I am having trouble understanding the wiring diagram for the vss circuit on my 95. I am trying to correct a small spedometer issue. In the diagram there are two wires for the vss circuit. My questions are does the PCOM or the RABS module send a voltage signal to the vss on one of the wires? And does the return signal pass through the RABS module first or is it in a series and go to the RABS and PCOM together? Thanks in advance!

 

A variable frequency signal is induced by the exciter (tone) ring moving passed the VSS sensor itself. Think of the circuit as a mini-generator. The RABS module and PSOM are electrically in parallel with each other. The RABS is first to receive the signal as the VSS signal is making it's way to the PSOM. From there the buffered signal is sent to the PCM

 

Thanks for your help. So if the RABS was faulty, could the signal sent to the PSOM be wrong? I am asking this because when I am at highway speeds the speedometer will jump around one or two miles an hour and when the speed control is activated I can feel a slight jerking. Last night the ABS light came on for the first time for a few minutes and then shut off. The truck only has 49,000 miles on it.

 

Try unplugging your RABS module and see if the speedo works smooth. As long as the speedo is fluctuating, the cruise control is not usable.

I'm having the same problem on mine. Disconnecting the RABS module on mine allowed the speedo to operate smoothly. I have been driving it that way for quite some time now.

 

If it turns out to be my RABS module can I still purchase one of them? I like using the cruise.

 

The RABS module has nothing to do with the Ford Speed Control.
You can use your Speed Control with the RABS module unplugged.
The PSOM and RABS module are wired in a parallel circuit not series.
Ford trucks do not have cruise control, only GM trucks have that.

 

Hey Subford. What would cause this issue if I its not the RABS? I replaced it with a new one and the problem is still there?

 

Are you saying that with the new RABS Module the speedometer is fluctuating and it is not fluctuating with the RABS Module unplugged?
If so I would guess that the new module is bad also. I do not think a bad fuse for the RABS Module would do this but you might check them.

Most of the time when the speedometer is fluctuating it is caused by a bad rear end parts but unplugging the RABS Module should not change the fluctuating.

I have also heard of the PSOM causing the speedometer to fluctuate. You would need to put a analog VOM on the VSS wiring to see if it is the rear end or the PSOM.

 

Sorry for the thread hijack.

Yes the speedo has a slight fluctuation like .5-1 mph all the time and 5-10 mph on the highway.

It did this with the original instrument cluster and the Lightning cluster I have now that has low miles and is in mint condition. So I don't think its the PSOM.

It also did this with the original RABS module, a junk yard module, and a BWD remanufactured one from Orielly.

I also replaced the VSS twice and that didnt help either.

When the RABS is unplugged the speedo works perfectly.