Yesterday the glow plug light on the 2.5 Turbo started coming on after the car had started. My wife let the glow plugs warm up, as usual, (the glow plug light was out) and after it started, the glow plug light came on and remained on on for a couple of minutes. It has done this on every start since. The car continues to start with no problems at all, just that the light will not come on during the warm up period and then comes on after the engine starts. Does this sound like a glow plug gone bad or is it something to do with the glow plug relay?

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Bob Morrison
92 300D 2.5 -197000 Miles
87 300D-170000 Miles The Californian
87 Volvo240DL-200000+MilesGrandson's Now

Disconnect and check the resistance of each glow plug. Sometimes one or more open elements will confuse the control unit... it happened to my 190D. I think they should each read about 0.7-0.8 ohms. A bad plug will read as "open" of course.

Clean and reseat the connectors on the control unit while you're in there.

This is a diagnostic mode. The car is telling you it has a burned out glow plug. The glow plug relay measures the resistance of the plugs - it is finding something it doesn't like - probably infinite resistance in one plug.

The car will still start and run just fine with one plug out. If you pay attention, you'll probably notice following the first cold start of the day the car runs a bit rough and smokes a bit much for the first few seconds. It's pretty much running on four cylinders until the fifth heats up enough the reliably ignite the fuel.

If you have all original glow plugs, your best option is probably to replace them all. They're not expensive - about $15/each from mail order vendors, nor are they difficult to replace. I'd also recommend replacing the 80 amp strip fuse inside the glow plug relay. They too occasionally fail due to age. It's available at your local MB dealer for a buck or so.

You'll be good for another ten years after this service.

- Jim

Thanks Guys, I'll order the glow plugs from Rusty tomorrow.

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Bob Morrison
92 300D 2.5 -197000 Miles
87 300D-170000 Miles The Californian
87 Volvo240DL-200000+MilesGrandson's Now

Morrib,

Here's another theory:

If one light bulb burned out in you house, would you replace all the light bulbs?

Same goes for glow plugs (GP). Just because one goes bad is no indication that the others will. I would suggest buying 2 and save $45. One to replace the bad one and a spare to put in glove compartment. Remember, GP are not show stoppers. When one goes bad, the only result is miss firing on one cylinder for a short time after starting. Once the engine is runnng, the GP are essentially inert.

If it ain't broke, don't fix it. If it still works, don't replace it.

P E H

I used to agree with PEH on the "if it ain't broke" when it came to glow. Now if one goes I will replace all or at least have all I need to replace in a pinch. My #1 went bad last year in the fall. Had no GP light period but the car started ok. Replaced that one plug and all was well, or so I thought. Took the car to the middle of nowhere in Feb of this year, and I mean nowhere. No cell service, no major roads, no houses, no nothing for a good 30 miles and had two little kids (ages 8 and 10) with me. Hiked back to the car on a very cold north FL morning with temps in the low 20's. Car flat out would not start. Would "almost" fire but not enough to get it going. Had to open the hood and push it out from under the trees to let the sun heat the engine as much as possible. (don't laugh I figured heat is heat) and let it sit for about 3 hours. Temps did go up to around 40 and the engine did get a little warm to touch and it DID start but not like it should have. So when I got home I ordered 5 new plugs and replaced all of them. No problems at all so far even in temps below 15f in Ky. a couple of weeks ago. Car is a FL car and has no idea what a block heater is so when I travel north the glow system HAS to work.