I know this has been hashed to death but I recently bought a pristine 1996 F150 Eddie Bauer from orig owner. Garage kept since new. My OD light is blinking. Scan shows speed sensor bad or erratic. Sensor replaced but light still starts blinking above 45 mph. Scan again & get same code. Shifts perfectly, lockup is spot on. Speedo is all over the place. Could this be a wiring issue? Thanks for any help. These computer controlled vehicles are more trouble than my 46 2 dr Sedan Deluxe.

 

I will guess you have an 4R70W transmission.
So the PSOM output will cause the blinking but the shifting will be normal as the PCM will shift the transmission off the OSS and not the VSS.
The PCM may use the VSS if it is there but with it gone it will use the OSS.

But being speed related I would think your problem is more than likely in the rear end.

 

PSOM issue I'd bet.

 

Best I can figure the vss in the diff is fir rabs. Is that correct? The vss in trans is for speedo is what I can gather. As my user name & avatar suggest I'm into old school cars.

 

Both speed sensors are new & trans works as it should. Only thing is speedo is funky & od light blinking. Yes it is a 4r70w

 

Yes and no.
The one in the diff is for rabs, speedometer, speed control and information to the PCM.

The OSS in the trans just sends information to the PCM.
I do not know the logic in the PCM for shifting with an 4R70W but I do know my 4R70W will shift OK with the speedometer (PSOM) removed from the truck. I do not know if it will shift with just the information from the VSS and nothing from the OSS. But it will shift with just with the OSS.
We do not get much information on this site from owners that have the 4R70W and have a speedometer problem.

It is hard to say if you have a bad PSOM or the rear end has a problem or bad wiring between the two.
So yes it could be a wiring problem.

The only way to know without replacing parts is to put a meter or scope on the VSS signal from the rear end.

 

There is more than one speed sensor. What code did you get? I need the number, not the description. And where was the sensor you replaced?

 

0503 speed sensor A. It has been replaced as well as the vss in the diff.
Any help would be appreciated. Thanks Mark. Truck shifts as it should but after about 40mph the speedo goes nuts & od light comes on.

 

P0503 is vehicle speed sensor intermittent. That's the sensor on the rear differential. An intermittent code usually means there is a problem in the wiring to the sensor, not a problem with the sensor itself. You're going to need to electrically diagnose what's wrong in the harness.

 

Thanks Mark. Where would I find resistance and things to check? Would the crazy speedo have anything to do with it?

 

The crazy speedo is probably due to the problem that set the code.

There is no resistance value. It's a wire. Each wire should be close to zero resistance from one end to the other. And it should be open circuit to chassis ground and to power.

 

Thanks...........I hate electrical issues. Rather build an engine, LOL

 

Deleted Deleted

Posted in wrong thread.

 

It isn't the OSS. The OSS does not affect the speedometer.

 

I got this thread mixed up with a different thread.
The Thread it was supposed to be posted in was this thread:
https://www.ford-trucks.com/forums/1...hifting-2.html
Sorry I do not know how I posted that in this thread.