On diesel compression-ignition vehicles, spark plugs are replaced with glow plugs. These devices glow red-hot for a predetermined amount of time, delivering the required amount of heat to the engine's cylinders before cranking. Cold temperatures can affect how long the glow plugs stay on. Faulty glow plugs can cause the engine to not crank at all or run poorly when cold. In order to fix this problem, the glow plugs must be removed, tested and replaced if necessary.
Use the multimeter to test the voltage of the battery. Apply the positive and ground leads to the positive and negative terminals of the battery. The voltage of the battery should be 12.5 volts with the vehicle off and 13 volts when the vehicle is running. Lower voltage readings usually indicate a faulty battery or alternator.
Use an automobile repair manual for your vehicle's make and model to locate the glow plug relay. Remove the connectors and check the connections for rust or corrosion.
Connect the ground lead of the multimeter to the negative battery post. Use the positive lead to test each socket in the connector. Check the values shown in ohms on the multimeter against the proper values given by the repair manual.
Locate the metal strip that serves as the glow plug fuse. Visually inspect it for hairline cracks and corrosion. Use the multimeter to test both sides of the fuse. Readings for both sides should display around 12.5 volts.
Locate the glow plugs near the intake manifold and remove them with a socket wrench. Place one lead on the glow plug wire and the other on the glow plug harness socket to test continuity.
Visually observe the glow plug to insure that the tips are not broken off. Place one lead on the body of the glow plug and the other on the threaded tip where it attached to the harness socket. If the multimeter displays a reading of 0 ohms, the plug is shorted internally and requires replacement. If you have a jump starter, you can test the glow plug by connecting the negative lead to the body of the plug and the positive lead to the threaded tip of the plug. This will send power through the glow plug and cause it to glow red hot at the end that is inserted into the engine. Remove the positive lead and allow the glow plug to cool down before attempting to handle it again.
Test the new glow plugs with both the multimeter and jump starter. Insert the glow plugs in each socket and tighten them with the socket wrench. Reattach the harness socket and glow plug wires to the glow plugs. Test the connectors once more to insure that the relay is working properly.
Reattach the connectors onto the glow plug relay and turn the ignition. The glow plug indicator on the instrument panel should remain on for 10 seconds, depending on engine and outside temperature. Crank the engine -- it should start instantly and run smoothly at idle.
Exercise caution when testing the glow plugs. Allow the glow plugs to cool down in order to minimise the risk of burns or electrical shock.