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The preglow light almost always turns off too soon, especially when the engine is cold from sitting overnight. With a cold engine I always glow at least 10 seconds regardless of what the light does.
The duration of the preglow light is set internally by a time-delay circuit. This circuit has no idea whether any or all of the glow plugs are good or bad. Later models actually look at the engine temperature but I believe your model looks only at the ambient temperature (temperature in the preglow relay).
A second time-delay circuit turns off the glow plugs after about 30 seconds. If it isn't too noisy where you live, you should be able to hear the preglow relay click off. There's no point in waiting any longer as the plugs will then begin to cool. If the engine runs rough then it is likely that one or more plugs are bad.
Jeremy
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"Buster" in the '95
Our all-Diesel family
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"Oh lord won't you buy me a Mercedes Benz."
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