I just drove my truck to WV and while driving down highway running 80mph the ABS & Brake light came on! Message center said Service Brake System. The lights would go off after awhile driving above 65mph and then once I slowed down between 40mph and 65mph the speedo would take those 10 to 20mph bounces and then the lights would come back on! I could deal with the lights and speedo bouncing however it was causing my tranny to downshift and upshift each time the speedo bounced! I was pulling a loaded down 24ft enclosed trailer full of UTV & ATV'S! Got to WV and took to local shop, they replaced the speedo sensor! This did nothing! Not sure if this matters but just 2 weeks ago put some 20"wheels with 305/55/20 on it so it tossed my speedo off about 8mph! While I was driving down highway I went to the tire setting on my Edge Attitude display and increased the tire size on this trip, cannot remember how long before issue started. Dash cluster has just been replaced 4 months ago and driver side wheel bearing which would have replaced the wheel speed sensor for that side I guess. Has anyone on here had this happen or anyone have any ideas to help?

 

Haven't been inside a Duramax rear axle lately, but I believe the speedo and rear ABS pull their signal from the tone ring in the rear differential. The tone ring is either part of the ring gear or it is pressed on to the carrier. Sounds to me like there is a chipped ring gear tooth, or something else that has left metal chunks in the rear diff and is throwing off the magnetic pickup of the sensor. I'd pull the rear diff cover and see what falls out...

Then again, I may be totally wrong.

Anyways, I hope this helps.

 

My rear diff has no wiring or sensors at all on it at all. The rear sensor for the back is mounted on the rear tail of the tranny. Basically is monitors the yok, which I did replace that rear sensor. It did have some real fine shavings on it, but as soon as I put the new one on and drove it did same crap. I'm hooking up machine to it tomorrow to see what each sensor is doing. I tested barkes today and the ABS is not working and the truck will not go into 4-wheel drive when those brake lights are on. Wierd!