93 e350 non turbo diesel....came into my shop with complaint of erratic shifting along with speedo problems. Previous shop changed speed sensor at rear axle...no help, then it came to us. This version does not use a tranny speed sensor, just rear axle.
upon test run,(fluid is fine) speedo goes erratic at 15 mph, tranny goes into limp mode. we had a donor cluster so swapped that out...then it did fine up to about 55 then went nuts. ..found faulty wire at axle connector and replaced with new connector and wire up to frame main harness (soldered not crimped). After that, trans shift is good but speedo still jumps a bit around 45. ...assuming I installed a faulty donor cluster initially...I swap back to original...now it faults at 15 mph again....along with this, gas gauge was erratic a few times. considering the lesser of 2 evils, I swap back the donor cluster but no change this time at all...at 15-20 its erratic and tranny kicks down and sometimes limp mode...also ABS light is on and off and there was light metal shavings on sensor.
Jacked up axle and it has no issues at all up to 75 mph....road test has problems.
******I feel I'm chasing a ground issue or the slight possibility of a faulty (new) sensor.***** are there any other locations to look for a common fault in harness or ground? could ABS be the culprit? throw some ideas at me

 

Could it be a fuse?

 

a fuse wouldn't explain no fault running on stands but faults on the road...like a vibration caused fault. but fuses checked out

 

I agree with the wiring being the possibility. Hook up an oscilloscope and take some readings.

 

Might not be anything to it, but I would investigate the metal shavings on the VSS a little further. Pop the cover and see what's going on in there.

 

The shavings will definitely be investigated along with tone ring, metal on this one wasn't enough to effect signal...in my opinion. thanks

 

what am I looking for with scope...standard hall effect square wave? possible noise on the power supply leg?

 

I would put the VSS that came with the truck back in and if you do not have that one get one from a Ford dealer and have him look up the right one with the VIN.
The VSS from auto part stores do not work so well. There seams to be more than one for the rear end and a Ford dealer sells both of them.

If the signal is week you can also unplug the ABS module and see what happens as you drive it.

Bad speedometer jumping over 55 has also turned out to be a bad PSOM.

 

Posted by subford in another thread: https://www.ford-trucks.com/forums/1...l#post14567591

 

There is not a power supply to the VSS circuit. The VSS generates its on AC power.
The VSS is not a hall effect device.

The signal between the VSS and the PSOM/RABS modules is a sine wave. The signal will be 667 Hz at approximately 30 mph and 3.5 Volts AC.

There is an article on the PSOM and VSS in this PDF link below that you might find helpful:
http://www.atraonline.com/gears/2007...2007_01_04.pdf

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Don't you just love it when people say throw a scope at it?

Very helpful response and good info in that ATRA article. Reps sent