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Old 03-20-2000, 07:05 PM
Posts: n/a
In my '83 300D,(257k miles) which has been running flawlessly up until yesterday morning, it seems my glow plugs are not glowing enough. I've measured the resistance between the glow plug and the block (with the electrical lead attached) and each plug is about 0.5 ohms. The glow plug indicator light on the dash stopped coming on when this problem arose but I can hear the relay clicking behind the driver's side headlight.

I measured voltage and found about 10V at each glow plug for about 25 seconds or so with a cold, but not cold soaked (overnight) engine. I seriously doubt all of my glow plugs failed simultaneously. In the morning, I can get the engine to start by cranking the hell out of it.

The engine starts normally (<1sec cranking) if not more than one hour has elapsed from the previous run and I wait for what would normally be the preglow time with the key in the preglow position. Ambient temperature about 35degrees F.

Then engine starts with more difficulty (2-3 seconds of cranking) than usual after about 3 hours have elapsed. Ambient temperature about 35degrees F.

The engine is difficult to start after sitting overnight in 25degree weather (30 seconds of continuous cranking), and emits a large cloud of dense grey smoke as it begins to catch.

The fuse under the cover of the glow plug relay is okay. When I first checked it, one of the screws holding it in place was almost completely loose. The connectors in this area were all clean and free of corrosion.

I'm starting to think that my glow plug timer relay is limiting current to the plugs, thereby making the car difficult to start when cold. Remember, the indicator light in the dashboard has stopped coming on when this problem arose. Is this thing rebuildable or otherwise serviceable? Do they normally fail without warning as this one seems to have done? All help is greatly appreciated.

Thanks in advance.

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Old 03-20-2000, 09:40 PM
Registered User
Join Date: Mar 1999
Location: Suwanee, GA, USA
Posts: 4,712
Take the top off of the glow plug relay and look for a fuse. Push on it and see if there is a crack in it. It is 80A I think.

1981 280GE SWB
1987 16V

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Old 03-20-2000, 10:31 PM
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
The real test on this system is to monitor the current flow during glowing with an inductive pickup. Resistance reading below one ohm are tricky. By measuring the current you are judging the dynamic system working through the real resistance with the effective voltage (the other side of the story).

That system starts at over 100amps instantly. The current drops as the plugs heat up to around 70amps. These plugs were called quick glow when they came out because they were designed to have very temp dependent resistance. Thus the high initial current and the quick reduction to not burn them out. The circuit should stay lit for 45 secs if no starting attempt is made.

If the current is down or if you can't measure this, measure the voltage at the fuse. The battery voltage shouldn't drop much lower than 11.5v during glowing.

I am actually guessing there, as I imagine your temp conditions are causing your battery to be lower than I'm used to seeing. I always wondered how you Yankees got them diesels going (bg), we have enough trouble in N. Florida. (three days a year). You have to have a hot battery to start a cold diesel.

Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician

[This message has been edited by stevebfl (edited 03-20-2000).]
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Old 03-20-2000, 10:55 PM
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Join Date: Jun 1999
Location: Los Angeles, Calif, USA
Posts: 521
The glow plug relay is more or less a diagnostic tool. In your car (81?? or newer)when the 80A fuse is bad, it turns the indicator off. Also, if there is one (or more) glow plug drawing different amount of electrical current than the #1 glow plug (I think the #1 plus is used as reference), the electronics in the relay box turns off the indicator light.

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Old 03-20-2000, 10:56 PM
Posts: n/a

I wish I had the means with which to measure the current to each plug. I have been doing a little research on this system, I purchased the CD's from the Partsshop a while back.

Anyway, what I have learned is that the glow system uses glow plug number 1 as a current reference for the other four plugs. If the other four plugs are out of whack in comparison to number one, the comparison within the plug relay fails and the other plugs don't get the juice (I think) and the glow light on the dashboard fails also.

The corollary that I am inferring is that plug number one always gets the juice since its on the other side of the current comparison. I think I have observed this because as I begin to crank the engine when cold, one cylinder always fires, long before the others begin to get involved. I'm guessing this is cylinder number one, though I haven't confirmed this. I guess what I'm getting at here is that I think something within the glow relay circuitry has failed and doesn't allow current to flow to the other four glow plugs. Is this possible? I've seen a reed switch referred to in the repair literature.

What if two of the four "current-tested" plugs have failed? Will this be enough to upset the comparator and shut down all but number one?

Should I buy five new glow plugs or just go for the relay?

I think the battery is in good shape as I measured about 12.5VDC at the fuse strip on the relay. Yes, we do have to have hot batteries and decent compression to get these buggers going. And, as luck would have it, it has only snowed what seems like three times this season and tonight is one of them (our weather out here has been really messed up for the last two years or so) and messing with this thing in the cold and snow is no fun. I guess I shouldn't be griping too much because this car's been starting (reluctantly) without being plugged in and with what seems like one good glow plug (and a new starter if this keeps up for much longer).

Thanks for all replies.


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Old 03-21-2000, 06:31 AM
Posts: n/a
Excuse me...but if you measured each glow plug and got 0.5 ohms and they're all connected together, dosent that mean that you got a parallel reading?

If one of the glow plugs has shorted or gone high then that would affect the circuit resistance and therefore by Ohms Law the current to the plugs ...........HUH?

But if your engine is getting old ,low compression can hurt cool or cold starts over time.
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Old 03-21-2000, 07:27 AM
Posts: n/a

I have been through this firedrill more than once. All electrical measurements are okay, but if you will pull a few glow plugs, I'll bet you find at least one with a hole burned in the outer shell, exposing the heating element inside.

I know this doesn't seem to make sense, because it would seem that only one would cause problems in one cylinder. But my experience has been, replace 'em all and the problem is fixed. Let us know if that corrects your problem.


You are correct about the parrallel circuit, however, if you unplug the connector inside the top of the glow plug relay, the glow plugs are isolated and can be checked individually with an ohmmeter. As a matter of fact, you can check them all from the plug you disconnect at the relay much easier than probing underneath the fuel injection lines.

Good Luck,

Larry Bible
'84 Euro 240D, 516K miles
'88 300E 5 Speed
Over 800,000 miles in
Mercedes automobiles
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Old 03-23-2000, 11:08 PM
Posts: n/a
Benzmac, Steve, David and Larry,

Thanks for all of your very helpful suggestions. You guys were right on the money. Today, I replaced all of the glow plugs (#3 was a real pain) and the thing runs like a champ. Again, my thanks.

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Old 11-25-2000, 04:30 PM
Registered User
Join Date: Oct 2000
Location: Lansing, Michigan, USA
Posts: 49
Sorry to ressurect this old post, but the sypmtoms that JDEN had were exactly what I am having, except, I just replaced all the glow plugs and nothing. It is only around 34 degrees here, melting as we speak, but still no life whatsoever. Doesn't even sound like it wants to start.

Any other ideas? I have an 81 300D. fuel filter is full of fuel, the old glow plugs had fuel and oil on them (except the second from the back (#4?) which was completely dry, and my battery is 3 days old.

Any ideas would be great - I am out of where to look next.

Thanks, Josh.

81 300D 250K+ (odo doesn't work!)
Want another one.
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