DTC 29 - Glow Plug Relay Circuit Fault: Notes

WARNING: This page does not describe the selected car, but rather 10 other vehicles, including the 1994 GMC Yukon, 1994 GMC Suburban, 1994 GMC Pickup, 1994 GMC Cab & Chassis, and 1994 GMC C3500 HD. However, it is still accessible from the selected car via links, so may be relevant.

The glow plug system is used to assist in providing the heat required to begin combustion during engine starting at cold ambient temperatures. The glow plugs are heated before and during cranking, as well as initial engine operation. The PCM controls the glow plugs by sending a battery voltage signal. PCM will default to hard start. DTC 29 will set when:

Glow plugs are commanded on and glow plug voltage is less than .8 volt.

Glow plugs are off and glow plug voltage is greater than .8 volt.

System voltage exceeds glow plug voltage by 2 volts.
NOTE: Test numbers refer to numbers on diagnostic chart.
1. This step determines if DTC 29 is the result of a hard failure.
2. Scan tester may not display system voltage. It is important that the PCM is cycling voltage on and off indicating the PCM is okay.
3. This step checks the relay feed circuit.
4. This step checks the relay ground.
5. This step will determine if an open is present in the glow plug relay control circuit.
6. This step checks the relay and wiring.