Glow plugs, dash light, relay?
 

I know, I know, another glow plug thread, but I'm doing my research on this using the search feature. Maybe I just haven't successfully found the thread about this.

Machine: 1985 300D Turbo, w123, 617, Bosch GPs

Problem: Car starts smoothly if I wait 8-15 seconds in pre-glow before starting. But the dash light does not come on. I know the GP's are working. But why doesn't the light come on? So I checked the following.

Continuity, voltage, resistance: The resistance on the plugs is under .7 ohms. The voltage on the fuse and in the relay sockets and pins checks out. The GP dash light bulb works when the relay plug is disconnected.

Cleaning/Reaming: Took a glow plug out today using some thin 8mm and 12mm wrenches. It had some carbon buildup on it that I cleaned off. I don't have a reamer, so I tried using different drill bits in there. Nothing really came out. I'm going to take all the plugs out and crank the engine and see if any carbon shoots out. But most likely, the dirt doesn't seem to be the culprit.

Checking each plug: Still need to get a jump box and run current through the plugs to make sure they get hot.

What a certified MB tech thinks: I talked to the mechanic who sold me this car, and who knows a hell of a lot about these machines. He said if the light on the dash goes out, but the car still starts smoothly, then I shouldn't waste money replacing the plugs. I can just count to 10 during pre-glow like I do now, and in the Chicago winter I'll have to count to 20. He thinks the problem is with the relay timing. Is that true? Can replacing the relay fix this problem? I've read on this forum that if the voltage checks out fine in the relay box, nothing is wrong with the relay. Has anyone had this problem and just left it alone?

What I haven't done:

• Hook up each GP to a jump box, see if it glows.
• Take each GP out and test resistance across it.
• Check the resistance on each GP's wire.

I didn't think I needed to do these things because http://dieselgiant.com/glowplugrepair.htm only checks those things out for plugs where there is a continuity problem. Should I do these checks anyway?

Try this, turn key to get GP's going, wait and listen, you'll hear a click, roughly 30 seconds, engage starter. If it starts, as you've described it does, there is a dash light problem somewhere. But it sounds like the relay etc. are working, except for the light, see above. Hope this helps.

I did this. I turned the key to pre-glow. Got out and put my ear up next to the relay box. I waited for 30 seconds and heard no clicking. Then I engaged the starter and it started smoothly. It is about 75 F in the garage, but I've tried starting without doing the pre-glow at all before, and it was much harder.

EDIT #1: Less than an hour later, I got someone else to turn the key for me while I listened at the hood. I only heard that first initial click when you turn the key to pre-glow setting.

So, no clicking, no dash light, but smooth starting.

P.S. So just because the dash light bulb turns on when the relay plug is off, doesn't mean that the bulb "works"?
So if the bulb is good but doesn't work when the relay plug is on, then the timer probably isn't working... ?

EDIT #2:
Also I wanted to mention.. If one GP is bad, that may not make it hard for the engine to start, but it may be enough to make the dash light not work. That's why I thought checking each plug with a jump box might be necessary.

better get that ***** repaired before your move to illini;P

wish i had some info. i just upgraded to a gp relay wiht afterglow, and i'm very happy with it. you should do the resistance check with each plug like dieselgiant shows. it's quite easy, and you'd be surprised with the results (maybe). i had a bad (intermittently operating relay, and 3 out of 5 plugs bad) news when i did that test........good luck.

be sure to post your results.

Mojool,

You said you had 3 bad GPs out of 5. Did you have reason to suspect that before removing the plugs to test the resistance? Before you tested the resistance on each plug taken out, did you test the resistance in the relay plug? And were they all under 1 ohm?

I'm curious about checking the actual GPs when the resistances are good within the relay plug itself.

Won't hurt. I had a bulb difficulty, checked ohms at relay, one was bad, replaced all, I'm funny that way, fine since.

Change the #1 glow plug. Put in a brand new one.

If that doesn't solve the problem, the relay is the culprit.

BTW, the SD has the same problem right now..........need to check the same things.

My 240D with turbo engine (and old loop style relay) has the same issue. All GPs work, but the light does not come on. I tried using a old relay I have and the light works fine, so there must be a internal problem with the relay. Unfortunately the old relay is sticky and needs a swift tap to turn on the plugs so I am just living with out the dash light.

:dizzy2:

Brian,
Why #1?
Did your SD just stop clicking during pre-glow?
Do you think that is controlled by GP #1?

Is it absolutely certain that the GPs are not bad if the resistances are low in the relay plug?

#1 has a direct effect on the light. If the resistance isn't proper in #1, the light wont illuminate. You previously measured resistance, but, it's possible that the measurement under load is not adequate.

Best as I can tell, the glow works fine on the SD. No four cylinder starts..........but, I have not checked anything yet.

That's interesting, my GP light also stopped coming on a few days ago, but it still seems to be starting on all five. Haven't had a chance to check the GPs or the bulb yet because I'm out of town this week (WA state).

We've got glow plug light disease..........apparently it's spreading.:eek:

LOL, as long s we find a cure before winter. :D

Now you've piqued my curiosity. I'm going downstairs and check it out. I'll advise later...........

Cool, let us know.