The indicator lamp in the instrument cluster lights when the glow plugs are operating. After the lamp goes out, the engine can be started. Voltage is supplied to the glow plugs for a few seconds after the indicator lamp comes on. If no attempt is made to start the engine after the lamp goes out, the timer disconnects voltage from the glow plugs to prevent the battery from discharging and the glow plugs from overheating.
On models with 1.7L engines, the fuel filter is also equipped with a heating element, which is installed between the filter and the casing and is controlled by the pre-heating system. The heating element is switched on if the temperature of the fuel passing through the filter is less than 5°C and is switched off when the fuel temperature reaches 16°C.
Examination
Connect a voltmeter between the glow plug supply line and ground. Turn on the ignition and check that voltage is supplied to the glow plugs at the appropriate time interval.
At an air temperature of 20°C, the time intervals for supplying voltage to the glow plugs should be 3 seconds, during which the control lamp lights. The time the control lamp lights increases at lower temperatures and decreases with increasing temperature. If voltage is supplied to the glow plugs, then the relay and wiring are in good condition.
Using a tester, check the integrity of the glow plugs by checking the resistance between the contact at the top of the plug and the body of the plug. The resistance of the glow plug is less than 1 ohm and if the tester shows more resistance, the glow plug is faulty.
Text copied from an online resource opelbook.ru
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