A bad rear axle speed sensor can cause a lot more than a dead speedometer on a 1994–2003 7.3L Powerstroke. The rear VSS signal is used for vehicle speed, ABS operation, and automatic-transmission shifting strategy. When that signal gets weak or drops out, you can see a bouncing speedometer, ABS light, flashing O/D light, harsh shifts, delayed shifts, or cruise-control problems.

The bigger issue today is the sensor itself. Ford discontinued the original F85Z-9E731-AB VSS sensor, and we've seen plenty of aftermarket replacements that fit correctly but don't produce a clean, consistent signal, or even don’t work altogether. They can be hit or miss, and that's why we developed our own.

As always, this article should get you pointed in the right direction. Diagnostic know how is still required. Use your best judgement and don't rely soley on what tips we're sharing here.

 

Common VSS Symptoms

• Bouncing or dead speedometer
• ABS warning light
• Flashing O/D OFF light
• Hard or delayed shifts
• Cruise-control problems
• P0500 vehicle-speed code
• C1230 rear wheel-speed circuit code on applicable trucks

If several of these show up together, we'd start at the rear axle sensor and pigtail before digging into the transmission, a much bigger and more specialized job.

 

Why Some Replacement Sensors Don't Work Well

The VSS mounts on top of the rear differential and reads a tone ring inside the axle. The sensor has to maintain a clean signal across the air gap between it and the tone ring. As that gap increases, the signal gets harder to read.

On a truck that's more than 20 years old, there are plenty of reasons that gap may not be perfect anymore. Corrosion can build up under the sensor, the mounting bore can get dirty, bearings and other axle components wear, and metal can collect on the magnetic sensor tip.

We've also seen customers go through multiple parts-store sensors trying to fix the same speedometer, ABS, or shifting problem. The frustrating part is that the sensor can bolt in perfectly and still be the problem. Fitment and signal quality aren't the same thing.

 

Why We Built Our Own VSS

Once Ford discontinued the Motorcraft sensor, we weren't happy with many of the replacements that were available. So we developed the Riffraff Diesel VSS Rear Axle Speed Sensor 94–03, part number RDP-F85Z9E731AB.

We used newer electronics rather than simply copying the original Ford design. In our testing, the sensor maintained a clean signal at more than twice the air gap and RPM range of the original Ford sensor, without signal degradation. That's especially useful on an older axle where everything may not be sitting exactly where it did when the truck rolled off the assembly line.

More sensing margin means fewer situations where a marginal air gap turns into an intermittent speedometer, ABS, or transmission problem.

 

Don't Forget the Pigtail

Before replacing the sensor, look at the connector and pigtail. It sits on top of the axle where it gets water, dirt, salt, vibration, and suspension movement year after year. Again, as these trucks age, wiring does too. Wiring has become a very common problem with these older trucks, go figure...

 

Here’s what to look for:

• Broken locking tab
• Green or corroded terminals
• Loose terminal fit
• Brittle insulation
• Broken wires near the connector
• Old or poorly sealed repairs

If you can move the connector and make the speedometer jump, you've probably already found the problem.

We also carry the Motorcraft VSS Rear Axle Speed Sensor Wiring Pigtail 94–03 so you don't have to plug a new sensor into a worn-out connector. Ford still offers it, and it is currently a quality part. Of course, if that changes, you can probably guess what our next new product will be (a new pigtail).

 

Quick Diagnosis

A quality scanner can help prevent playing “parts darts” and replacing parts unnecessarily.

Before replacing anything:

• Scan the PCM and ABS module.
• Watch vehicle-speed data during a slow road test and look for inconsistencies.
• Inspect the sensor connector and wiring.
• Wiggle the harness while watching vehicle speed (NOT while driving, of course).
• Pull the sensor and inspect the tip for metal.
• Make sure the sensor bore is clean enough for the sensor to sit fully flush.

Of course if a scanner is not available, do the best you can. If the wiring looks good and the signal is still dead or erratic, the sensor is the next place we'd probably go.

 

Codes to Watch For

P0500: This is a vehicle-speed signal fault. 

C1230: On applicable Super Duty trucks, C1230 points toward the rear wheel-speed circuit (circuit, again, not necessarily the sensor). 

P0720: P0720 is an output-shaft-speed sensor circuit code. That's a different sensor and a different diagnostic path. Don't install a rear axle VSS just because you have a speed-related transmission code. They can be related, but don’t assume. Diag further.

 

OBS Trucks Are a Little Different

On OBS trucks, the rear axle speed signal passes through the PSOM, or Programmable Speedometer/Odometer Module. That adds another possible failure point. If the rear sensor and wiring check out but the speedometer or transmission still has a vehicle-speed problem, the PSOM and the signal coming out of it need to be checked.

 

Bottom Line

If your 7.3L has a bouncing speedometer, ABS light, strange shifting, or a P0500 code, we'd check the rear VSS and pigtail early in the diagnosis process. 

Ford's original sensor is gone, aftermarket replacements are hit or miss, and that's really why we built ours. We wanted a replacement that didn't just fit the original hole and connector. We wanted one that produced a stronger, cleaner signal and had enough extra sensing range to work reliably on trucks that have been on the road for more than two decades.

Parts Links:

Motorcraft VSS Rear Axle Speed Sensor Wiring Pigtail 94-03

https://www.riffraffdiesel.com/vss-rear-axle-speed-sensor-wiring-pigtail-94-03/

Riffraff Diesel VSS Rear Axle Speed Sensor 94-03 7.3L

https://www.riffraffdiesel.com/riffraff-diesel-vss-rear-axle-speed-sensor-94-03/

 

 

 

Disclaimer: The information provided on this blog is for informational purposes only. We share our knowledge and experience, but we are not liable for any damages, injuries, or losses that may occur as a result of using this information. Situations are rarely cut and dry in the automotive world. Your situation will likely be somewhat different than what we describe here. Use your best judgment and always consult a qualified professional for automotive repairs and modifications. Your safety is your responsibility.