TREK

My overdrive usually kicks in around 55 but hasn't worked at all yesterday and today. This happened the first time a couple of weeks ago after I developed a transmission leak and the fluid level got low.

RamGuy84

First off check fluid in Netural then clean the TPS on the top of the pump tward the front of Motor

j_martin

The overdrive shift is electrical. This problem is probably not related to the low fluid incident in any way.

I don't believe in the "this is what mine was, change that" approach to fixing things. It is the most expensive way to get the job done, if it ever gets done.

The OD solenoid is driven by the Power train Control Module (PCM). Let's go down that route.
The PCM needs signal from the (Crankshaft Position Sensor) to start up. If the alternator and tachometer are working, the PCM is cycling normally.

For OD, the PCM needs signal from the Vehicle Speed Sensor (VSS). If the speedometer is working, the VSS is OK.

The PCM also needs input from the coolant temperature switch indicating the engine is warmer than Numb, Alaska. If the switch isn't closed, the OD will not function.

The final input the PCM needs, and the one most likely at fault, is the TPS signal. That is an analog signal that should be around 1v at closed throttle, and 3.5V at full throttle. Both those readings can vary a bit, the PCM will compensate, but if they are wildly off, or more likely the wiper arm of the potentiometer (in the TPS) is dirty and opening the circuit, then the PCM refuses to act on it.

Attempt to adjust the TPS. Use an analog meter, and see that the needle sweeps smoothly when the throttle is moved through it's range. Here's a snapshot of the FSM procedure:
http://dens-site.net/Dodge_CTD/tps-adjust.jpg

hope it helps

TREK

I know how to set the voltage of the TPS but how do you clean it?

RamGuy84

Spray it off with Brakeclean after taking the 2 screw's out lift it up and see if it's dirty . Also try some electrical cleaner on the connection for the wires .

Jim5870

hopefully you don't have this problem...... but I guess this was a common failure in the early days of the 518 overdrive sections.

https://www.dieseltruckresource.com/...rdrive+failure

Angry Johnny

What J said. The TPS is sealed and there is no way of really cleaning it inside. There are some posts in the sticky to adapt a cheap gas unit TPS to the diesel. That is what I have done and it works fine. I suspect you've had a problem with the TPS all along as mine goes into O/D at about 35 with light throttle.

cougar

With a small sharp scratching tool dig out the potting compound that secures the cover.



Carefully lift out the center drum. Be aware of the delicate contacts and the return spring. Using a cotton swab ans contact cleaner clean the contacts on the drum and the carbon ribbons on the inside of the body. I then place a thin layer of dielectric grease in the ribbons and a dab of lithium grease on the bearings. Reassemble and seal the cap with RTV.

cougar

I eventually wound up going to a switch for OD. I have a problem in my wiring harness that I couldn't work around.