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Old 07-02-2012, 03:21 PM
SoCal Diesel's Avatar
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Glow Plug Time on an '87

How long should the glow plug light stay on if the engine is cold? I am in Southern CA so that is around 70 degrees f. Mine stays on for only 2 or 3 seconds.
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Old 07-02-2012, 03:33 PM
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The light is a purely synthetic indication. When the light goes out, does your engine start?
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Old 07-02-2012, 04:20 PM
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Location: Mount Holly, NC
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it all depends on the year of the car, and the outside temp.
as long as the light comes on, the GP's should be working. you need to wait as long as the car needs to get sufficient glow. up to around 30 seconds on a really weak motor at 70F...
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Old 07-02-2012, 06:03 PM
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The light goes off in response to a timer but the glow plugs stay on; in the '87 there is a timer circuit and a temperature sensor in the preglow relay. As the sensor warms up the light stays on for less time. Two or three seconds at 70F is normal (but see the next paragraph). Later engines have a temperature sensor in the engine itself. All pre-glow relays keep the glow plugs on for approximately 30 seconds or until you start the engine; later engines have an afterglow circuit that keeps the plugs on even after the engine starts. The length of time depends on the model-year; Mercedes made a number of changes. Your car has no afterglow.

In theory the engine should start once the light goes out; however, the OM603 engine seems to like a minimum of 10-15 seconds preglow regardless of what the light says. The '87 also has a circuit in the airbag electronics that lights the "SRS" light for ten seconds (later cars for only four seconds). I used the SRS light to tell me when the ten seconds was up and I then started my '87. I also added a home-made afterglow circuit and the engine really liked that.


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