Hi Friends, ive searched forums and posts for weeks and have read many, many posts about this pesky abs brake light issue but havent found symtoms quite like mine. My abs lights come on at the same speed all the time(68 mph) I have rwal system and Ive changed the sensor in my 2001 ram 5.9 differential twice. My speedometer has never fluttered or stopped as described in some posts but the lights only come on when i hit 68 mph. What am I missing? Is there another sensor somewhere (transmission) or do i believe the modulator or pump is gone ? This 68 mph thing really seems like a clue, no? Thanks

 

Moved to 2nd gen ram section.

 

There is one in the tranny. Try cleaning the connectors first.

Meant to say ON the tranny not in.

 

The Vehicle Speed Sensor is not used after 97 ... it's still there but serves no purpose except to throw a CEL when it dies. At any rate, the VSS wouldn't effect the ABS anyway. The RWSS does serve double duty but the VSS didn't.

Ummm ... 68 mph ... yeah, that does sound like a clue but how ... no speedometer error. That pretty much rules out the RWSS as a culprit. A problem there will surely mess up your speed sensing. About the only things left are the CAB (Computer, Antilock Brake) and the ECM itself. Bad wiring would act the same as a bad sensor so, again, no speed error = good wiring.

Lemme think about this for a bit ...

 

The speedo is not always messed up when the rear sensor is faulty. And another thing you might look into is the fluid level and the brake lite switch on the pedal. As far as the fluid goes, it is common knowledge around here that the brake line above the gas tank could have a tiny pinhole, and will also cause the lites to go on. The brake lite switch is a rare cause but I have seen some cases where it was the culprit.

 

How to break this gently ...

Peh ... the RWSS uses the hall effect (it's just a coil of wire) to send a series of pulses to the ECM. The string of pulses are read by the computer which sends a control voltage to the speedometer. Faster string of pulses = higher speed. No (or degraded) pulses = no (or low) reading. So if the RWSS is messed up there is no way your speedometer can work. This only applies 100% to 98 and newer trucks. 97 back still had the VSS tied into the system but I've never cared enough to study it (beyond the fact that my 98 doesn't use it).

A leak in the system can cause a brake light, but not ABS. The brake pedal problem shows up in cruise control issues.

 

yeah man.. like my 96 theres a sensor on the tranny.. but 98 and newer i have no idea.. my only other advice is to ask Montec hes a prof. tranny technician.. maybe he knows something that we are all missing...

 

Well if that is the case, then the numerous posts in the past on here where the lites were on but no speedo problem was present, and after replacing the sensor on the rear end, fixed the prob, then I guess it was just a faulty/corroded contact in the plug. And when the sensor was replaced, it corrected that problem and made it look like the sensor itself was bad.

 

The 98 and up retain the VSS on the tranny but it's output is ignored. The ABS signals are not generated in the transmission. They come from the RWSS, go through the ABS controller and into the ECM.

No, silly. Up until 97, the vehicle speed data was generated old school ... by a gear in the tranny (not to be confused with the VSS). The RWSS was only used for RWAL data so it's entirely possible to see ABS/BRAKE lights with working speedometers in these older trucks as the data came from different places. In 98, the RWSS was tapped for speed sensing as well, abandonding the age old gearing system. Since the OP claims he has an 01, if his RWSS (or associated wiring) were bad, the speedometer/odometer data would be faulty as well.

 

OK, so now tell me how the abs controller doesn't notice a pressure loss if the rear brake line has a pin hole in it?