In overdrive doesn't shift down
 

Hi,
My car, a 1989 Firebird Trans Am is running normally when I drive in the streets, he shift up and down fine (between 1st and 3th gear) . I always use the overdrive lever position.
The problem is above 40-50 miles/h when the automatic transmission gets the 4th gear, so the overdrive, the car ins not be able to shift down. If I climb up in the road the speed and rpm are very low but the car doesn't shift down, even if I try to use the kick down with the throttle, the problem is the same. He only shift down when I reduce the speed to STOP.
Any idea of what happen to my car?
Thank you so much and Merry Christmas!

I am assuming you are using a THM700-R4 transmission...
If not please tell us what transmission you are using.

You are going to have to start with taking line pressure readings in all gear ranges at idle and at 1,500 RPM or so.
Then you are going to do the same with the Throttle valve cable pulled tight to see how the line pressure rise is acting.

Depending on how things are looking pressure wise...
-The TV plunger/ TV system may have to be looked at.
-Or the 3-4 shift valve-train may have to be looked at.

It would also be nice to see some photos of the valve-body and valve-train components, so that I know what I am working with.

Hi vorteciroc,
Thank you for your response, yes my transmission is a THM700-R4.
Our first step will be to drain all the lubricated oil & filter of the transmission and change que kikdown cable, then we can begin your suggested tests.

It would also be nice to see some photos of the valve-body and valve-train components

I assume these are internal body transmission parts, so I cannot take photos. Am I right?

Best regards,

It’s not a kickdown cable, it’s a throttle valve cable, and I wouldn’t mess with it. It just sounds like you have a sticking lockup solenoid. Common problem. It’s called a “TCC solenoid” and you need to drop the pan to access it.

Thank you Sacrilege.

When we drain the lubricated oil & filter we will take a look a t it.

Best Regards,

Hi all,
After to drain all the old oil in the automatic transmission, the workshop did this with a recirculation system to make sure all the existing oil is drained, and was filled up again with new oil and filter. At the same time they changed the TUV cable and adjusted according the manufacturer instructions.
Now the car is shifting down well from the OD when I push the throttle to the maximum and the car is able to get the 5k -6k rpm.
Thank for all you comments, sorry for my English.

Excellent!

If the TV cable is ever badly out of adjustment or is disconnected...
It can cause enough damage for the transmission to be in need of rebuilding.

Great news that everything is operating properly again! :nod: