Thread: 1980 300SD
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Old 01-12-2001, 07:36 PM
Posts: n/a
Here is how to test the relay when the bulb does not light:

In the event of a problem, the lamp does not light despite car starting.

Pull 6-point coupler from preglow relay, turn key to position "2", bridge jack 1 and 2 of coupler.

If preglow indicator lamp is not lighting up, check bulb or replace. If the bulb is okay, then you need to check the blue/white wire that goes from jack 1 to the bulb for an interruption.

If the bulb is lighting up when you bridge "1" and "2", then you need to test the green wire going from jack "8" of the 8-point coupler to the the temperature sensor for interruption.

If that wire is okay also. Then you need to test the temperature sensor.
The resistance of the sensor should be:

At 0 deg Celsius: 8200 ohms
At +25 deg Celsius: 2440 ohms
At +80 deg Celsius: 290 Ohms

If you do not get similar readings, replace the temp. sensor. If every test is okay, then you have a bad relay.

This second test is for a light not lighting and the car NOT starting:

To test for problem when the glow plug light does not come on, and the engine cannot be started:

Pull 6-point coupler from relay, turn key into position "2"
test voltage of jack 2 against ground with a voltmeter

If no voltage(aprox. 12 volts)check the red and black wire going from the fuse to jack 2 for interruption

If you have voltage, connect voltmeter to jack 2 and jack 4 and test for voltage.

If no voltage,(aprox.12 volts), test brown line from jack 4 to ground for interruption.

If voltage is indicated, relay is defective.

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