The glow plug circuit is used on diesel engines to initially start the engine from cold. The glow plugs heat up the combustion chambers prior to cranking the engine. This heat, combined with the first squirt of fuel from the injectors and the extremely high cylinder pressures, fires the engine during cold starts. After normal operating temperature is reached, the water temperature sensor wired in the glow plug system changes the system's electrical resistance and cancels glow plug operation during hot starting.
The fast glow control units on 810 and Maxima diesel engines have multiple functions, controlling various components of the glow plug system. The unit has a total of ten terminals:
- Determines the pre-glow time (approx. 4 to 12 seconds)
- Stops after-glow operations when a voltage of more than 7 volts is detected after pre-glow operation
- Determines the period that the warning lamp remains illuminated (approx. 1 to 9 seconds)
- Determines the after-glow time (approx. 5 to 32 seconds)
- Stops pre-glow operation when coolant temperature is higher than 50°C (122°F)
GLOW PLUG REMOVAL & INSTALLATION
LD28 Diesel Engine
- Disconnect the glow plug electrical leads. Remove the glow plug connecting plate.
- Remove the glow plugs by unscrewing them from the cylinder head.
- Inspect the tips of the plugs for any evidence of melting. If even one glow plug tip looks bad, all the glow plugs must be replaced. This is a general rule-of-thumb which applies to all diesel engines.
To install the glow plugs, screw the glow plugs into the cylinder head just like spark plugs. Install the glow plug connecting plate. Tighten the glow plugs to 14-18 ft. lbs., and the glow plug connecting plate bolts to 12 inch lbs.
Glow plugs are tested by checking their resistance with an ohmmeter. The plugs can be tested either while removed from the cylinder head or while still in position. To test them while removed, connect the ground side of the ohmmeter to the threaded section of the plug, and the other side to the plug's tip as shown in the illustration. If a minimum of continuity is shown on the meter, the plug is OK. If no continuity whatsoever is shown the plug must be replaced. To check the glow plugs without removing them from the cylinder head, connect the ground side of the ohmmeter to the engine block (or any other convenient ground) and the other end to the glow plug tip. Likewise, a minimum of continuity shown signifies that the plug is OK; a lack of continuity and the plug must be replaced.
GLOW PLUG CONNECTIONS
A diesel engine's reluctance to start can often be traced to the glow plug busbar (the wire connections to the plugs.) Because diesel engines have a certain degree of vibration when running, they tend to loosen the glow plug busbars. This causes hard starting, as the plugs are not receiving their full current. Periodically tighten the wire connection to all glow plugs.
- Connect a test light to the blue/yellow wire leading to the glow control unit. Measure the length of time that the test light is lighted.
- Standard operation (except restart operation within 60 seconds):
Restart operation (within 80 seconds): The length of time the light is ON should be less than 6.5 seconds. For example, when restarting the engine 5 seconds after the ignition switch is turned off, the lamp should be ON for 1.5 seconds (with engine coolant temperature below 122°F and glow plug voltage 10.5 volts).After-Glow Operation
- Connect a test light to the blue/red wire leading to the glow control unit. Measure the length of time that the light is lighted. In the normal condition, when the ignition switch is turned ON from ST or OFF, and the ignition switch in ST, the test light is on continuously. Refer to the accompanying illustration.
- After the pre-glow system turns off, check the operation of the test light (2) in the accompanying illustration; (test light no. (1) in the illustration is OFF).
The water temperature sensor is connected to the fast glow control unit. Sensor resistance varies with changes in the temperature of the engine coolant.
The sensor is tested by measuring resistance while removed from the engine and inserted in a vessel of water as shown in the illustration. Replace the sensor if resistance figures vary greatly from those shown.