ksems

Hi all -

what would cause an auto Th350 to shift too early.
Example....by the time I hit 25 mph , my car is in DRIVE :(...just aint right )
It bogs a little and clunks into gear.:(
It is putting a load on the engine and causing some knocking.
The problem is not there while the car is warming up.
The problem is there under light acceleration after the car is warmed up.
At this point it is also shifting into gear rough.....but shifts fine when its still cold and warming up.
Under heavy throttle it seems much better.
Trans was rebuilt a year ago.
What should I be looking at here?
Ive got a couple different troubleshooting chores going on and Im trying to figure out where to start on this one.
Is there a sensor that might be bad once the car warms up?
Would a bad HEI module cause this once the car warms up...
Would incorrect engine timing cause this?...:mad:

Thanks
Kevin

ksems

also it feels like the car goes from 1st to 2nd
then 2nd to third...then 3rd to another gear.
The car kind of sits back after its in 2nd ( drops down into another gear )
but then it goes into drive...:(....thats 4 gears .....huh?

the vehicle is a stock 1981 with Auto trans

Thanks
Kevin


[Modified by ksems, 7:26 PM 2/13/2002]

lbell101

modulator?
Hose to the modulator?

ksems

hey thanks - may be.....would that be temp related...?

Xracer

Mine does that too. I just thought it was a function of being a smog car. I will look into the hose as well.

ksems

xracer....

shouldnt do it.....mine didnt use to do it....ran like a champ after I got the trans rebuilt a year ago....smooth and strong...:).......but not right now :(

lbell101

boy... I'm not sure if it would be temp related. But the symptoms seem to be what happened several times to me when the mod went bad. They sell adjustable ones so you can change the shift points some.
Worth a try unless someone else here has an idea or more insight.

ksems

maybe it has to do with more vacuum while the car is warming up..
less vacuum after it is warmed up ??

ksems

TTT :D

Aaron76

.It only seems like 4 gears. The last gear you feel is the lockup torque converter engaging. It was installed to give you a direct link from engine to tranny under certian conditions eg. high way driving It was suposed to give better gas milage by elimiatiog the slip that auto tranny's are known for. The down side of this is that you can't install an shift kit on this tranny.
I hope this helps.

1980 razz

i had this same problem..my freind played with the linkage under the hood beside the carb. and adjusted it to fool the trans to shift later..he over did it and reajusted again yesterday and now its perfect..when he over did it the first time the vette would chirp toooo loudly when changing from 1st to 2nd at heavy foot and clunk at slow speeds..now its smooth and fast...

thamzid

I just bought a B&M Shift Kit for my 81 and I sure hope you are wrong about installing the kit on this Tranny. Has anyone installed a B&M Kit on a stock 81 Tranny and if so did it work OK and do you like it.
Later
Ken

ksems

one more symptom...

after driving 65-70 for about 10 minutes...then slow driving at 5 mph in stop and go traffic,
the car would bog on initial acceleration very consistently...:( highway speed seemed smooth )


Kevin

Aaron76

if you have a th350C it is the one that has a lock up tourque converter you can check by looking at the bottom some where there should be a strange 4 prong elec plug if you have this plug then you have a th350c and according to " How to restore and modify your corvette" you can not install a shift kit in this tranny.
good luck.

ksems

Aaron - you seems pretty knowledgable about these transmissions...what do you think
is happening with mine?....or maybe its NOT transmission but something else?

Thanks

Ingar, Norway

There is a manifold mounted temperature switch on the vacuum line to the lockup switch which cause lockup only when engine is warmed up. (Emission gear, not really necessary)
There is also a switch in the transmission that grounds the solenoid at a given speed..

The following conditions must be met to engage lockup:

Brake pedal not in use
High ported vacuum (will be low in idle)
Engine warm
3 gear engaged
Speed above 35mph


You find all details in the shop manual at my site.

ksems

interesting - thanks

I looked at your site quickly, saw the info on the TV detent but didnt see the manifold temp switch stuff.
I will look more thoroughly later.

Kevin

Ingar, Norway

Ok, that might be since the manifold switch is a air valve, not electrical. Just follow the vacuum hose from the electrical switch and you should find it.

Aaron76

The reason I know about this is because when I built my car I knew that I wanted an automatic with a shift kit.
I had one on a previous Vett (79) and I found it to be flawess and I won far more races with it than I did with the 4 spd.
You just can't beat perfect shifts 100 % of the time, although I do miss the fun I had with the 4spd I guess it was a trade off.