Speedometer issues.

[celticwarlock]

I have no idea where to post this, so I'm putting it here.

I could use some help with the speedometer issue I am having on my 1999 Ram 3500 van. As soon as I go above about 45 miles per hour or so, the speedometer needle begins to falter occasionally, and the ABS and brake warning light both come on. When the van is turned off and re-started, the problem goes away again and the Iights both go off until I approach around 50 miles per hour, when it happens again.

The van has 2-wheel ABS, with a wheel speed sensor in the differential. Everything there is functioning as it should (I replaced the sensor twice, and the wiring has been tested). The speed sensor for the output shaft in the transmission was also changed, but didn't solve the problem. I also removed the anti-lock brake controller, inspected all the wiring, clean the connections, and neatly put everything back together. None of this has worked. Could it be the anti-lock brake controller itself? I hesitate to continue throwing parts at this problem, and I'm hoping somebody here can point me in the right direction. Thanks in advance.

 

[Davea Lux]

Tires that are unequal sizes on the ABS axle can cause this type of problem, the tires need to be the close to the same circumference or the ABS controller can read the difference in rotation as unequal braking if the lights are coming on when braking. You should also check to be sure that the brakes are properly adjusted and that they are properly bled.
A speedometer needle that is faltering along with an ABS warning is usually indicative of a bad speed sensor, but you have already replaced that item so I doubt that is the issue. You might also want to check the battery cables for corrosion as a bad ground can cause a lot of random computer malfunctions that generate false codes. If you have an OBD sensor, check for stored codes. Finally check you charging circuit with a voltmeter to be sure the system is not overcharging as that also can effect computer reliability.

Dave

 

[cbarge]

Common on that generation of trucks.
Its in the cluster circuit board.
If you really want it fixed change the cluster.
You pretty much eliminated other related items.
Hope 4his helps

 

[celticwarlock]

Tires that are unequal sizes on the ABS axle can cause this type of problem, the tires need to be the close to the same circumference or the ABS controller can read the difference in rotation as unequal braking if the lights are coming on when braking. You should also check to be sure that the brakes are properly adjusted and that they are properly bled.
A speedometer needle that is faltering along with an ABS warning is usually indicative of a bad speed sensor, but you have already replaced that item so I doubt that is the issue. You might also want to check the battery cables for corrosion as a bad ground can cause a lot of random computer malfunctions that generate false codes. If you have an OBD sensor, check for stored codes. Finally check you charging circuit with a voltmeter to be sure the system is not overcharging as that also can effect computer reliability.

Dave

Thank you for the suggestions.

I have checked the wiring extensively, including the grounds and battery connections, and nothing is amiss. The breaks are also properly bled and actually work very nicely. I bled them not long after this started to happen to make sure it was not a hydraulic issue, and I also checked for leaks, and did not find any. Also, there are no codes stored in the computer. That was actually the first thing I checked, but I did not mention it in my original post.

 

[celticwarlock]

Common on that generation of trucks.
Its in the cluster circuit board.
If you really want it fixed change the cluster.
You pretty much eliminated other related items.
Hope 4his helps

As far as I know, the ABS system interprets a signal from the tone ring and the sensor in differential, not the speedometer itself. If I'm right about that, then the ABS wouldn't be disabling itself and turning the warning lights on; the speedometer would still be erratic, but the ABS wouldn't react negatively.

Also, because the fault occurs regularly only when certain conditions are met, I'm inclined to believe the problem is independent of the speedometer (and therefore the cluster).

The schematics that I have reviewed in the service manual have not been of any help, unfortunately. If I knew more about how the systems actually operated, and where the signals originate and terminate, it would be much easier to trace the problem.

 

[Davea Lux]

The GM and Mopar pickups of that era did have a lot of issues with the instrument clusters. On GM models, parts of the boards had been outsourced to China and most failed. The problem on the GM units was with the magnetic motors that ran the gauges being of poor quality. I am on my 3rd cluster in my 2004 silverado. The Mopar boards were also outsourced and the had defective circuit pathways that caused shorts in the assembly to where idiot lights would come on at random without codes being generated. You might want to try finding a used cluster to see if that solved the problem. Failing that, the problem probably resides in the ABS management system as you have replaced everything else. A dealer with the proper equipment could probably run a diagnostic on the ABS system, problem is the hookup charge to run the diagnostic is sometimes more that the replacement part.

Dave

 

[celticwarlock]

The GM and Mopar pickups of that era did have a lot of issues with the instrument clusters. On GM models, parts of the boards had been outsourced to China and most failed. The problem on the GM units was with the magnetic motors that ran the gauges being of poor quality. I am on my 3rd cluster in my 2004 silverado. The Mopar boards were also outsourced and the had defective circuit pathways that caused shorts in the assembly to where idiot lights would come on at random without codes being generated. You might want to try finding a used cluster to see if that solved the problem. Failing that, the problem probably resides in the ABS management system as you have replaced everything else. A dealer with the proper equipment could probably run a diagnostic on the ABS system, problem is the hookup charge to run the diagnostic is sometimes more that the replacement part.

Dave

Absolutely. A replacement ABS unit is about $50, and the diagnostic procedure will be considerably more.

Still, I'd rather know than just guess. The lights are coming on, but go off again when the van is re-started. My supposition is that the ABS unit would trigger the lights if it was not capable of properly processing the signal, which in turn would make the speedometer behave strangely. I know for a fact that if you put an ABS unit in your vehicle that has the wrong values programmed into it (like if you pulled one out of a vehicle with different gearing than your own), your speedometer will function, but it will not be accurate. This tells me the ABS unit is at least partially responsible for speedometer function, though I do not fully understand their relationship to one another.

 

[celticwarlock]

Not that this thread is active, but I figured I'd provide something resembling an answer just in case this helps someone in the future.

After much research, it appears the ABS module is almost certainly at fault. I only have to find one with the exact part number and install it. I can't guarantee it, but it seems likely that this will solve the issue.

The signal does indeed go through the module before going to the cluster. When I cut power to the module, the singal never reaches the cluster, so the cluster can't be at fault.