Repost from the Vintage Forum

Hello,

Having some trouble with the glow plug circuit and now I turn to the geniuses here for help.

Replaced the 5 GP's recently after sensing some hard starts and for good measure (old plugs, long sitting etc). Realized the Preglow relay was not operating correctly and replaced that with a new one from MBZ. (the GP light would not indicate at key position 2 but would light up while cranking. Was getting 12V at pin 3 and 5 of the relay connector.)

After replacing the 5 GP's, the insulator caps and the relay, I'm still having starting issues. The glow plug light in the cluster does not extinguish in a timely manner like it should. The squiggly metal conductor between GP 4 and 3 gets bright orange, glows like a SOB. (GP 5 is at the firewall for reference).
The voltage at the threaded tip of each GP is as follows, roughly:
#5 9.5V
#4 7.75V
#3 1.4V
#2 1.3V
#1 .2V

So you can see a significant voltage drop from GP 4 to 3. I don't have a meter that goes up to 60 amps and above so I can't supply current readings.
But, my thought is that there is either too much resistance or too much current between GP 4 and 3. If there was a short in the circuit, I would think there would be 0V at the GP's in line after the short, but there is still a bit of voltage getting through. But, I don't know why there would be excessive resistance in the conductor that would kill the voltage getting to GP3. It seems I reassembled the conductor/insulator sequence properly. I have the FSM and the way it goes back is somewhat obvious.

I don't see any obvious shorts or groundings of the circuit from the outside.
BTW, I reamed each hole with the aftermarket reamer, grease in the flutes. . .

Is there a possible defect, shorting inside the pre-chamber of #3?
Also I switched up the GPs of cylinder 3 and 4 with the newer and old GPs so I don't think that's an issue.

Thoughts suggestions greatly appreciated.

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63 220S W111
76 300D W115
2013 VW JSW TDI M6

previously-
73 280 SEL 4.5
86 300E 5 speed
2010 VW Jetta TDI M6

First off....on a car with loop plugs, you cannot check the voltage at each plug....you can only confirm that plug 5 is getting 12v....the wires between that are resistor wires....thus they drop the voltage...

You should have upgraded your plugs to the newer pencil style, they work 100% better and if one dies....you don't lose all the plugs...

Since you have reinstalled loop plugs, did you confirm the ground wire is connect to plug 1?

Have you also confirmed the temperature sensor for the glow plug relay is plugged in? and not lose, missing or unplugged? This is between the head and the injection pump...there are two sensors, one for the gauge and one for the glow plugs..

http://dieselgiant.com/mercedesdieselloopglowplugrepair.htm

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Actually, checking the voltages this way tells you quite a bit. There should be a consistent voltage drop from plug to plug. I don't have the manual for the old series system, but it looks like 1.5V is what you should expect. Here you see a big drop between 4&3, and no drop at all between 3&2. What I would suggest is to remove all of the bridges, set your meter to measure resistance, and measure each plug individually. They should all measure approximately the same. The one that's different is your problem. My bet is that 2 will need to go. Before you reassemble, clean the ends of the bridges with sandpaper. You really can't measure the current draw at each plug, even if you had a 60A meter.

The Voltage at the Wire coming off the Fire Wall is 9.5-10 Volts. As the Voltage goes through each Glow Plug there a Voltage drop of .75-1.25 Volts; meaing the Voltage drops every time it passes through each Plug.

What type of Glow Plug Relay was used? If a Glow Plug Relay from the Pencil Type Glow Plug was used I would not expect it to function properly.

If you used a Pencil type Glow Plu Relay you might as well have invested in the updated Pencile type Glow Plugs to go with it.

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84 300D, 82 Volvo 244Gl Diesel

Thanks for the replies. I will test the resistance at each plug and hope that solves it.

BTW, to properly test resistance I assume I would measure at the threaded tip and then the contact area for the bridge that carries current to the next GP? Can I leave them all in place for the test?

I have the correct relay 001 545 18 32 ordered from the Classic Center.
I don't think the thermo time switch is involved since the GPs at 1 and 2 aren't getting enough voltage to warm up the switch to tell it to extinguish the GP light. I swapped out gp 3 and 4 but didn't think to swap 2. Fingers crossed.

I don't mind the old loop plugs, just part of the cars' quirks. And I thought I read something about the pencil plugs drawing too much current for that system. Anyways, its a 115 what's the rush? Can't get anywhere fast anyways

__________________
63 220S W111
76 300D W115
2013 VW JSW TDI M6

previously-
73 280 SEL 4.5
86 300E 5 speed
2010 VW Jetta TDI M6

On the 1976 ones the udated Pencil Glow Plugs are not compatiable with the Glow Plug Relay used on that Year. Also I believe your system uses a 50 amp Fuse instead of the 80 amp one.

When the updated Pencil Glow Plugs came in a Kit the Kit had another Glow Plug Relay to replace the old one. However, you also had to do some wiring.

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84 300D, 82 Volvo 244Gl Diesel

After some further testing I'm pleased to report that the problem is solved. Tested each plug for resistance but nothing stood out. Tested all the old plugs and conductors for comparison as well, nothing.
Knowing something hadto be wrong I set the multimeter to beep mode, grounded one end on the block and tested each GP at the threaded tip and found a closed circuit on #2. The loop was slightly mishapen and was making contact in the combustion chamber. We're talking about a very small deformation.
I haven't put it all back together and started it (waiting for new plug now) but I'm 99.9% sure it is good to go. Will sand the contact surfaces clean as advised.

Thanks!!

__________________
63 220S W111
76 300D W115
2013 VW JSW TDI M6

previously-
73 280 SEL 4.5
86 300E 5 speed
2010 VW Jetta TDI M6