The other measurement you can make is w/ red probe on glow plug and blk probe on ground, w/ plug in and "glow on", see if you get ~12 V across each glow plug. You only have ~20 sec each time. That is the best way to judge that your car is doing its job supplying them.

The prior test was of each glow plug itself. Putting 12 V on a failed glow plug doesn't help.

The very best test is with the injectors (and heat shields) out, to view each glow plug tip and see it glowing orange. Do that whenever you remove the injectors, but not just to test glow plugs. People have found some to glow in the middle, instead of the tip, which you wouldn't know otherwise.

Re ground, I usually use the fuel filter mount since close, but last time I got less resistance using the power steering pump. That matters more for the ohms test, not the volts test. Funola is correct that the head is theoretically ideal, but might be harder to get good electrical contact. Could even touch the threads of the glowplug if you can get 2 hands in there. I don't fuss about the readings. A bad plug is infinity. Found one last time I did it a week ago, and guess what - a Bosch again.

Re the GP relay box, I was going to swap the one out in my 1985 today, but I couldn't find the spare box I had set aside. It is getting tricky to get the GP lamp, and usually (always?) when I don't get the lamp it truly is hard to start. It could be the key switch is going since seems I must get it in just the right spot to get the lamp, but swapping the GP box is much easier than the key switch (3 dang screws are hard to get at).

I'll check that when I get home from work this afternoon.

Good info, thanks. I've been using the fuel filler mount for ground as well.

- Peter.

__________________
2021 Chevrolet Spark
Formerly...
2000 GMC Sonoma
1981 240D 4spd stick. 347000 miles. Deceased Feb 14 2021
2002 Kia Rio. Worst crap on four wheels
1981 240D 4spd stick. 389000 miles.
1984 123 200
1979 116 280S
1972 Cadillac Sedan DeVille
1971 108 280S

Over about a year I tried to sort out similar intermittent issues on the glow relay of my 85 and in the end, determined it was a flaky glow relay, gave up and converted it to manual and couldn't be happier. Should have done that from the start and not waste the time spent. Internal schematic of the glow plug relay is not available and I was not able to trouble shoot and repair it (note: I have a degree in EE and electronics was a hobby).

__________________
85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked