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.

__________________
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

Your problem is pretty obvious, low voltage supply.
ALL GP's should read 12v or higher.

Id pull the connector to the GP's and with the key on, check V at the GP connectors in the relay box. If you dont have 12v there, head back. Make sure your fuse link is actually in one piece and doesnt have a split in it.

If you do have 12v at the connector, Id say you have one or more GP wires from the relay to the physical GP's that is making contact with something other than the plug in the motor.

__________________
1985 300D 198K sold
1982 300D 202K
1989 300E 125K
1992 940T

"If you dont have time to do it safely, you dont have time to do it"

"The democracy will cease to exist when you take away from those who are willing to work and give to those who would not."

That's actually not how the system works. The incoming voltage is 9 to 10 V at plug 5 and then there's a predictable "voltage drop" at each glow plug as it progresses.

Here's the advice from Diesel Forum in case anyone else has this problem

1976 300D Glow plug circuit issues

__________________
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

My input is based on later yrs 300D's

__________________
1985 300D 198K sold
1982 300D 202K
1989 300E 125K
1992 940T

"If you dont have time to do it safely, you dont have time to do it"

"The democracy will cease to exist when you take away from those who are willing to work and give to those who would not."

Ok, the later years were wired I'm parallel, right?
The early ones in series...

__________________
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

Yes, later yrs had 12v on each GP.

__________________
1985 300D 198K sold
1982 300D 202K
1989 300E 125K
1992 940T

"If you dont have time to do it safely, you dont have time to do it"

"The democracy will cease to exist when you take away from those who are willing to work and give to those who would not."