Good Morning Blazer folks, 2002 Blazer LS 4x4.

Got a good snow here in the ol' Mitten State so needed to use my 4wd again. After a quarter mile, the Service 4WD message popped up on the dash. When this happens, 4WD works, but it won't let me go back to 2WD unless I cycle the key. The last time this happened, my coworker suggested to look for corrosion on the connectors for the 4WD module in the pass side kick panel. I did that, and found corrosion. Cleaned it all up and applied dielectric compound. Haven't used it since then, until this morning. Also, when doing the connector there were no codes stored in the vehicle. My t-case is the stupid 4 button Auto case which I really dislike. What other things could be triggering the light, and why are there no codes stored? I'm stumped. TIA!

 

Ok, I got updates. There was a history code (C0305) for the front propshaft speed sensor. This is an auto t-case and naturally, GM's verbage and diagrams are unclear. The manual shows it where you think it'd be by the front propshaft, but the parts list shows a left and right side on the rear output case. Either way, I have to lift and inspect.

 

Front prop speeds sensor should be located just above the front output shaft on the front portion of the transfer case body.

The rear prop speed sensor & VSS (vehicle speed sensor) are located on either side of the rear output. I can't recall which is which at the moment.

 

Turns out, I read the callout incorrectly. All 3 sensors are the same part number!! You are right, the front sensor is by the front prop shaft...right where you'd think it'd be. On the rear, the left side is the rear prop shaft, and the right is the VSS. I have a new sensor on the way....wiring is good end to end from sensor to TCCM, but it was reading zero RPM. Sensor itself tested open...spec is between 1300 and 2700 ohms. Hopefully this fixes it.

 

Sounds like you have it tracked down. I was pretty sure that all three of the sensors were the same part. I always had to look at wiring diagrams for the wire colors to tell between the VSS & rear prop speed sensor.

 

Yes, it's now fixed. I do the same thing....verifying wire colors. Can't always trust the service manuals....that's why my Uncle got into technical authoring.