Section 00-00: Power Stroke Diesel Engine
7.3L DI Turbo Workshop Manual

Glow Plug System Operation

10. Glow Plug System Operation

Relay Operation

9 12 VoltsSpec.Measurement
Relay On Time10-120 seconds 

Glow Plug Operation

Glow Plug NumberGlow Plug to Ground 0.1 to 2 ohmsConnector to Relay 0 to 1 ohm


To determine if the glow plug system operation is sufficient to permit starting.

A heavy duty glow plug relay was introduced in March 1996.

Loose connections at the high current connections on the glow plug relay can lead to failure of the relay (over-heated terminals.)

If the glow plug relay is replaced, the nuts must be tightened to 70 lb-in.

NOTE: OBD II (California) diagnostics are on pages 229-233.

Test Procedure: Relay Operation

Connect a DVOM (Figure 15.) 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 engine. Note the time in seconds from when the key is turned 'ON" and the glow plug relay energizes until the glow relay de-energizes. The glow plug relay will make a loud click noise which is easily heard when it energizes and de-energizes. The dome light will dim and the dash voltmeter will dip when glow plugs are drawing current form the battery. "ON" 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 not present, check for B+ at power supply terminal (terminal with single large wire). Power for glow plug power supply is supplied from starter relay through two fusible links at solenoid.

Test Procedure: Glow Plug Operation

Disconnect all of the glow plug/injector connectors from both valve cover gaskets. (4 connectors) With the pigtail harness adapter installed (PN 01-00935) (Figure 16.), measure glow plug resistance to ground (preferably B-). A resistance measurement of .1 to 2 ohms indicates a good glow plug.

Test Procedure: Glow Plug Harness Continuity

NOTE: Incorrect measurements will result if all glow/injector connectors to valve cover are not disconnected. (4 connectors)

Measure for continuity from the connector harness to the glow plug feed terminal on the glow plug relay. Resistance should be less than 5 ohms. (Figure 17.)

Possible Causes

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 does not turn "ON," and remain on for 10-120 seconds, the problem may be a:

If the glow plug resistance to ground is high, the most likely causes may be:

Tools Required

Rotunda DVOM 105-00050, pigtail connector (PN 014-00935), stop watch or equivalent.