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Heres the procedure:
Recommended Procedure: Relay Operation
Install a digital multimeter on the glow plug feed side of the glow plug relay (large stud with two wires connected). Turn the ignition key to the ON position but do not attempt to start. Note the time in seconds from when the key is turned on and the glow plug relay energizes until the glow plug relay de-energizes. The relay does not come on if EOT is above 55°C (131°F) for F-Series and 30°C (86 °F) for Econoline. The glow plug relay makes a loud click noise which is easily heard when it energizes and de-energizes. Compare the times measured to the table (time will be affected by engine temperature, battery condition and vehicle altitude). The voltage at the glow plug feed terminal may vary from 9 to 12 volts depending upon battery condition.
If battery voltage is not present, check for B+ at the power supply terminal (terminal with single large wire). Power for glow plug power supply is supplied from the starter relay through two fusible links at solenoid.
Disconnect all of the glow plug/injector harness connectors from the valve cover gaskets. With the Rotunda Glow Plug Injector Adapter 134-00132 or equivalent installed, measure glow plug resistance to ground (preferably B-). A resistance measurement of 0.1-2 ohms indicates a good glow plug.
Glow Plug Harness Continuity
Measure for continuity from the connector harness to the glow plug feed terminal on the glow plug relay. Resistance should be less than 0.1-1 ohm (Federal).
Measure for continuity from the connector harness to the Glow Plug Control Module (GPCM). Resistance should be less than 0.1-1 ohm (California).
For California only, the glow plug system monitor will start when the glow plugs are commanded on for over 30 seconds and the battery voltage is between 10-14 volts. A glow plug code will be stored in continuous memory if one or more glow plugs are not being powered.
Note: Look very carefully for poor connections, burnt looking or loose fitting pins that will cause high resistance and set a code.
The Glow Plug Monitor Self Test (California only) is a KOER functional test of the glow plug system performed on demand with the engine running and the A/C off. The PCM will activate the Glow Plug Control Module (GPCM) which monitors the glow plug. The pedal may be used to increase the engine speed to increase voltage if needed. A fault must be present at the time of testing for the test to detect a fault. The trouble codes will be sent to the PCM on the diagnostic line and then output to the NGS.
California or Federal
The glow plug on time is dependent on oil temperature and altitude. The glow plug relay (Federal) or GPCM (California) comes on between 1-120 seconds and does not come on at all if oil temperature is above 30°C (86°F) for Econoline or 55 °C (131 °F) for F-Series. By using the NGS you can run the Output State Self Test which will cycle the glow plug relay on for 5 seconds the first time only that the accelerator pedal is pressed. This self test does not set any codes.
You can verify glow plug on time by monitoring NGS PIDS: EOT, GPCTM. The wait to start lamp ON time (1-10 seconds) is independent from glow plug relay/glow plug control module (GPCM) ON time.
Here's another picture to help you
Insufficient glow plug ON time will not allow enough heat to accumulate in the combustion chamber to easily facilitate starting. If the glow plug system ON time does not meet any of the specifications in the accompanying chart the problem is most likely a faulty wire harness connection, ground connections or glow plug relay.
Note: Look very carefully for poor connections, burnt looking or loose fitting pins that will cause high resistance.