How to Test a 7.3 Glow Plug Relay

Written by dan swords | 13/05/2017
How to Test a 7.3 Glow Plug Relay
Glow plugs aid in starting a diesel engine during cold weather. (detail of diesel engine image by Petr Gnuskin from

Glow plugs are an important component in the Ford 7.3 litre Powerstroke diesel engine. The glow plug, along with the glow plug relay and temperature control devices, heat the cylinder so proper combustion can take place. There are four connections on the relay. The two smaller wires energise the coil, or turn the coil on. The two larger wires are the connections from the battery to the glow plugs. Successful troubleshooting of the glow plug relay can be accomplished with a digital voltmeter and someone to help start the engine.

How to Test a 7.3 Glow Plug Relay
Verify all fuses are in good condition. (car fuses image by Witold Krasowski from

Inspect the fuses in the fuse box. Replace all damaged or blown fuses. Inspect the connections to the fuse box. All connections should be tight and free of corrosion and damage. Clean the corroded contacts with baking soda, water and a wire brush.

Inspect the wiring and connections to the glow plug relay. The relay is located in the engine compartment behind the fuel filter. Verify that the connections are tight and free of corrosion, damage, abrasion, oil, dirt and debris. Clean the corroded contacts with baking soda, water and a wire brush. If abrasion or exposed wires are found, temporarily wrap the damaged wire with electrical tape. When possible, permanently replace the damaged wire.

How to Test a 7.3 Glow Plug Relay
Use your digital voltmeter to measure the voltage at the glow plug relay. (digital multimeter image by dinostock from

Measure the voltage across the glow plug relay coils. Place the positive probe on terminal 85 and place the negative probe on terminal 86. Have someone turn the key switch to "On" while you measure the voltage. The voltage should be approximately 12.6 volts. If the voltage is not correct, inspect the wiring to the glow plug relay. Replace any loose or broken connections.

Test the glow plug relay by temporarily connecting a 14 gauge wire from the positive battery terminal to terminal 85 on the glow plug relay. Use black electrical tape to temporarily secure the connection. Connect a 14 gauge wire to the chassis ground, but do not connect the other end at this time. Connect the positive probe of the digital voltmeter to terminal 87 and place the negative probe to the chassis ground. Temporarily connect the 14 gauge jumper wire to terminal 86. The relay should turn on and the glow plugs should get warm. Disconnect the jumper wire immediately. If the glow plug relay does not turn on, the relay is defective and must be replaced.

Things you need

  • Digital voltmeter
  • Baking soda
  • Water
  • Wire brush
  • Black electrical tape
  • Replacement fuses
  • 14 gauge wire

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