Diagnostic Aids

WARNING: This page is about a different car, the 2014 GMC Yukon XL 1500, 2014 GMC Yukon, 2014 Chevrolet Tahoe, and 2014 Chevrolet Suburban 1500. However, it is still accessible from the selected car via links, so may be relevant.

A TRUE B+ has enough current to light a test light.
A DIAGNOSTIC B+ is a low current circuit that DOES NOT have enough current to light a test light.
A short to ground on the 4WD HI indicator control circuit will cause the 4WD HI indicator to remain ON at all times.
An open or high resistance on the 4WD HI indicator control circuit will cause the 4WD HI indicator to be inoperative during the 2 second bulb inspection.
An open in the indicator dimming control circuit will cause the 2WD HI, 4WD Auto, 4WD HI and 4WD LO mode indicators to be inoperative during the 2 second bulb inspection.
EL-49741 harness is a good source for 9 V.
EL-49742 harness is a good source for B+ and ground
With key REMOVED - opening a door puts a true B+ on the indicator dimming control circuit, TCSCS X1 pin 4. Other possible voltages are listed below:
- Door open, key removed - indicator dimming control circuit voltage = true B+.
- Door open, key in, key OFF - indicator dimming control circuit voltage = true B+.
- Door open, key ON, engine OFF - indicator dimming control circuit voltage = diagnostic B+.
- All doors closed, key OFF - indicator dimming control circuit voltage = B+ (high current - enough to light a test light).
- All doors closed, key ON, engine OFF - indicator dimming control circuit voltage = diagnostic B+.
- Key ON, engine OFF. Place a shop light on the DRL sensor. When the DRL sensor commands the headlights OFF, the indicator dimming control circuit voltage, TCSCS X1 pin 4, will go from a diagnostic B+ to a true B+.