Table of contents: Description 🠇 Testing 🠇
Description
Each swirl chamber has a heater plug (commonly called a glow plug) screwed into it. The plugs are electrically operated before start-up by the preheating control unit.
A warning light in the instrument panel tells the driver that preheating is taking place. When the light goes out, the engine is ready to be started. If no attempt is made to start, the timer then cuts off the supply, in order to avoid draining the battery and overheating the glow plugs.
Testing
If the system malfunctions, testing is ultimately by substitution of known good units, but some preliminary checks may be made as follows.
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Connect a voltmeter or 12-volt test light between the glow plug supply cable and earth (engine or vehicle metal). Make sure that the live connection is kept clear of the engine and bodywork.
Have an assistant switch on the ignition and check that voltage is supplied to the glow plugs. Note the time for which the warning light is lit and the total time for which voltage is supplied before the system cuts out. Switch off the ignition.
If all is well, voltage will be supplied and the warning light will stay on for approximately 5 to 6 seconds.
If there is no supply at all, the control unit or associated wiring is at fault.
To locate a defective glow plug, disconnect the main supply cable and interconnecting wire from the top of the glow plugs.
Caution: Be careful not to drop the nuts and washers.
Use a continuity tester, or a 12-volt test light connected to the battery positive terminal, to check for continuity between each glow plug terminal and earth. The resistance of a glow plug in good condition is very low (less than 1 ohm), so if the test light does not come on, or if the continuity tester shows a high resistance, the glow plug is certainly defective.
If an ammeter is available, the current draw of each glow plug can be checked. After an initial surge of around 15 to 20 amps, each plug should draw around 10 amps. Any plug which draws much more or less than this is probably defective.
As a final check, the glow plugs can be removed and inspected as described in Section 2.