH400.

OK in that case, there will be no cable at all attaching the carb or throttle. The TH350 uses a cable for detent downshifts - the 700R4 for both detent downshifts and modulator functions (the Throttle Valve Cable or TV cable), the TH400 uses an electric link for detent downshifts. There is a possibility the solenoid inside the transmission has failed, causing the transmission to be stuck in a detent downshift - passing gear - in continuous usage. Check that if everything else checks OK.

1981chevyc20 · #5 03-11-2007, 12:30 AM

Thanks. I did replace the vacuum modulator. I hate to say it the only thing it did is calm the shift a bit but it still shifts hard; and I may have to have it rebuilt. Just for the simple fact when I bought this from my father-in-law, no telling how long the truck was be used with a broken, bad trans mount. It probably caused internal issues. If I have any other issues can I contact you through this forum?

Thanks

1981chevyc20

silicon212 · #6 03-11-2007, 12:46 AM

It sounds like a passing gear (detent) issue, the more I think about it. As was stated earlier, the TH400 has an electrically operated detent valve, so check all of the connections. Unless the transmission has had its governor modified (i.e. reduced counterweight mass or increased spring tension), a stuck detent valve is likely to be the culprit. You may need to drop the pan as the solenoid that operates the valve may have failed, or the valve itself might be stuck in its bore. There's also a possibility that there is a current on the wire that connects the transmission - pull this wire off and drive the truck and see if it shifts differently.

1981chevyc20 · #7 03-11-2007, 12:52 AM

I appoligize you maybe repeating yourself, but where is this wire located? That you said to pop it out. And the detent valve where is it located? I am a shade tree machanic but want to learn more about my truck. Are they located on the drivers side or on the passanger side. If I remember I seen a cable on the drivers side and the vacuum mod. on the passanger side along with two lines and the dipstick. Could a dirty filter be the issue? Please help I am confused and my head is beginning to spend. I want to be able to fix my truck with my hands. I enjoy working on it. But I need the guidance.

Thanks

silicon212 · #8 03-11-2007, 01:46 AM

Quote:

Originally Posted by 1981chevyc20

I appoligize you maybe repeating yourself, but where is this wire located? That you said to pop it out. And the detent valve where is it located? I am a shade tree machanic but want to learn more about my truck. Are they located on the drivers side or on the passanger side. If I remember I seen a cable on the drivers side and the vacuum mod. on the passanger side along with two lines and the dipstick. Could a dirty filter be the issue? Please help I am confused and my head is beginning to spend. I want to be able to fix my truck with my hands. I enjoy working on it. But I need the guidance.

Thanks

The detent valve is in the valvebody, which is inside the transmission on the other side of the pan. I would suggest a technical manual if you've never done this before; these are invaluable even to those of us who have done several over their lives.

It's been a long time since I had a TH400, but IIRC the wire's on the driver side near the manual valve (shift arm). It'll be a single conductor wire, the only wire on the transmission.

A dirty filter, one which is restricting flow, will actually cause the opposite problem - the shifts would be soft and mushy, with plenty of slip.

Dont worry about your present lack of experience - remember, we all started out somewhere and that goes for me as well. I think you will enjoy learning these if you are having fun with it now - but keep in mind that there will be some mental slips as well as other blindsiding things, so be prepared to have to pull something back out and redo it. I've been working on cars in some capacity since 1979, age 10. This mostly involved things such as PM, fluid toppings and stuff like that until I got my first car at 17. I'm still learning and I'm still having fun. However, my last screwup was over this past summer when I put together a TH700R4 - I put one of the roller clutches on backward and that caused the trans to actually bog down in 4th - OD. It was as if I had hit the brakes on the car.

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