1999 GMC Forward Control 5.7 R, AutomaticFACTORY OEM SERVICE PROCEDURE

Glow Plug Relay

  1. Ensure coolant temperature is less than 80°F (27°C) before beginning test. Place ignition switch in RUN position and allow glow plugs to cycle. After 2 minutes, crank engine for one second. It is NOT important that engine starts. Return ignition switch to RUN position. Glow plugs should cycle on at least once.
  2. If glow plugs do not cycle, disconnect glow plug relay (relay is located on top rear of engine). Connect test light between Pink wire terminal of harness connector and ground. With ignition switch in RUN position, test light should be off. Test light should illuminate when engine is cranked.
  3. If test light does not illuminate as specified, repair short or open in engine harness Pink wire. If test light illuminates as specified but glow plugs do not cycle, replace glow plug relay.
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When to See a Mechanic

Stop DIY work and contact a certified mechanic immediately if any of the following apply:

  • • You smell fuel, burning insulation, or see smoke.
  • • Brakes feel soft, pull hard to one side, or make grinding noises.
  • • The engine overheats, stalls repeatedly, or misfires under load.
  • • You are missing required tools, torque specs, or safe lifting equipment.
  • • You are not confident in the next step or safety outcome.

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