First time to the forum out of desperation. My truck will not go into overdrive, but tranny checks out working good. Must be electrical? Computer? I'm stumped.

 

Welcome to the forum. I am no auto expert, but it seems that many times it's the Crank Position Sensor CPS or the Throttle Position Sensor TPS. There are many threads on how to test and adjust them, Good luck...Mark

 

I changed TPS already. I didn't think about CPS. Thanks. At least that's a new start.

 

If the tach works, doesn't that mean the CPS is OK?......Ben

 

I don't see where he said that the tach works ...Mark

 

He didn't...I'm just offering that as a quick test, if my assumption is right......Ben

 

Bypass the 4th gear grounding circuit at the trans. You're grounding the wire to the 4th gear solenoid, while driving above 35mph. If you have power with the key on, and you ground the other wire while cruising above 35mph, your trans is either cooked, or the solenoid is shot.

There's a couple threads, one I started Which helped me diagnose a toast 4,th gear.

 

I have already had the tran checked out and ALL is good with it. I have been investigating for a couple weeks with some professional folks. Man, great ideas...anything deeper?

 

This is the best way to diagnose if 4th gear is actually engaging or not. Like NJTman says it could be a toasted solenoid.

Out of curiosity how did your professional folks determine that the trans is all good?

 

I did this test, and even replaced the solenoid. The trans was cooked inside (mechanical failure), and nothing I did worked.

Here's the thread.
https://www.dieseltruckresource.com/...-330573/page2/



Go underneath. Pull the two wire connector out of the solenoid switch (driver side behind trans cable). Turn key on but don't start it.

One wire (orange with green stripe) is switched ground (which means it's turned on by the pcm once 35MPH is achieved), and the other wire (pretty sure it's blue) is the constant power wire with the ignition switch turned on. It de-powers with the ignition key off not to have constant power on the solenoid , night and day.

So, you have power at the blue wire Yes or no ? If yes, continue.

. The other wire (orange with green skinny stripe) is your switched ground that comes on over 35mph. The orange wire is TEMPORARILY connected to a lead that runs up your engine bay, or through the door vent window (which you fabricate yourself). You take another wire that's long enough to reach the wire that you just connected to the orange wire at the plug end in the trans, and you have both wires sitting next to one another in the cab. I draped them over the multifunction switch. Plug the connector back into the trans.

Got into the truck, and hopped on the highway. when the truck was over 35mph, I took the two wires and I held them together with my left hand. This completes the ground to the transmission and if it shifts into 4th gear, you have an electrical issue either with the pcm or other. If nothing happens (which is what happened to me), either the solenoid in the trans pan is shot, or there is a mechanical failure inside the 4th gear assembly in the trans. Of course, it couldn't be just a simple solenoid for me, as I had to have the trans rebuilt.

You're bypassing the PCM and other sensors to see if it's actually the transmission or an electrical issue. Lots of miles ? Eh, I'm betting trans issue. With any luck, you can replace the solenoid, fill it back up, and be done. No luck for me, though

 

Within a year of my purchase of my '91.5 my transmission started acting oddly in OD, then 1st and reverse. In my case, a bushing in the overdrive failed and that caused a cascading effect on the rest of the transmission and it had to be rebuilt. The same thing happened to my '89 Dakota and it also affected some of the Chrysler Corp. minivans.
I hope this doesn't end up being the case with yours.

 

When my Crankshaft Position Sensor ("CPS") began acting up (intermittently), the in-dash voltmeter would show discharging, the air-conditioner would quit, and the transmission overdrive would turn off. I originally blamed the Powertrain Control Module ("PCM") but the fix was quite simple. See these two old threads for details:

https://www.dieseltruckresource.com/...oblems-328710/

https://www.dieseltruckresource.com/...sensor-323729/

Good luck.