Is the CEL on? If so, what were the exact codes pulled? Has this vehicle been in an accident before? Are there any aftermarket mods installed? Who did the tranmission rebuild? Did they rebuild or install a used transmission instead?

Are you sure the OD button is not activated? Have you tried cycling the button?

 

No modifications have been made other than maintenance items. I'm not getting any check engine or other warning lights so I haven't scanned it yet. The transmission rebuild was done by a reputable shop. And yes, I've tried cycling the OD button on and off! I don't think it's a problem in the transmission itself at this point. Something in the sensors/computer is not signaling the OD to engage.

 

The things that could possibly not allow the OD to engage would be engine coolant temp, speed (around 51mph), lock out button/circuit, or speed sensors/speedometer.

Do you have an OBDII reader/scanner? If you do, what is the engine coolant temp (ECT) being reported to the ECU when this issue is happening? If this sensor is faulty and reporting a lower temp that it actually is, this condition will prevent the transmission from shifting into overdrive. A faulty ECT should not set a code and illuminate the CEL.

Is yer speedometer working correctly? If the speedometer cable is broken/disconnected, the ECU would have a difficult time to shift the transmission based on speed. I understand that if the cable was disconnected or broken, the CEL would light up and set the codes fer the speed sensor.

Is yer ATF at the proper level? The ATF level is checked on level ground, the engine running, and after all the gears have been cycled through. The proper level should be between the marks fer the HOT section on the transmission dipstick.

I don't think the ATF temperature sensor would keep it from engaging as that sensor reports the ATF temp to the ECU which doesn't allow the transmission to shift to 2nd gear until the ATF is above the lower threshold temp.

From my factory EWD book...

Shifting into the 4th speed (Overdrive) takes place when there is no continuity to either No. 1 or No. 2 solenoid.

4. OVERDRIVE CIRCUIT
* O/D main SW on
When the O/D main switch is on, a signal is input to TERMINAL OD2 of the engine control module and engine control module operation causes gear shift when the conditiond for overdrive are met.

*O/D main SW off
When the O/D main switch is turned to off, the current flowing through the O/D off indicator light flows through the O/D main SW to GROUND, causing the indicator light to light up. At the same time, a signal is input to TERMINAL OD2 of the engine control module and engine control module operation prevents shift into overdrive..

Does the O/D Off light turn on when you switch the O/D switch? If it doesn't, yer bulb could be burned out. Does yer gear selection lights work? Have you checked the fuses to make sure they are functional (especially the 15A CIG and 10A GAUGE fuses)? These fuses are located in the interior fuse panel.

 

If you guys can shed any light on this... I've "fixed" the problem a few times but it keeps cropping back up on my 1997 with AT. Here's a quick rundown:

1. Original issue, would not go into overdrive last winter. Had the entire unit rebuilt at a cost of $2800, was working fine for about 6 months but I don't drive this truck a lot, not my daily driver. Overdrive again stopped engaging.
2. The shop inspected it, diagnosed as a faulty ECU. Replaced with a reman unit at a cost of $400. That worked for about a month again on light duty.
3. Since then, I've replaced the Throttle Position Sensor (new part) - no change. (BTW, the 1997 TPS does not adjust, it's plug and play)
4. Replaced Transmission Fluid Sensor (used good unit) - no change.
5. I replaced the thermostat with a new OEM part, overdrive DID work for a short time, but then stopped engaging again.

So now I'm at the end of things to try and money to spend. I can get the shop to replace the ECU since it's under warranty but is there something else that could cause the problem?

I know how to solve your problem I have the same problem with mine my mechanic ran a jumper wire to the solenoid that shifts the overdrive basically I have a push button on my dashboard I push it to shift into overdrive it works perfectly little annoying but it never fails me I tried everything like you did nothing seem to work never seem to have a real answer why well definitely work I put
over 150,000 miles on the truck since we did this