Do you have 12V on both sides of the contacts on the large relay?
If you do then the problem is probably between the relay & the GP's.

__________________
Grumpy Old Diesel Owners Club group

I no longer question authority, I annoy authority. More effect, less effort....

1967 230-6 auto parts car. rust bucket.
1980 300D now parts car 800k miles
1984 300D 500k miles
1987 250td 160k miles English import
2001 jeep turbo diesel 130k miles
1998 jeep tdi ~ followed me home. Needs a turbo.
1968 Ford F750 truck. 6-354 diesel conversion.
Other toys ~J.D.,Cat & GM ~ mainly earth moving

I read what I think is the high amp supply on both sides and it was indeed 12v. Thick red wire goes to the relay and purple wire with white stripe goes to the plugs. I checked the supply wire that goes to the plugs and it has perfect continuity. Testing that wire against the battery ground when the key is in position 3 and the relay is closed shows no voltage though.
I have no clue where to go from here.

Phil Forrest

__________________
1972 220D "Trudy," named by a friend.

"The 220D sounds good... I suspect it is the only car that you need a calendar for, rather than a stopwatch, when doing acceleration tests."
Tom Abrahamsson

It sounds like a connection problem. As soon as you put a decent load on, it breaks down. Check all the connections involving the thick wires. There is a bad connection some place along those wires.

__________________
Grumpy Old Diesel Owners Club group

I no longer question authority, I annoy authority. More effect, less effort....

1967 230-6 auto parts car. rust bucket.
1980 300D now parts car 800k miles
1984 300D 500k miles
1987 250td 160k miles English import
2001 jeep turbo diesel 130k miles
1998 jeep tdi ~ followed me home. Needs a turbo.
1968 Ford F750 truck. 6-354 diesel conversion.
Other toys ~J.D.,Cat & GM ~ mainly earth moving

Lst night I read through the FSM on wiring and testing of the glow plug system, went to sleep, abandoned my frustrations and woke up with a good feeling about today's hunt for the electrical gremlin.

I just tested all the poles on the relay again, this time against the battery positive and another test againt the ground. Switch on, switch off and a test independent of the battery.

Supply from the positive pole of the battery to the high-amp pole on the relay is good with no resistance so there's no break in the line there.

Testing the battery ground against the ground on the relay (supply to GPs) showed ~4.5 Ohms which accounts for the glow plugs (car is dead cold right now) and lets me know that all my plugs are good and the wiring is right.

So I found that on the ground side of the relay, which provides current to the glow plugs, there is zero resistance across the relay gap. While the gap does close and the electromagnet works, as well as the release after the starter stops, there is a short somewhere in the relay I'm almost certain. At least, this is what I'm thinking. When the relay closes the circuit, I think I've been measuring low-amp signal which is 12v instead of the high-amp supply. The fact that there is zero resistance across the relay gap even when the car is off tells me that there has to be a short in that relay somewhere.

The supply from the battery is sound. The ground from the battery is sound. This means there are no breaks in any connections up to the relay so the relay is the weak link.

Am I on the right track with my testing and is there anything else I'm missing?

Thanks all,
Phil Forrest

__________________
1972 220D "Trudy," named by a friend.

"The 220D sounds good... I suspect it is the only car that you need a calendar for, rather than a stopwatch, when doing acceleration tests."
Tom Abrahamsson

I tested the high amp supply to the glow plugs with a jumper (as if the relay were closed) and the car started right up so I installed a backup glow plug switch using a pull-starter knob. The supply for the new switch is just a few inches upstream from the relay plug so it doesn't get in the way of installing a new relay when I find one.

Once I got the new switch wired up and installed in the dash, I tested it out for voltage then heated up the glow plugs and my car came to life immediately. The only thing I'm missing is the pre-glow indicator lamp so I'm going to have to be on-point about remembering to kick the pull starter switch off once the motor starts.

I'm really glad to have this problem figured out for the time being so I can reliably start the car in the morning.

Now on to getting the suspension done!

Thanks to all for the help.
Phil Forrest

__________________
1972 220D "Trudy," named by a friend.

"The 220D sounds good... I suspect it is the only car that you need a calendar for, rather than a stopwatch, when doing acceleration tests."
Tom Abrahamsson