Rick's Free Auto Repair Advice

Service 4WD light on GM

Service 4WD light on, trouble code C0388, C0374

General Motors has issued a Technical Service Bulletin #05-04-21-003C to address a problem with their 4WD vehicles where the Service 4WD message may appear or the 4WD light stays on. The TSB applies to the following vehicles.

2004–2007 Buick Rainier
2003–2006 Chevrolet Avalanche, Suburban, Tahoe
2003–2007 Chevrolet Silverado Classic, TrailBlazer, TrailBlazer EXT
2003–2006 GMC Yukon, Yukon XL
2003–2007 GMC Envoy, Envoy XL, Sierra Classic
2003–2004 Oldsmobile Bravada
2005–2007 Saab 9–7X with Four Wheel Drive or All-Wheel Drive
and Active All-Wheel Drive (RPO NP4) or Active Two-Speed (RPO NP8) Transfer Case

In addition to the message, you may notice erratic operation of the 4WD or AWD after driving through rain, snow, or a car wash. The cause may be a corrupted speed sensor signal. GM has discovered openings in the speed sensor wiring insulation that may short circuit the signal. This occurs most often on the rear speed sensor.

The fix is the fabricate a new harness or seal the existing harness. Or, for full-size pickup and full-size utility models, replace the pigtail harness that runs from the C151 connector under the hood to the transfer case with harness, 15832722 or 15224663, depending on vehicle equipment.

Another problem with the rear differential lock coil may also cause the message

To diagnose that, disconnect the rear differential lock harness from the rear differential lock coil connector.

Use a digital volt ohm meter (DVOM) and measure the resistance of the rear differential lock coil and pigtail from terminal to terminal.

The resistance should be 2.2-4.3 ohms and not shorted to ground

If your readings are out of spec, replace the rear differential lock coil. Unfortunately, the rear differential side bearing must be removed to replace the lock coil.

This problem shows up as a 4WD light stays on or Service 4WD

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© 2012 Rick Muscoplat

Posted on by Rick Muscoplat


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