Overdrive problems...again

[dan brooks]

Under light throttle it won't shift into OD until about 45 MPH. After adjusting the TPS a little over a week ago I thought I had the problem fixed, but it's acting up again today. I checked the TPS adjustment and it's within . 01-. 02V of where I set it. The voltage rises smoothly as the throttle is opened.



Any ideas?

 

[cerberusiam]

There are four areas that can affect the OD shift. TPS, speed control, crank sensor, and trans temp sender. The PCM takes a read on TPS position, speed, rpm, and trans temp to decide if and when to shift to OD. I believe the trans temp is high/low signal but the rest can effect shifting. I have had more problems with bad connections and speed sensor than anything else and there are a couple articles on here about noise on the crank pickup affecting OD shift. If the TPS signal looks good you might try greasing all the connections in the circuit. If all else fails try the car wash trick and see it temporarily solves the problem. Good luck.

 

[dan brooks]

Thanks - I'll try cleaning and greasing all the connections and keep my fingers crossed. It seems to be intermittent, but I haven't been able to pin down any particular cause.



BTW - What is the car wash trick?

 

[cerberusiam]

Car wash trick - steam clean or water clean the underhood and under carriage where all the wires are. Go drive the vehicle and see if the problem condition resolves itself. If it does, pretty good indication there is a bad connection somewhere that the water is completing.

 

[cerberusiam]

Mine did the intermittent thing for 6 months before failing and it was the speed control piece on the trans. Replaced it and the problems went away. Mine would just arbitrarily drop out of OD running down the road and not shift up occassionally. Had a PCM go bad also and it caused all kinds of weird things with the OD, charging system, and speedo. Updated PCM works a bit differently than the original but its better. Good luck.

 

[dan brooks]

Thanks.



Yesterday morning, I cleaned and "greased" the TPS, speed sensor, and trans temp sending unit connections. Haven't had a chance to see if it did the trick or not, but I'm keeping my fingers crossed... don't even want to think about having to buy a new speed sensor, TPS, or worse yet, the ECM.

 

[cerberusiam]

Speed sensor is about $60 depending on dealer. Echlin is supposed to make a replacement but have not tracked that down yet. TPS is $180 from Cummins, little spendy but not as bad as dealer. PCM's are available thru Shucks, AutoZone, and probably a few others and run anywhere from $180 to $250. Not cheap but don't even go to the dealer for PCM, you will think you bought truck again. If the TPS is giving good smooth voltage readings it is probably not the problem. If all your other functions are working right I doubt PCM. My guess would be speed sensor or connections to it. There is a shaft in speed sensor adaptor that sometimes can get worn and give weird readings but you should see speedo bouncing and acting flaky. The temp sensor can be jumped across the pins and see if that makes a difference. That has been a few problems.



With 3 different trucks it has been the speed sensor in all cases. Just my experience so far. Hope you find problem and it is minor.