08 F450 erratic speedometer

[Jeff_]

Hey guy, I've been fighting one for a couple days with hotline and it seems all they end up doing is helping me load the parts cannon with the wrong parts.

 

This 08 F450 6.4 pickup has a problem with the speedometer intermittently going nuts. When it happens, it is usually while driving, but it did it once for me parked at idle.

When it happens the VSS pid goes nuts. VSS is an output of the ABS module based on rear axle and front wheel speed sensor readings. I monitored the pids for those input sensors and they seemed ok when the concern happens, but the rear diff signal had a slight glitche once in a while so I replaced the Diff speed sensor - no change.

 

Hotline had me drive the truck with the wiper motor disconnected - no change, and the 4x4 module disconnected - no change. We then thought that since VSS originates in the ABS module, and the VSS pid goes freaky when it's inputs are ok that it may be the ABS module... NOPE.

 

So we now have it narrowed down to either the cluster causing feedback through the VSS wiring, or the PCM, or RFI, or a harness issue.

 

I'm wondering if any of you have had this before and if so what fixed it.

Thanks in advance,

 

~Jeff

[Jeff_]

Well I guess you're all confused too, that's ok it's fixed.

 

AFTERMARKET WIRING which was T'd into his VSS ckt for his nav system fell behind the engine and melted to the up pipe. It was picking up interference from the spinning turbo's and messing up the VSS signal to the pcm.

 

Is it beer:30 yet?

[Brad Clayton]

Good catch, probably wont see something like that again, but still one to remember.