robox

A few weeks ago my truck started intermittently dropping out of overdrive. It got progressively worse. On a hunch, I pulled off the TPS and cleaned it out pretty well with electrical cleaner. Overdrive came back..... but not for long. I cleaned the TPS again; overdrive came back again, but for an even shorter time. Sounds like a TPS problem, right?

Well, I followed the advice of several folks on this forum and installed a potentiometer instead. This, however, did not work. I was not able to get OD with any setting of the pot. I checked the pot with a multi-meter and it was smooth and good.

Then I re-connected the TPS and checked it also with a multimeter. That also checked out good, moving smoothly from .86 volts to 3.5 volts. I even ran wires from the TPS into the cab which I hooked up to my multi-meter so I could watch what it was doing as I drove. Totally fine. But by this time I have no OD whatsoever.

It's not the VSS. That is relatively new, the connection is good, and the speedo works just fine.

It's not the trans overtemp switch. I pulled that out a long time ago and jumpered the wires together. I just re-checked that connection and it's fine.

I read 25 or 30 threads before posting; I didn't want to waste anyone's time. The only discussions I noticed containing potential leads mentioned the crank sensor and the PCM.

What's a crank sensor? Where is it? Is it expensive? Could it be my problem?

Regarding the PCM... I found a link on how to test it, but where is it? Could it be my problem? (My grid heaters work, my heat/AC works.....)

Any other ideas?

For those of you out there who put in a manual OD switch, (grounding the solenoid) don't you find that a pain in the edit?

Thanks a bunch,
Christian

cLAYH

On my old '92 I used a floor mounted dimmer swt(like older cars used for high beams). When I wanted OD I tapped my foot, when I wanted it off I tapped it again. You get used to it, like driving a stick. Might be useful to hook one up just to test a see if you can manually force the tranny into OD to rule out any problems inside the tranny.

funcruise

As an added feature to the dimmer switch you can use the centre post for ground and the right hand post to the tranny, then take the left hand post and run a wire up to a light or led to ground it. Then when tranny is in od the light is off and when out of od the light will come on. Much easier to tell that way.....

th_racer

Try grounding the wires out that go to the od unit, don't worry with a switch until you know you can make it work by tieing the wires together, it could be the silonode in the trans bad, also make sure all of your grounds are good. The radio shack pot worked great on mine and from the sound of things your factory pot is good so it could be the wire or tranny silonode

cLAYH

My swt only had two posts. So I just "t'ed" in a wire that grnds a light mounted on the dash, the other side is hooked to 12V ign. When its in OD the light is on. WAY less confusing that way.

wannadiesel

Like th_racer said, try grounding the solenoid to make sure it's OK.

It doesn't sound like the PCM or the crank sensor.

ChrisLib

Sounds alot like what my truck was doing, see here,
https://www.dieseltruckresource.com/...threadid=72661
and the fix here
https://www.dieseltruckresource.com/...ad.php?t=74673
Hope it helps!

wannadiesel

Man, I forgot all about that, Chris. That was a weird one.

robox

Well, just to follow up on my overdrive problem....
I re-checked all the connections, located the PCM and cleaned the connector for that, and still no OD.
Rigged up a temporary wire to manually ground the solenoid, and..... Got overdrive back!
So, I did what others have suggested and put in a foot-switch (old car high-beam switch). It's working fine. I still need to put in a light somewhere on the dash to let me know when I'm in OD. I may in fact use one of the two message lights below "Wait to start" and "water in fuel" to let me know I'm in OD. (Those 2 messages never light up.... I don't even remember what they say!)
So I guess my OD solenoid is fine. To be honest, I'm quite happy to be done with the PCM / TPS / VSS cabal. They just couldn't seem to get it together.
Once again, thanks for all the help!