Ford F-150 / 250 / 350 / 450:

Speedometer not working

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# 1  9/3/2013 5:21 PM

New member
Registered: 9/3/2013
Posts: 1

Speedometer not working

I have a 2000 F 250 super duty I recently purchased. While driving home I noticed ABS light was on and speedo not working. I replaced the VSS and ABS light went out but still no speedo all other gauges work. I have pulled all connectors at firewall, G.E.M. and ABS fuse panel etc. Looking for corrosion, found nothing. I checked voltage coming from VSS while axle was spinning wheels off ground and got an AC voltage, both at the VSS and the gray/black wire at PSOM. So I guess my question is where does this AC signal get changed to DC. To run the speedo at the ABS, ECM, or PSOM? Also while I had the gauges out I tried to energizing the speedo with a AA battery and it went up to 40 mph, so I believe the stepper motor is OK. Please help



# 2  9/3/2013 11:21 PM

Registered: 11/24/2009
Posts: 1299

Re: Speedometer not working

If the speed sensor on the truck fails it will affect the anti-lock brakes, transmission shifting, engine performance and speedometer.

It will most likely be the speed sensor at the rear differential but more information is needed so I can help me make sure it is the problem.

Use this information below to help you out. If it doesn't work then scan the truck and check out the vehicle speed sensor and anti-lock wheel sensors with a scanner to verify their proper operation. Also any trouble codes stored in the computer memory.

Below is an explanation of the circuit for the speed sensor.

On vehicles equipped with 4-Wheel Anti-lock Brake Systems (4WABS) , the vehicle speed signal is generated by the rear anti-lock brake sensor and sent to the 4WABS module.

The 4WABS module sends the vehicle speed signal via circuit 679 (GY/BK) to all systems that require a vehicle speed signal input.

Vehicles equipped with Rear Anti-lock Brakes (RABS) generate a vehicle speed signal from the rear axle speed sensor.

The Generic Electronic Module (GEM) receives this signal (VSS_GEM) for internal use and then distributes it to the appropriate other users (i.e., the Powertrain Control Module [PCM] , speed control module, and speedometer).


I hope this information helps you out. If not then get back to me and we can diagnosis the problem from there. Get back to me when you can.
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