My 1996 F250 Powerstroke (2wd automatic) seems to have a speed sensor problem; changed the one in the differential and it fixed part of the problem. Now I have a 2-3 mph speed sensor variation (showing in the speedometer and when I engage the cruise control) prohibiting cruise control use. I understand that will require replacing the sensor in the dash cluster. Any advice? My truck has just over 98K miles, and well cared for.

 

What was the original issue? I suggest you put the original VSS back in, especially if you used an aftermarket replacement.

 

Apologies; originally I was pulling a big 5th wheel and my speedometer went "crazy", flipping back and forth and the tranny started erratically shifting. I was able to get to a shop and we replaced the vss (this is the sensor in the differential). The problem with the shifting and speedometer erratically spinning was cured and I was able to pull for several hundred more miles, but was not able to use the cruise control due to the slight variation in the speed regulation. I had the vss changed again but it didn't cure my problem. My present diagnosis is based upon a discussion with a Ford mechanic, but if they are correct then I will have great difficulty correcting my problem. That is why I am looking for experienced Ford owners who have struggled with these problems to offer some reflection on what you all did in response to a circumstance like this. Their answer is that the parts are obsolete and not available from Ford.

 

So the new VSS reduced the speedometer fluctuations is how I read the end result. When the original VSS was replaced was it dinged or covered in metal shavings? If either is yes it may be bad carrier bearings in the rear end.

Was the replacement(s) Ford or aftermarket?

Right now I would say the PSOM is good. It's the converter behind the speedometer.

 

The electrodes were clean; no sign of metal shavings. So tell me about the "converter". Where is it located and what will it take to make the repair?

 

You are jumping the gun here. The PSOM converts the raw VSS signal to something the computer (PCM), speedometer and Speed Control can use. Since 1992 the rear axle VSS serves two purposes. The first is the Rear Anti-Lock Brake System (RABS). The second are the functions listed above. The raw output needs divided down to a usable signal, that's where the PSOM comes into play.

A varying speedometer reading is typically a VSS issue.

Cut from a Ford TSB:

TSB 96-21-11 Programmable Speedometer/Odometer Module Pointer Waver

Publication Date: OCTOBER 7, 1996

LIGHT TRUCK:
1992-1996 AEROSTAR, BRONCO, ECONOLINE, F-150-350 SERIES

ISSUE: The speedometer needle may waver and/or a light surge may occur on some vehicles when speed control is used at highway speeds between 80-113 km/h (50-70 mph). This may be due to slight dents/chips in either the exciter ring or the Vehicle Speed Sensor (VSS) and air gaps between the VSS and the exciter ring.

You can check the VSS signal into the PSOM at the RABS test port using a multimeter.

1983 Ford Bronco PSOM picture | SuperMotors.net


The test port is located near the driver side hood hinge on a gas engine truck. Should be the same for a diesel.

For reference from a gas engine truck:

courtesy of subford

 

Based upon the very helpful info. sent in response to my circumstances I pulled the vss from the differential and found powdery material, like graphite but nothing that gritted between my fingers. It wasn't fuzzy or course. So, is that metal shavings. Should I do the trace test to see the quality of signal from the sensor? Or do I have bearings out?