I submitted this a few days ago. Hoping maybe there is a new set of eyes that can help. I have 2008 F250 5.4L automatic supercab. I have an issue with a couple things. The speedometer will jump around when the vehicle is not moving. Sometimes it happens immediately when I turn on the key and sometimes it will jump once when I start the engine. Generally when it jumps around a lot after just turning the key on the red brake light will also stay on along with the ABS light. I have done a lot of pinpoint checks and found nothing wrong so far. One thing I did notice was that if I disconnect one front ABS sensor, it can be either side, the speedometer does not move when starting or just turning on the key. I do not know why the front sensors have any impact on the speedometer since the VSS on the rear diff is the speed input. I have done a test rotating the front wheels to prove out the front ABS sensors and they are fine. I have checked voltage to the sensors and it is fine. Anyone out there with any thoughts on this?? The speedometer seems to operate fine when I start driving.

I have the Mitchel wiring diagram and I see the Yellow Blue wire going from the ABS VSS pin to the PCM VSS pin. There is a splice (S130) that shows a takeoff to the 4x4 module and to the wiper motor. I have no idea why there would be any connection to the wiper motor. Is there anyone out there that can explain that one?

 

Without knowing how it was designed to work Its hard to get a grip on it. Did you check resistance of each front sensor?
Maybe something is shorting out or open intermittently.

 

Thanks for the reply. Help is always appreciated. I tried a resistance check and it was infinite on both sensors. I have a new sensor so I checked it and it was 750,000 ohms. All seemed extremely high but Ford's Pinpoint test asks for greater than 10,000 ohms with no max. Passive sensors should be around 600-1500 or so ohms but I never did find a resistance number for active sensors. Ford uses active sensors on that truck. So I tried another method of testing the front sensors. I disconnected the sensors. I connected an LED in series with the sensor and a 12 volt low milliamp power supply. I jacked up the truck and spun the wheel. The LED pulsed as the the wheel was turned so that tells me the sensor is reacting to the tone ring. My take on that test is the sensors are fine. A voltage test on the harness side connectors was within specs as well.

I am leaning toward the ABS module itself. But before I spend the time and $$$ on one I want to be sure I am curing the problem, not just spending time and money. Another issue with replacing the module right now is my Forscan extended license is expired and I have not been able to renew it.