Hi, I am new to the forum. I have a 99 F350, the ABS light is on, and the speedometer does not work. Before everyone yells at me to search the problem here is what I have done so far. I replaced the driveline ABS sensor on the rear, light still on, I measured resistance on the new sensor to verify it was good, it passed. Check wiring with a DMM from sensor to the plug on the left side wheel well, 0 ohms. I have an OTC genisys scanner and it will not communicate with ABS system to clear trouble codes or view data. There are no other codes stored (engine PCM, GEM, Air bag) All the fuses and relays are good. The truck runs and stops fine, but I need to get the ABS light off so it will go through inspection. Also is there a seperate computer for the ABS or is it run through PCM? and if so where should it be on the truck? All the other gauges work except the Speed-o. What else can I try or test? Any all help would be appriciated, thanks Rob

 

Many of the times, a speedometer and ABS issue will be related to a bad or dirty speed sensor located on the top of the rear differential.

I am pretty sure that the ABS codes are all stored in the PCM but need to be pulled via a different system call, at least that's the way I think that the AutoEnginuity scan tool works.

 

The sensor has been replaced alredy and was clean upon removal (no metal shavings, etc) The only code stored was a P0500 VSS failure. Also a good wiring diagam would help, thanks

 

there is another speed sensor on the transmission

 

scanner shows vehicle speed, coming from speed sensor on trans, thanks


Rob

 

Do a search.

Just kidding, we're not like that here. The P0500 means the problem is related to the VSS on the rear diff, which you already changed and tested. The ABS is controlled by the PCM, so no need to look further. I'm not familiar with your scanner, but I'm wondering if there may not be more codes stored that it cannot see. These trucks are not OBDII compliant since they were made for the medium duty platform. That means all scanners won't work, which could explain you not being able to clear the code.

You might try clearing the codes the old fashioned way. Just unplug both batteries to clear the memory. That will give you a memory code, but it will be a soft code that won't set the check engine light (CEL).

Welcome to FTE.

 

The pulse train output from the speed sensor in the diff goes to the GEM , and that's where the Speedo gets its signal from and the speed data is also sent from the GEM to the PCM to run the shifting program for an auto tranny. The GEM is located deep under the dash where a leak from the windshield can drip on it and cause issues such as yours, especially if you've let a pile of snow sit there and slowly melt!