Hello,

My 04 FX4 is throwing an ABS light. And for some reason this causes the truck to go into limp mode after a certain speed.

I struggle to grasp the logic of the engineering as the top speed of the truck isn't affected it just Induces its own misfire etc and has no power (gets around 5mpg). I thought this mode was for overheating problems and other things which may damage the engine, I think I can manage to drive without ABS.. Not to go off on a tangent I clear the DTCs and everything runs fine until the ABS light comes on again.

Anyhow, I had the truck scanned and changed the rear differential sensor (paid $160 from Ford, ouch). I still have the ABS light and the only code is U1900 which is a network failure. From the scanner looks like the ABS module sees all the sensors but the PCM isn't getting that info.

The pin point test checked out on the wiring. So I guess that leaves me with changing an ABS module or PCM?..

Anyone have any ideas here if this is logical and how might I determine which end has the network fault between the ABS or PCM?

Thanks

 

In case anyone is following along, I did some more diagnostics on the truck today.

We used the IDS software to test the network and all the modules passed the communication test. We can see the VSS output in the ABS module but not in the PCM. When driving the truck I have no speedometer either and it will enter limp mode as soon as you get rolling at any speed.

It seems like the VSS is not being passed from the ABS module to the PCM even though they are now showing passes on the network test.

I don't know what to say I guess I'll get a junk yard ABS mod first and see what happens..

 

To follow up, even though I appear to be alone on this one maybe someone will search this in the future...

I put an Oscilloscope on the CAN bus with the PCM and Instrument cluster disconnected and could see the ABS module transmitting clean pulses. I checked the live output and confirmed all the sensors were functioning.

Oddly enough the 4x4 module could see the rear differential sensor but still the PCM has no VSS input. This is the only item that appears to be malfunctioning on the PCM.

Tomorrow I'm going to price new PCMs... Peace..

 

This really scares me. I wonder if the problem is big brother telling us we aren't smart enough to not drive without ABS. Kind of like my Tire pressure Monitoring on the new cars....Keep us posted....I drive an 04 FX4 myself and keep a link to all the things you guys post relating to it.

David

 

Very interested in what you find. Are you sure there are no bent or missing pins on the PCM harness connector?

Sent from my XT1080 using IB AutoGroup

 

The problem has been resolved.

Basically the U1900 code was misleading. I'm not sure where it came from (The scan tool perhaps). The Ford tech that was helping me was under the impression the VSS signal (per his scan tool) came from the rear diff sensor. This is not the case on this particular truck and transmission it comes from the Output shaft speed sensor OSS. The OSS was in fact left loose after a transmission job done previously and would intermittently rattle out of the hole and loose signal. I replaced a faulty differential speed sensor not long ago which was causing the ABS failure.]

This problem can also appear to be an issue with the electronic throttle control because of the forced limited power. All I can say is spend the time to get the diagnosis right before expensive parts changing.