The preheating system warms the swirl chambers prior to engine start-up. The length of time the plugs are on is determined by the coolant temperature. A coolant temperature sensor sends this data to a control unit which regulates the engagement time of the glow plugs.
When the ignition key is in the ON position, the system is engaged and the dashboard light comes on. After the dash indicator goes out, the plugs remain on for 1-14 seconds, depending on conditions. When the engine starts, the glow plugs are switched off. There is a safety circuit built in which will turn the circuit off after a predetermined amount of time if the engine is not started. This prevents the glow plugs from melting and damaging the engine.
TESTING & REPLACEMENT
These tests must be carried out with the engine coolant below 100°F (38°C). During any of these tests, remember that the glow plug circuit will shut off after a period of time. It will be necessary to cycle the ignition switch to OFF and then back to ON to continue testing.
Since the glow plug circuitry uses the normal 12 volt electrical system of the car, it is fairly simple to test with a test light. A voltmeter can also be used.
To check for power to the glow plug, connect the test light to ground and touch the other end to the plug terminal. Turn the key to the ON position and look for both the test lamp and the dashboard indicator to illuminate. If both lamps light, all is well up to the glow plug. If either or both don't light, check the control relay as explained below.
Remove the test lamp and turn the key OFF . Disconnect and remove the ground bar which runs between the glow plugs. Connect one end of the test light to the positive (+) terminal of the battery and touch the other end to the end of one glow plug. If the test light comes on, that plug is good. If the glow plug must be replaced, do so by simply unscrewing it and installing a new one. Reconnect the ground bars between the plugs.Glow Plug Control Unit (Relay)
See Figure 1
On cars with the D24 engine, the control unit is located under the left side of the dashboard. On Turbo models, it is found under the hood on the left shock tower. For D24T vehicles, disconnect the control unit plug and turn the ignition ON . Ground one end of the test light and check for voltage at terminal 15. If no voltage is present, a wiring fault is present. Also check the terminal of the orange wire. If both the dash indicator and the test light come on, the control unit is failed.
Also with the ignition ON , check for voltage at terminal 30 on the relay; no voltage shows a wiring fault between the battery and the control unit. Check terminal G for voltage; if no voltage is present, check the 80 amp fuse on the bottom of the relay. If voltage is present, the control unit is OK. Remove the test light and reinstall it to the positive battery terminal. Check the control unit ground by touching the test light to terminal 31 (black wire) on the control unit. If the test light does not come on, the unit is not grounded.
On the D24 family, leave the connector on the relay and check for voltage at terminal 15 (blue-with-red wire). No voltage shows a wiring fault between the fuse box and the relay. Under the hood, check the smaller glow plug relay (on the left shock tower) for voltage at terminal 86. Connect the test light to the positive battery terminal and check the glow plug relay for proper ground at terminal 31. If the test lamp lights up, the relay is bad. Remember this is all being done with the engine cold, the ignition ON and within the operating cycle of the glow plugs being on.
Back inside the car, check for voltage at terminal G on the control unit. No voltage indicates a faulty control unit. Connect the test light to the PLUS terminal on the fusebox (+) and check terminal K on the control unit. If the test lamp lights up, you have either a defective dashboard indicator light, a wiring problem between the control unit and the indicator light, or a fault in the printed circuit behind the instrument panel. If the test light does not come on, either the control unit is bad or the wire between the control unit and the temperature sensor is grounded when it shouldn't be.