The OverDrive button/switch is used to prevent the vehicle from switching into a top gear. This increases the RPM of the engine and gives better performance. Overdrive means shifting the transmission to the Top Gear.Mostly this is done when driving the vehicle on highway speeds.
A lot of transmissions get fried because they were flushed.
In the future Just change the filter don't flush it.You should be able to find a used transmission pretty cheap though.
Make sure the transmission fluid is at the proper level that could cause it to not shift properly.
Check the coolant temperature sender for the A/T computer, and make sure the engine is reaching proper operating temperature. If not, the A/T computer will not allow overdrive.
Other possibility can be faulty overdrive sensor.
By far and large the number one culprit for not shifting is cold temperature.
Vehicle will not shift into OD until the transmission fluid is up to operating temperature.
There are few possibilities that can cause this problem:
Cold Temperature
Fouled out spark plug.Clean the dirty carbon layer collected on the plugs.
Bad Coil Pack or it can be Broken Coil Pack Connector
Vacuum Leak,get the vacuum pressure tested.
No gas cap or worn out gas cap.Inspect it now.
Bad Knock Senor
Or a issue of Bad O2 Sensor.
For start up you get it on a good transmission capable scanner and see what is going on.
what needs to be done is monitor the live data on the scanner and see whether 4th gear is commanded by the computer.
Ensure that scanner needs to be on it while driving and when O/D falls out see if the computer is still commanding it. If you are getting the command then your problem is most likely in the 2/4 band servo. Most likely a leak in the 4th piston. -
This testing will help.
Thanks.
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