Long time lurker, first time poster. Any help is appreciated.

2000 7.3 4x4 Ex. 295k miles. Original engine and transmission.

Occasionally get P0500 code (Vehicle Speed Sensor A) while driving. Using a cheap scanner, I can trend the measured speed of the vehicle while driving.

Trended speed, Speedometer, and Tachometer all perform as they should until 50 mph. At 50 mph, the speedometer and tachometer still work correctly. The measured vehicle speed on the scanner slowly drops from 50 to 0. It does not change from 50 to 0 instantaneously.

Cruise control also works up until 50 mph.

I disconnected the sensor in the rear differential, and took it for a drive. ABS Light came on (as it should), and the trended speed, tachometer, and speedometer all performed the same as before.

My initial understanding was that the sensor in the rear differential triggers the P0500 code. It appears that is not the case.

What is the trigger for the P0500 code?
What is the vehicle using to measure/trend the vehicle speed that I am seeing in my scanner?

Other than the Service Engine Soon light turning on, and cruise control not functioning at speeds greater than 50 mph, the vehicle drives and operates as it should.

Is there something obvious that I am overlooking? I would like to resolve this issue. Any help is appreciated.

Thank you

 

Trying to remember, but IIRC on the 4x's there is a sensor in the t-case also...? Could be one of them is gunked up... causing a mis-match in the readings? Or, I could be way wrong, chased something similar in a friends truck quite a few yrs ago, was 7.3 and 4x, seems my brains hard drive needs to be de-fragmented.. files are everywhere :

 

The P0500 code is truggered when the PCM doesn't see vehicle speed. The vehicle speed comes from the sensor in the rear differential. No other sensor can set this code.

When the vehicle speed signal is lost the PCM will create one from readings from other sensors. It will then display it as if it were reading the VSS on the rear differential..

You have either a bad sensor on the rear differential, debris on that sensor (inside the differential,) or a damaged wire between that sensor and the PCM.

 

Thank you for your reply Mark. I have seen your replies on other threads here and have a deep appreciation for the knowledge you share on this forum.

I will investigate further.

Thank you!