Overdrive on transmission won't work

Tedor

Registered User
I have a 1991 SC Auto. When I drive in 50mph and the overdrive gear is supposed to shift in the car stops accelerating and only the rpms goes up. Just like on a Manual when the clutch is slipping much. So if I want to go over 50mph I can kick down or I do as I do now, I drive on the other "Drive" gear without the overdrive gear which causing higher revs on the highway.

I have changed the transmission oil and it looked good. Car got only 50k miles on it.

Only thing that isn't right is just like a wrote, the overdrive otherwise the shifting is smooth.

What could cause this? I Would be happy if I don't needed to take the trans down, at least until the winter.

Thanks
 
Sounds like the TV cable is rigged wrong at the throttle body. Do not drive with the shifter in the OD position until this is corrected or you will burn up your trans. It's a simple adjustment but can cause a world of hurt if not right.
Alan
 
Sounds like the TV cable is rigged wrong at the throttle body. Do not drive with the shifter in the OD position until this is corrected or you will burn up your trans. It's a simple adjustment but can cause a world of hurt if not right.
Alan

I am not certain you should drive the car at all with a TV cable that is either disconnected or maladjusted.
 
Okey, I found the a post here from TCCOA on how to adjust.

For me it just happend all of a sudden. It didn't get worse by time just one day it didnät want to put the overdrive in.

Thanks
 
Sounds to me like your OD clutch is burned up. That's the main problem with the AOD transmission. :rolleyes:

Definitely check your TV cable first though.
 
Sounds to me like your OD clutch is burned up. That's the main problem with the AOD transmission. :rolleyes:
Actually, the direct clutch.. and that means 3rd isn't far behind. It is the smallest of the clutches, if you have even the smallest drop in pressure, and those plates start to slip, they overheat quickly.. as in just a few miles quickly.
 
Actually, the direct clutch.. and that means 3rd isn't far behind. It is the smallest of the clutches, if you have even the smallest drop in pressure, and those plates start to slip, they overheat quickly.. as in just a few miles quickly.

I have rebuilt a number of AOD's and I have found that the single leading cause of failure is heat. The stock cooling unit that is integrated as part of the radiator just plain sucks to start with and gets worse with age of the radiator. In many cases the radiator isn't cooling as well as it did when new and second the coils get restricted over time and do not allow as much tranny fluid to pass thorugh the cooler.

If your transmission is toast, as suspected, I would highly recommend bypassing the stock radiator trans cooler and installing an aftermaket transmission cooler.
 
Definitely a good idea, but it won't stop the direct clutch from frying. There really isn't much fluid flow through any of the clutch drums. They are designed to be on/off devices and not designed to remove heat. In fact, anytime you rebuild one, you need to be generous with the prelube as it will take a lonnng time for the fluid to find its way to the frictions.
 
Definitely a good idea, but it won't stop the direct clutch from frying. There really isn't much fluid flow through any of the clutch drums. They are designed to be on/off devices and not designed to remove heat. In fact, anytime you rebuild one, you need to be generous with the prelube as it will take a lonnng time for the fluid to find its way to the frictions.

Lots of prelube and soak your friction plates in ATF for a long time. You are correct about the fluid flow but the cooler definitely helps. It keeps the overall tranmission temperature down and does keep the drums cooler via heat transfer along the hard parts. If the rest of the transmission gets too hot, then the drums get too hot then it's all over for the clutch packs. :)
 
Thanks guys! :)

I adjusted the TV cable tight so it was right before to restrict the throttle from going wide open and the trans still won't put it in over drive....

Is there something more to check or adjust?

Im very supprised beacuse it happend all of a sudden, I have never ran the car with the cable loose and I think the adjustment were good from the begining beacuse the tranys shifting didn't change as I adjusted the cable.

Can I drive the car in the other "drive" gear(the one that have 1st, 2nd and 3rd) without the overdrive? Or will this hurt the trans more? I have allready put like 150 miles on that drive with no issues, changed the trans oil 10 miles ago and it looked like new. The plan is to take the trans apart this winter. Right now I want to enjoy and drive the car.

/Fredrik
 
Can I drive the car in the other "drive" gear(the one that have 1st, 2nd and 3rd) without the overdrive?

/Fredrik
You should absolutely do that. You want it to keep from trying to shift into OD at this point. You should be able to drive it like that for months.
 
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I had same problem on my 93.Solution was to replace clip on the kick down cable at the throttle body,it may have fallen off or broke.check that b4 you spend alot of time and $
 
Isn't the TV cable the same as kick down? I zip tied it to the Throttle Body so it sits there...
 
Isn't the TV cable the same as kick down? I zip tied it to the Throttle Body so it sits there...

No. The TV Cable controls the transmission line pressure which in turn controls shift points and shift firmness. That is not the same as a kick down cable. The issue is if that cable becomes maladjusted or pops off completely then you don't have enough line pressure and you will damage the transmission components.
 
Isn't the TV cable the same as kick down? I zip tied it to the Throttle Body so it sits there...

Your SC's throttle body will have three cables attached to it - thottle, TV, and cruise.

If your TV cable is zip-tied to the throttle body, that needs fixed...it's a very sensitive adjustment. If it has been that way for awhile, that may be a large contributing factor to your OD problems.
 
On the AOD, the TV valve serves the same purpose as the "old school" vacuum modulator valve. That is it changes pressure to make the trans shift. Its however, more precise than the vacc valves. The TV cable, operating the TV valve, also serves the same purpose as the kickdown cable. However, it does so by compressing the throttle valve to the end of its stroke causing the transmission to downshift.

As long as its adjusted and attached with a zip tie, it will do what its supposed to do. I've driven my 2 autos for years that way with no problems. :D I now have the brass bushing, but its really just for style. :cool:
 
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