VSS- PCM- ABS Module ques???

[dieselbmet]

Drove to the golf course and after 18 started my 03 F350 7.3L SRW crew cab and my speedometer, odometer, cruise control did not work. My OD light was flashing and my ABS light was constantly on. hooked my tuner/reader to the OBDII and got P0603 and P0500. I replaced the VSS consistent with code P0500 and no dice. I took it to my localFord Dealership and they troubleshot this down to the ABS Module (not covered of course!) Question: Can a faulty ABS module cause the failures above or am I being snowed? Bottomline..I don't see how the ABS module can change how the VSS and PCM operate the speedometer.

help.....

[SmokeyWren]

Originally Posted by dieselbmet

Question: Can a faulty ABS module cause the failures above or am I being snowed?

Your tech has it right. Yes the ABS module can cause those problems.

Bottomline..I don't see how the ABS module can change how the VSS and PCM operate the speedometer.

In the rear diff there is a "tone ring" with 120 "teeth". Looks sorta like a light-duty gear. That tone ring is attached to an axle.

Also in the rear diff is a variable speed sensor (VSS). That VSS counts the teeth in the tone ring, and every time it gets to 120 it knows that your tires have turned one revolution.

The VSS sends the count of tire revs (and partial revs down to 1/120th of a rev) to the ABS module. In that module is stored the parameter of how many revs/mile your tires make. So different size tires have different revs/mile. Stock for your 2003 is 655 tire revs/mile.

The ABS module then uses a tiny computer to determine distance traveled, and sends that data to the main computer (PCM). The PCM has a clock and knows the time, so it then uses the distance data from the ABS module and computes time/speed/distance and sends the appropriate commands to the speedo, odo, tripmeter, cruise control, etc.

So without the ABS module, nothing using time/speed/distance is going to work right. And yes, that includes the cruise control, which uses speed to maintain your speed.

The use of the ABS module to store the tire revs/mile parameter is not the method used by most other vehicles. But all '99-up SuperDuty trucks use that method. That's one reason a cheap scanner will not work on a SuperDuty truck - they will read the PCM but not the ABS module.

[dieselbmet]

Thank you for the quick reply!

Understood. I thought the VSS sent all info straight to the PCM and not via the ABS module. your explanation was great. I guess I will make sure the same tech works on my rig the next time I am forced to the dealerships door.