2000 Astro-Service AWD-dash light

[Lockdoc54]

I have a 2000 Astro AWD, with 115,000 miles on the vehicle, the 8th digit in the vin# is W. The yellow 'Service AWD' dash light has just come on, and it will go out for a day or two, and then come back on. Unfortunately, the Haynes manual I have, covers 1985-2003 Astro and Safari vehicles, but not the AWD models.
Can anyone, give me information, as to what type of services might be required, to resolve the problem. What are the types of issues, that cause the indicator light to come on? Does the sensor indicate a problem in the transmission, transfer case, rear end, or all of the these. As for leaks, I've not noticed any fluid, leaking on my driveway, and the underside of the vehicle, shows no signs of a leak. Thanks in advance for any suggestions.

[Blue Bowtie]

There are three sensor on your transfer case, not including the encoder (actuator) motor.



They are under the blue plastic caps in the photo. The rear/upper sensor is used for the road speed (VSS) input, the rear lower sensor is for AWD (Transfer Case Position Sensor), and the front/upper sensor is for ATC (front axle speed) to verify traction loss. Make sure the connectors are clean and tight.

It might be best to scan the PCM and ATC, but in my experience scanning and diagnosing these systems, the codes can frequnetly be misleading. Have all the codes recorded, but don't allow anyone to replace anything unless they guaranty that the part they replace will repair the problem. They will be more than happy to spend your money guessing at the problem, and may not find it. The dealerships are especially good at this, since they are instructed to replace all suspect parts, but the almost NEVER actually test suspect sensors with a power supply and meter or oscilloscope.

I had one on MY personal vehicle that showed various codes - C0222 (Right Front Wheel Speed Signal Missing), C0237 (Rear Wheel Speed Signal Erratic), C0245 (Wheel Speed Sensor Frequency Error), and a C0246 (Transfer case lock system performance). All the sensors were just fine (as proven by a multimeter, NOT a scanner) and the problem turned out to be an intermittent contact in the ECBM Power relay (for the ABS system, not the ATC). The problem was so brief that NO codes were set for the ABS system.

The signals from all three sensors are used together in multiple systems, and the failure of one can manifest itself as an error code in a seemingly unrelated system. Even the factory service manuals are not very clear on this, as I found.