I lost my overdrive. I was making a quick run to town, got about 2 miles and the overdrive kicked off. Was going about 55, overdrive had been on for a minute before kicking out. Any ideas?

I already checked whats obvious. TPS plug hooked up. No fuses blown. No cooked fuseable links. The overdrive off light comes on and off with the button. The plug is firmly connected. I climbed under truck and tranny line temp sensor is hooked up. The plug into the top of tranny is connected. I can't find any rubs or shorts on what parts of the wiring I can see.

Any tips on troubleshooting or isolating the problem would be really appreciated.

Thanks

 

Sounds like your tps, same thing happened to my truck. You should pull the TPS out, smash it with a hammer, then wire in a toggle switch for o/d. Thats what I did, except I havent smashed the tps yet

 

Does the TPS effect anything else other than overdrive? With the switch in do you get a 1st-OD at any speed or do you have to shut it off around town.

 

Same question. What does the TPS affect as far as the OD operation?

In order to check the TPS operation, what can I do to "hot wire" a toggle to isolate it as my problem? In other words, I remove TPS from circuit. Wire the plug so system acts like TPS is saying "OD" and then check. If it works. . .TPS is problem and I need to wire permanent switch.

 

The TPS controls the shift point of the OD. HERE is a thread on how to fix it cheap and control the shift point from a **** on your dash. The toggle switch might work fine - if you don't mind flipping it on and off every time you need OD. That's too much work for me.

 

TPS is the problem. To check if it's bad connect the +ve & ground through your volt meter. Reading should be 1.0-1.5. Connect 3rd wire& ground (colors may vary according to how much home made wiring on your truck). While connected move the throttle w/key on, engine off. You should get a reading of 3.5-5. This will tell you if it's the TPS or the wiring that's gone.

Unfortunately, 9/10 times it is the TPS. Not terribly expensive though, and free if you follow the advice of the guys above and toggle it. Good luck with it.

 

TPS, speed sensor, PCM, or OD solenoid. Had all of them at one time or other. Check the voltage on center wire of TPS plug. Should be around 1v at rest to 3.5v at WOT with a smooth reading up and down with no jumps.

TPS is about $160 at Cummins. Speed sensor should be about $60. PCM is $200 to $350 depending on where you get it.

You can wire a mystery switch to test OD operation but if you leave it on it will stay in OD all the time. A potentiometer from radio shack will fix it great for about $5 also but no kick down from OD to drive.

 

Tomorrow I'll put a meter on the TPS. Is the speed sensor you're refering to the one on the back of the tranny that runs the speedo?

Do the TPS's just all together die? I thought they started acting up first. I had a 95 that did weird things at a certain throttle position. It affected the TC lockup. Would jump in and out. If you went up or down with the pedal it would be OK. I took it apart and cleaned .. . .no problems. This time, it just dropped out of OD and hasn't come back, any pedal position.

I suppose if everything else fails I can hot wire a switch to the solenoid itself. If still nothing, than I know its the solenoid.

 

Yes the speed sensor is the on on the tranny, in your case the t-case with gear reducer.

The TPS on my 92 was workng fine and just quit one day. Broke something inside and voltage went to 4.5v all the time. Still couldn't get things to work right with new TPS. Replaced The PCM and al the adjustments and correct shifting came back.

 

Just a quick note. Same thing happened to me 2 years ago. Driving down the interstate and the OD just went bye-bye (at 60 mph). Mines on a toggle switch!!! Needless to say, I drove home at 50 mph. It turned out to be the connection (plug) on the Tranny. Just unpluged it, and pluged it back in, and bingo, it's worked flawlessly for 2 years. Loose, dirty, I don't know???

 

Still trying to get my OD back. I checked the TPS. Getting .9V at idle position. It goes to 3.75 at WOT. Smooth transition all the way. I checked the temp sensor by hot wiring plug, still no OD.

Finally I tapped a wire into the ground side of solenoid wire and grounded it while going down the road.. ... . OD came on! So the solenoid is OK. I've got voltage going to it, just doesn't ground when it needs to.

What else do I need to check? How about the OD off button on the dash. Can that fail and leave the solenoid circuit open? Is the solenoid ground routed through that switch? because I also disconnected that plug and still no OD.
Where is the computer that runs the OD? Maybe the plug needs cleaned there.

Any other ideas? I know I can hot wire the solenoid, just not interested in manually turning the OD on and OFF.

 

The PCM is between the battery and the fender.

The ground for the OD solenoid comes straight off the PCM. Since you are not reporting any other problems, I'm assuming the PCM is working OK. The problem is being caused by one of the inputs that the PCM uses to decide when to ground the OD solenoid.

You didn't say the speedo was broke, so the VSS is OK.

The charging system is working, so the PCM is in "engine running" mode.

There are a few things that can lock you out of OD. First is the switch. It's a momentary deal, it grounds a wire to the PCM. The PCM is supposed to light the light in the switch when it gets the signal to lock out O/D. If the light works as it's supposed to (goes on and off with button pushing), the switch works.

There's a coolant temp switch located behind the fuel filter mount, it should be closed above 60 degrees. There's the transmission thermo switch, it should be closed unless the trans is way too freakin' hot. I would check these out just by pulling off the plugs and jumping them one at a time to see if you get O/D.

Make sure the truck won't start in Drive. The PCM looks for voltage from the NSS, if there is voltage it assumes you are in neutral and don't need O/D.

 

Well, I went out tonight. Direct wired the temp sensor on the engine and went for a drive. Still no overdrive. .. .but have a new problem (or maybe the original problem) to sort out.

Its dark now and when I went for a drive the lights are surging. They are slowly strobing or dimming. The charge guage on the dash is low most of the time with all the lights on but sometimes it jumps up and the lights come up brighter. If I shut everything off and just idle the alternator appears to be working. Guage reads about halfway. I put a meter on the battery and it flucuates quickly from 12-ish to 14-ish. I cant tell exactly because meter is digital.

I'm guessing my internal regulator is dying. I've also read that if the crank pickup sensor is dying it can cause the alternator to not charge either. Is that right?

I've read the tech article about converting the internal regulated alternator to an external regulator. The article said it would take care of the bad crank pickup as well.

My question is, will putting an external regulator on solve my NO OVERDRIVE problem as well? regardless of whether the regulator or CPS is failing???

Anyone know?

 

Probably not. Some have said the CPS would effect OD but on my 92 you could unplug the CPS and still have OD. The only thing the CPS provides info for is the charging system and AC on my 92. What is in other PCM's is a guess.

The lights flickering like that is usually a bad connection, either a ground or battery cables. Pull your grounds, scrape and dielectric grease them, then clean up the battery cables and grease them. I have had that happen twice. Once was a bad ground and loose battery cable. The other was a bad connection in the positive battery cable. The lack of a good ground and flaky charging will definitely screw up the OD shift.

Also check your harness under the dash and make sure it is not rubbed bare on the dash supports on the left side of the steering column. You could have an intermittent ground there and causing the OD to be locked out or causing a drain on the charging system.

 

Obviously the outside temperature in your area has dropped. The intake heater is cycling on and off. The heater draws about 90+amps and your lights will dim when the heater element kicks in and the lights will brighten when the element cycles off. This will happen for the first little while after you intitially start the truck and usually stop before the engine reaches operating temperature. It is normal for this to happen.