Recently we moved into the mountain area just outside Yosemite. In the past week or two, the temperature slowly worked its way into the mid 20's. Anything lower than about 45, made it near impossible to start our 83 300SD, which in 50 degree plus weather started immediately. In researching the glow plug system, I found out that the relay has a sensor that determines the air temperature and controls the time that the glow plugs have juice sent to them. In the past I would go through 2 or 3 complete cycles to try and fire it up. In checking my plugs I found some out of specs and some dead.

I changed them with Bosch units and now no matter what the temperature is, (been as low as 24) as soon as the light on the dash goes out, I turn the ignition key and within 2 seconds it is purring. I didn’t ream or do anything special….just replaced the 5 glow plugs.

Don

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1980 300TD-T (82 Turbo and Trans) 159,000 Miles "Jackie-O"
1983 300SD 272,000 Miles "Aristotle"
1987 Jeep Wagoneer Limited - keeps the MB's off the ice and out of the snow
1994 BMW 530it

exactly what I did. Many people here always troubleshoot all the different components that deal with glow plugs but the first thing that I would do is to check the glow plugs. Heck, I would not even check them, new ones are around 10 bucks so screw messing around with anything. Just buy new ones and you are set for a while.

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Audi TT

It's worth replacing all five or six. It doesn't cost much and then you know where you stand. I just did all five of mine. Starts A WHOLE LOT better in the cold weather!

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1982 300SD Light Blue
2002 Honda Accord SE
1974 Toro Wheel Horse Tractor
2000 Toyota Tundra Pickup

BigRed,

The GP relay does time the GP, but not as you said.

The GP are timed to stay on about 45 seconds all the time on a 300SD unless interrrupted by turning the key switch to the start position. The timing of the GP dash light is proportional to the temperature, but when it goes out, the GP are still energized ~45 seconds .

So its best just to wait longer before trying to start the engine than to start the GP cycle again and again.

If engine doesn't start, its also better to crank for short periods of time and then start the GP/start cycle again since the GP are turned off and start to cool as soon as the starter is energized .

P E H

To Quote the shop manual....Section 15-705 "Operation of Preglow System" Will support what I said.

"The duration of preglowing depends on tempurature of engine and on ambient tempurature."

Don

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1980 300TD-T (82 Turbo and Trans) 159,000 Miles "Jackie-O"
1983 300SD 272,000 Miles "Aristotle"
1987 Jeep Wagoneer Limited - keeps the MB's off the ice and out of the snow
1994 BMW 530it

I believe that the shop manual is referring to the duration of the preglow light. At warmer ambients and warmer engine temperatures the preglow light will remain on for a shorter duration. When the light goes out, M/B believes that it is time for you to start the engine.

However, M/B has never tried to start an engine with 150K on the clock at zero degrees ambient.

So, the advice given by PEH is correct. The glow plugs will stay on for about 45 seconds based upon a timer. You may take advantage of this extended glow time, after the light goes out, if you wish to have easier starting in very cold ambients.