Ford ABS/VSS gurus needed!

[SuperSonic]

ABS VSS Guru needed.
We are wrapping up our Fordodgeolet project and we are having a hard time figuring out an issue with the speedometer/odometer.
What we have is an early 99 F350 CC LB dually 2wd. Originally a 7.3/4R100/Dana80.
We stripped it down to bare frame and rebuilt it from the ground up.
It now has a 6.7 CR Cummins backed by a 08 Allison 6sp lct-1000 out of a 2wd Chevrolet 3500.*
We also installed a front and rear axle out of an 08 F450.
Front is a super60 style 2wd straight axle and the rear is a Dana/Spicer 110 4.30 ratio (soon to be 3.73)
Reason for the axle swap was for the bigger brakes and the heavier 19.5 tires and wheels.
When we brought the truck home, I believe that the speedometer was working, now it isn't.
The front VS sensors plugged right in and appear to be working correctly. If you unplug either the ABS light comes on immediately.
We had to change the plug on the harness to the VSS on the rear axle as they were different, but both are two wire, so we just switched the plugs. I checked the wiring schematics for both trucks yhrough alldata and made sure that it is wired correctly and even tried switching them to be sure that wasn't the problem.
Right now we are getting + voltage on both wires for some reason.
One wire has between 2.5-4.5 volts and the other is between 1.5-3.5 volts. Key on, sitting still.
The voltages are kind of jumping around randomly, it doesn't matter if all, none, or some of the other VS sensors are plugged in.
We have replaced the rear speed sensor, checked the tone ring, replaced the gauge cluster, and checked out the harness between the VSS and the ABS module that it runs to.
The only thing that we haven't replaced is the ABS module itself, but kind of doubt that it is the problem (it worked before)
It is odd that unplugging either of the front VSS will immediately throw a ABS light but unplugging the rear, even driving around for a couple days, yields nothing.
We thought there may have been an issue with the lack of transmission communication with the PCM or ABS module, thinking that maybe the PCM needed to see that the trans was in drive. We worked around that and verified that the PCM was seeing that it was in gear with no change.
Right now I'm at a loss. Amazing that after figuring out how to make the 6.7 and Allison standalone, that something as "simple"as the ABS system could get us screwed up.
Any input?

 

[me2]

Is the Ford ECM still in the truck ? If so if you connect a scan tool to it, you should see the ABS codes. Post what you find and I'll help. I have a '99 SD myself. It has a 5.9CR in it.

 

[SuperSonic]

Yep, anytime we unplug either of the front speed sensors, it throws a code.
But doesn't for the rear sensor.
I have a snapon modis scanner, did all ABS and GEM self tests, nothing shows up. When doing the GEM test it does run the speedo up and down, so I know the cluster itself is working. When sitting still, the ABS control module is reading 3mph at the front wheels, the rear nothing. We checked continuity and resistance on all circuits and didn't find anything wrong.

 

[mr4speedford]

Sounds like you have covered alot of potential options. I would check for interference with other electrical wires that run next to the vss wires from the differential to the abs module (aftermarket fuel pump, lights etc.)

From looking at the schematics it looks like the abs module doesn't communicate the with the pcm other than a vss signal from the abs module to the pcm.

The fact that you can un plug the sensor and not get a code, is rather strange, either there is enough voltage from something else to fool the module that it is still hooked up or possibly a bad abs module. It looks like you have covered everything else.

 

[stuckinmud]

Mr4speedford is on the right track with the rear sensor. That is where the abs module gets its speed from. The ABS is where your gauges get their signal from.

As pointed out your problem is that rear sensor is not throwing a code which means its not talking to the module, or the module is bad. Rear gear ratios have no impact as it's all based off a tone ring, but tire size does.

 

[G Racing]

Well I guess it all depends on if it was working to start with or if it wasn't. Is the sensor the same for all year models or did they change it. If its different you would need the sensor for that abs module. If its the same then you may have a wire chaffing issue or bad module as stated above. The front sensors should be showing o mph while sitting. If I recall you should have a 5v reference for the sensors.