2007 silverado 4x4 4.8 liter vin v

im having premature abs activation looking at scan data it shows the rear speed sensors dropping out. the speed sensor i replaced is on top of the tail shaft of the transmission. replaced it twice thinking we got a bad standard branded one. the speedometer goes from about 18 mph then just drops to 0 when the activation occurs. if i unplug the vss the activation quits but the abs is deactivated. is there a tone ring in the trans the vss reads??

That is the vehicle speed sensor the abs speed sensor is in the rear differential

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thats what i thought. dont see one in the rear diff or at any of the rear wheels. unplug the vss and the rear wheels quit reading all together.

The tone wheel is pressed onto the tailshaft bit I remember hearing that it can become loose on the tailshaft. But I have not personally seen one do that

how hard do you think it is to replace?? im going to scope it first though

just an fyi it has jf3 option. NOT jl4.

My first thought is a sensor air gap issue, but I don't know enough about the transmission internals to say how that's possible. The symptoms fit, anyway. What Bruce mentioned about the tone wheel loosening on the output is interesting, but I'm not sure why that'd only create an issue at low speed. I guess it'd be interesting to know if the sensor drops out completely at the same time as the scan data, or if the signal amplitude just gets too weak.

Speaking of, I'm having trouble tracing the VSS signal on the wiring diagrams (to see if it's a VRS or Hall sensor). According to the ABS diagram for JL3, the VSS signal gets to the ABS on a discreet circuit from the Radio/Navigation? :blink:

How cheap are these sensors? If they're cheap enough to be disposable, you could try grinding a few thousandths off the sensor shoulder so the tip will sit closer to the tone wheel. If the symptom improves, you're probably on the right track.

i will try that. but im hoping it makes the same signal as the front 2 abs sensors because im thinking of feeding one of those signals through the vss wiring.

does anyone have a good waveform of a vss on a chevy? im thinking it should be a sine wave most likely

Negative. I'd expect an AC sine wave, as well.