What exactly does the sensor on the rear axle do? 08' f350. I know it's a speed sensor of some sort, but what does it do or control exactly.

 

The VSS (Vehicle Speed Sensor) does a few things. It acts as a input to the ABS module to control the ABS for the rear axle. It also provides vehicle speed to the speedometer and helps determine shifting for the transmission if you have an auto. Removing this sensor, which I have done for troubleshooting an issue, will result in an ABS light, no speedometer input, and extremely delayed and hard shifts since it has to go on engine RPM alone to determine when to shift. On an '08 I cannot remember exactly how the ECU and the AT work together, someone with your year of truck could probably give you more specific info on that aspect of it.

 

Thanks for this information. Is there a particular sensor that could be going bad that would cause an intermittent 4x4 Low light to come on and off with you are in 2wd mode, and even though your 4wd works perfectly fine in 4h and 4l when to use the switch too. From my understanding, the light blinks because the PCM is losing its communication with the 4wd system. The light generally appears when the truck is braking, so I thought it would be related to the sensor on the rear axle.

 

I believe the sensor for that is on the xfer case, however, dont' go replacing it yet though. I say that because there has been a few cases of some folks on here having that happen as well. I don't remember what the exact cause was, I don't believe it was a sensor though and it was a wiring problem. I'm sure if you searched the forum though you will find a few threads discussing it.