1981 Transmission Acting Up

[Charlie P.]

I'm experiencing a weird problem in the extremely limited driving I do with this 7,300 mile car. I am not sure what it is doing, actually. It feels like it is shifting into third from first. If I step down even slightly, it revs like it is either slipping or down-shifting, and then clunks into third, I think.

I checked the modulator for a diaphragm leak, but it seems O.K. It could have 30 year old fluid in it, if that could be a factor. Any thoughts?

[Michael D.]

Re: 1981 Transmission Acting Up

Have you checked tranny fluid level?

[Jim T.]

Re: 1981 Transmission Acting Up

Charles don't know your knowledge of checking auto trans fluid, however the fluid level when hot should check full. Less than operating temperature the level should be a lot lower. Best checked after driven several miles at normal operating temp. My tubo 400 in my 70 is now officially 41 years old and still working great with over 160K miles. Several filter and fluid changes by original owner.

[Clem Z.]

Re: 1981 Transmission Acting Up

Charles don't know your knowledge of checking auto trans fluid, however the fluid level when hot should check full. Less than operating temperature the level should be a lot lower. Best checked after driven several miles at normal operating temp. My tubo 400 in my 70 is now officially 41 years old and still working great.

turbo 400 trans is the strongest one GM ever made. there are no "let us see if we can make cheaper parts" in it.

[Tom B.]

Re: 1981 Transmission Acting Up

On an '81 you will get full converter lockup at very low speed and rpm. You didn't mention if you were experiencing this at normal throttle or 50% plus throttle. My wife ordered her '81 new and it has always been this way. Low miles and never been apart for anything other than normal service work. You can pull away from a stop at, say, 20% throttle and it will shift to 2nd almost immediately. You may not even notice that shift. Then when it shifts into third at 25mph or so it will shift to 3rd and lock the convertor at the same time. There is a big drop in rpm and a pronounced hard jerk. It's the nature of the beast. This was the first year for computers and things were not as smooth as they are today.

Accelerate harder and it will shift smoothly through 2nd, 3rd, and then go to locked. I would suggest having your trans serviced with new fluid and filter if it's never been changed. Put some mixed road miles on it then re-evaluate. These cars need driven to stay sharp. 30 year old ATF is way past it's service life even with the very low miles. It seems like there was a tech bulletin about premature lockup at one time and the fix was a different prom in the ECM. I read this once years ago but have never been able to find it since. We had some ECM problems in the '90s with this car and after putting in a few remans we installed a new ECM with a new prom and it has been right as rain since then. Still shifts hard into 3rd locked though at light throttle but not as bad as before.

At lower speeds as soon as you give it a little more throttle the converter will unlock. This is what you are feeling is a downshift. Harder throttle and it will shift down to 2nd.

Tom

[Terry M.]

Re: 1981 Transmission Acting Up

I am thinking by 1981 the TH 400 was no longer used in Corvette.

[George C.]

Re: 1981 Transmission Acting Up

I am thinking by 1981 the TH 400 was no longer used in Corvette.

Last year in Corvette for the TH 400 was 1977.

[Charlie P.]

Re: 1981 Transmission Acting Up

Thanks all for all the input and advice. I will definitely change the fluid after I get back from the National. Hoping for a Bowtie, so I wouldn't want to put a new gasket in yet.