Low Speed ABS activation

[07sierra8.1]

Hello All,

    I have a 2007 Sierra 2500HD classic with 8.1L big block. For several years now I have had trouble with the ABS activating right before the truck comes to a stop. I figured I had a sensor going bad. I replaced both front sensors on the wheel bearing hub, that helped but didnt 100% fix the problem. I am aware that there is not independent wheel sensors for the rear wheels so in order for the abs computer to monitor the rear wheel speed it uses the speed sensor on the transmission/transfer case. Everything I have read or looked into about this issue, no one mentions that this sensor could be the culprit, everything always points to the front sensors. I have an Autel Scanner that I can monitor in real time the speed of all 3 of these sensors while driving, all sensors read the same while driving, however when slowing down, the rear sensor seems to be the first one to drop speed by about 1 second.  I cant actually see if a sensor drops to zero before stopping because the lowest read out it will go to is 3.7mph, even without moving all 3 sensors  read 3.7.    Could the rear sensor in the transmission be my problem? 

[tbarn]

Did you thoroughly clean the mounting pads when you replaced the front sensors? They tend to rust and it will raise the sensor away from the reluctor.

[Jsdirt]

When you're watching the speed PIDs by themselves, set them all to graphing, slow the graph speed down 2 "taps", then go for a drive. Whichever one flatlines before you come to a stop is suspect.

 

Normally this happens from "rust jacking" of the front sensors. Rust will lift the sensor up away from the reluctor just a few thousandths of an inch, decreasing it's amplitude just enough so that it can't be read by the computer at low speeds.. The fix is removing the sensor (very carefully - sometimes not possible without destroying it in bad cases) scraping, or wire-wheeling the rust off the mating surface, then applying grease to the surface to prevent future rust, and reinstalling the sensor. If you've replaced the front hubs recently, that's probably not the problem .. which does point to the rear.

 

Back there, I'd be checking the wiring to the sensor - make sure it didn't end up melting and shorting on the exhaust, or chaffing on the spinning driveshaft. Also check inside the connector for green, corroded and bad connections.

 

 

EDIT: Yep, what Tom said. I misread that you replaced the HUBS. You need to get that rust completely off of the surface before installing the sensors.