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Here is the only way to check Glow Plugs for correct operation!
1. Make sure the battery is fully charged.
2. At the GPR, located on the right valve cover, verify B+ at the battery feed wire. This is a large diameter wire, usually black/yellow or black/orange.
3. If OK, use a volt ohmeter to check for vehicle power to the GPR on the smaller red wire with key on, engine off (KOEO).
4. If OK, have someone cycle the key off and on, while you check the other small wire (generally pink/orange) for less than 0.5 volts. This is the control wire the PCM grounds for 30 to 180 seconds to energize the GPC relay. If the circuit is being grounded, an audible click should be heard confirming activation of the relay.
5. If no sound is heard, touch a standard 12-volt test light to the output terminal of the relay containing the two large brown wires (some later vehicles and California models will likely have one brown and one yellow) and check for voltage. If there is no voltage present, replace the GPC relay. If the voltage is OK, perform a voltage drop across the battery feed and output terminals of the relay by cycling the key off and on, and replace any GPR that exceeds a 0.5 volt drop across the contacts.
6. If the GPR checks OK, turn off the key and allow the glow plugs to cool.
Testing the actual glow plug operation requires more than a simple resistance test and presence of B+ at the components.
Note: Testing should be done using any device containing a high current inductive amp clamp.
1. Place the high current inductive amp clamp around the two output wires of the GPR.
2. While turning the key on, watch for the initial draw to read 160 amps or greater. Depending on the manufacturer, glow plugs will draw 17 to 26 amps each with a minimum recommended draw of 20 amps (20 x 8 = 160).
3. If the initial surge does not reach 160 amps, move the clamp to each glow plug supply wire at the valve cover connectors, cycle the key on and check for a minimum 20 amps each, replacing any glow plug that does not reach this spec.
4. Allow time to cool between testing each cylinder. On the 1994-1997 trucks, the supply wires will be the brown wires in terminals one and five of each of the four five-cavity connectors. On the 1999 and newer SuperDuty, the supply wires will be in terminals 1-2-10 and 11 of the two 11-cavity connectors. Note: The wires on the driver's side of the engine may be yellow.
This is the only way to tell if each glow plug is functioning correctly. The other ways only tell you if they are within specification, it does not tell if there is a wiring problem (ie lose connection or worn wire not allowing enough current).