Recently bought an '87 300D, and suspect a faulty glow plug--or maybe two. Between one and six key turns are required to light up the glow plug lamp. All plugs test good with an ohmmeter, and all pins at the relay have 12 volts. Is the next step a current measurement for each plug? Existing plugs have about 90,000 miles and 9 years on them. The holes were reamed when last replaced. The relay was replaced about a year before that, and there appear to have been problems with the GP light from then. I could also just replace them all and see what happens--maybe it's time?

Rob

hows the fuse?

http://www.dieselgiant.com/glowplugrepair.htm
http://www.dieselgiant.com/mercedesdieselloopglowplugrepair.htm
Second link is for hoop style GP... not on your car but while your discovering the wonders of GP ...
Have you tested the replay timer?
If the relay tests OK and the GP's test ok then you have the blub and ign. switch left to test.
I have not 'hot wired' (actualy a jumper) the GPs but I'm sure someone on this site has so you can verify everything works beside the ign. switch.
You may have a bad ign. switch.
There is a past post with details of bypassing the ign. switch.

Thanks for the ideas. I suspected glow plugs because of the rough starts, with clouds of smoke, when cold--coupled with the pesky GP light. Could the GPs be so sooted up that some have become ineffective? Maybe I've got two issues here: the relay and/or ignition switch, and the plugs themselves. There's a note with one of the maintenance actions that the light could be made to illuminate by tapping the relay, although I've not confirmed that myself. Still doesn't explain the hard starts, though.

Oh, yes. The test consisted merely of checking the resistance for each plug at the relay. Each plug tested the same-- .4 ohms, if my DVM is to be trusted. New plugs measure about the same, though, so the values must be about right. Measured voltage at each pin of the relay socket, and all measured 12 volts. And the fuse is good. My next step was going to be measuring the current to each plug by inserting an ammeter with appropriate shunt between the relay and the plug--one plug at a time. I still have to put together the shunt, and my resistors may not survive the load for long...

Something doesn't sound right. All symptoms point to a bad glow plug.......or more than one........

You've got plugs that have 90K on them and they all test lower than a brand new factory plug (typically .6-.8 ohms).

Your test with an ammeter might show some different results than one would expect with those resistance readings. The initial inrush current will be about 30 amps per plug for a brief moment if you actually have .4 ohms. But, I'll wager that one, or more than one of them is nowhere near this value.

Welcome to the forum.

Please provide details of how you performed this test and the results of this test for each glow plug.

The glow plugs should have been changed 60000 miles ago.

BenzDiesel

BenzDiesel,

Are U saying GP should be changed every 30,000 miles? Thats ridiculous.

Do U change your rlight bulbs before they burn out?

P E H

300TDT,

The GP light doesn't tell if the GP are working or not. Its mainly a timer. But if it blinks after starting, one or more of the GP are open (burned out).

If the GP test OK with the resistance test, they are probably OK. Did U check the fuse in the GP relay?

Do a search for cabin light and U will find out how to do a quick GP test.

P E H

That's not true on the (pencil type) 617, the GP light will fail to light if two or more GPs are open. On the 616, the GP light will fail to light if one or more GPs are open. I've never had a 616 or 617 GP light blink to indicate a bad GP.

I agree that the GP resistance test will normally tell you if they are functioning correctly (but there are exceptions).

Craig,

When I get the 617 back (wife has it) that the GP light always works I will do some experiments by disconnecting some of the GP and checking what the GP light does.

I have had the GP light indicate an open GP by blinking. That's what it is supposed to do so U know its time to replace the open GP.

P E H

I second the suggestion about the fuse. My car is pretty much like yours in terms of the GP light, 300TDT. If the light comes on it starts up fine as if all plugs are working. If the light doesn't come on, it's a hard start as if none of them are working. Usually the light will come on if I wait a minute and try again, but yesterday I was having no luck with that and used another vehicle.

I tested the system later in the day yesterday (what do you know, it was working again when I tested it) and everything tests out good. Plugs at 8 ohms, voltage to the relay 12.6 volts, just like the battery, and all the pins giving more than 12 volts to each plug.

At this point I suspect the fuse has some tiny cracks that are, at times, preventing the electricity from getting to the plugs. I hope I'm right as that's the easiest fix.

As a side note, my car may know things that I don't know, because on my way home yesterday I was rear-ended by a Ford Expedition. The damage to the pickup I was driving seems to be very minor. If I was in the lower-sitting Mercedes, though, I might be on the phone yelling at insurance people right now. There was no damage to me, either, which is good.

Edit: By the way, 300TDT, if you don't know all the ins and outs of testing the GP system, check out diesel giant's tutorial on the subject: http://www.dieselgiant.com/glowplugrepair.htm

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Ralph

1985 300D Turbo, CA model
248,650 miles and counting...

Rob,
Did you ever resolve the intermittent light issue? I've got the exact same problem. It used to be the third turn nearly every time, now it's closer to six. I'm going to check the plugs tomorrow night if I can, but I'm not convinced that they're related to the flaky light.

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1987 Mercedes 300D