I have a question regarding the ABS system that I hope somebody here can answer. The van is a '99 3500 with 2-wheel ABS.

Does the hydraulic control unit have anything to do with the speedometer signal? I know that the ABS controller takes the signal from the sensor in the differential, and then sends it to the PCM, which then sends it to the speedometer itself. I have a really weird problem I haven't been able to sort out for several months, and I am through throwing parts at it. I need a definitive answer about what is likely at fault.

My speedometer gets jumpy after reaching 45+ MPH, and the ABS and BRAKE lights come on. The lights go out when I turn the van off and start it back up again. The speedometer is stable and accurate below that speed. It only starts to misbehave once I go above 45 or so, and then the warning lights come on. The ABS module & differential sensor were changed, and the wiring has been checked. The ABS module connector was also thoroughly checked. None of this has made any difference.

I scanned the ABS system and was able to retrieve trouble codes 45 and 47, which appear to only be relevant to a four-wheel system (which this van doesn't even have). Here are the definitions for those codes:

45 LF isolation solenoid shorted
47 LF dump solenoid shorted

Are these solenoids inside the hydraulic control unit? Even if the system is saying that they are at fault, would they interfere with the speedometer signal?

I'm out of ideas. Any help will be greatly appreciated.

 

I almost forgot to mention....nothing appears wrong inside the differential itself. The tone ring is alright and the gear oil isn't contaminated.

 

Your van gets its cue for vehicle speed from the rear wheel speed sensor on top of the rear differential. I would be real tempted to plug in a decent scanner, and see what the sensor is reporting.... If the signal is indeed bouncing around, replace the sensor.

Why you are getting codes for valves you shouldn't have is a mystery to me...... RWAL doesn't have any control over the front wheels at all.

 

Precisely. I shouldn't be getting any messages like that given the design of the system. My experience, however, is not unique. I have heard of other people with similar anomalous issues with this system.

The van is on its third sensor. I have replaced it twice; once with a known good one, and then with another that was brand new. Neither one of them made any difference. I don't think there's any possible way that is the problem.

It is true that the speedometer signal originates there, but it passes through two units on its way to the speedometer - the anti-lock brake module, which I have already replaced, and the PCM itself.

 

Probably a bad connection somewhere in the wiring then.

 

This thread is comparatively ancient, but I thought I'd wrap this up anyway.

it turns out that the speedometer signal goes from the sensor in the differential through the ABS module before it goes to the cluster. There is no interruption or problem between the differential sensor itself and the wiring that eventually makes its way to the ABS module. This leads me to believe that the ABS module is doing something with that signal which is causing the cluster to jump around and be erratic. I am going to replace the ABS module again eventually, and then report back here with my findings just in case anyone runs into a similar issue in the future.

 

Thank you. I'm trying to sort out a non-working speedometer and a code 34/35 on my 1994 B350. I went through the fuse box today and noticed that I have never had a fuse where my manual says the ABS fuse should go. I tried to put one in and it wouldn't go in. It's interesting to know of this connection as I investigate further.

 

If the vehicle didn't come with ABS from the factory, then there will not be a fuse in the ABS slot.

 

Many later vans came with at least some form of ABS standard.....most were rear wheel only, but I'm not sure about the earlier 90's vans.