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Old 03-27-2006, 03:34 PM
ykobayashi's Avatar
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Join Date: Jun 2005
Location: Irvine, CA
Posts: 633
glow plug lamp light fix on 240D


Last night I had the displeasure of having my glow plug light not come on. The car (1982 240D) still started kinda okay but stumbled a bit to find a steady idle. The plugs had to be working I thought, so I checked the bulb in the dash. Good. But there was no signal coming to it when I turned the key. I got some small voltage fluctuation like 0.001V but nothing to light the bulb.

So I thought, listen to the relay. It clicked. I put the meter on each plug and saw 10.5V come on to each one when the relay clicked. Then being a hack electronic designer, I decided to open up the glow plug timer and take a look around. The indicator lamp was wired to a BC327 pnp transistor that looked like it was set up to light the lamp. Oh, I thought, dead silicon. But before I got out my solder iron, I checked the gurus at shop forum, yep somebody else had posted the same problem so it was probably that part. I mean, my plugs seemed to work, the wiring was okay, the relay clicked and I saw juice at the plugs but no light.

Then I saw a post about checking the resistance of the plugs - actually I saw many posts. Gee, is it this easy, you mean they are either 0.5 ohms or open? My meter should be able to see that. So I went out and sure as heck, 2 plugs are dead open!!!!!

I keep thinking there is some grease under the probe and I keep scraping the probe on the engine block but they are really open (1, 2 bad, 3, 4 good). I just swapped out the two with Bosch ones from Autozone ($9.99 Yeeehhhaww!!) and the light comes on!!!!

Hooooorrrrraaaaayyyyy!!!!! I was wondering how I'd wrangle a sample BC372 out of Onsemi or Fairchild. I thought over the design, and realized why that whole circuit is there. It only lights the light if there is sufficient current flowing in the plugs. If it doesn't light up, that's a warning that something is wrong. If they just wanted to design something that lit the light when the relay was on, you wouldn't need any electronics--just a wire.

Happy driving. Lesson is check the resistance of the plugs before the bulb, the wiring, the relay. My engine is so tight it only needed 2 plugs to light it off. My old 82 VW Rabbit diesel wouldn't start if one plug was out.

Oooohhhh...maybe that was because it was a series system? Cannot remember.

Good luck,

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Old 03-27-2006, 05:03 PM
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Join Date: Sep 2004
Location: cheeseland
Posts: 275
Interesting story. I have an '81 240D and my problem with the indicator lamp is that it will light up briefly and then go off, it does not stay on for the normal amount of time. I just count to 10 and start it up. Always fires right up and runs good, so I assume that the plugs are fine.

Sometimes when I'm just driving along the light and buzzer or just the light will come on for 5 - 10 seconds and then go off. I know the PO toasted the relay, replaced it and at the same time put in all new plugs. I bought the car immediately after that. I suspect he toasted a few other things but I have not wanted to tackle it as I have had bigger fish to fry with this car and all seems to function OK. Well, except for the lighting/buzzing issue

'85 300D - 206K (sold)
'81 240D - 149K (sold)
'03 Jetta TDI - (sold)
'79 300D - (sold)
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