I'm having some trouble with my W115 glow plug indicator - the "salt shaker" in the dash. The filament gets red/orange very quickly within just a few seconds (3) of applying the glow with the pull starter (gorilla knob). If I am not careful, it will blow the filament within 6 or 7 seconds.

The glow plugs themselves do not heat enough to start the engine and the bendy resistor wire between plugs 1 - 2 and 3 - 4 is only just a tiny bit warm to the touch.

To me this indicates a short somewhere. (Or a bad or wrong filament - ?)

However, if I remove the ground strap (which goes from the last glow plug to the engine block, made of braided copper) - then the filament doesn't light at all. So, it seems no short, because the only way the circuit is complete is if the ground strap is in place. Is my thinking correct there? If there was a short, with the ground strap disconnected, and current applied, the filament would glow, because the short would "complete the circuit.". But it remains dark.

I have tested each glow plug, removed them all many times, used the multimeter, I can detect no fault or short. None of the glow plugs show connectivity to the engine block, except when the ground strap is in place. I've swapped them with a new set (and a working set from my W110). I've cleaned the carbon out of each glow plug hole, they are pretty clean.

If I disconnect the regular wire to the first glow plug, and I use a separate wire directly from this glow plug to the positive terminal of the battery, all the glow plugs work, the heat quickly (and as far as I can tell, evenly). The two bendy resistor wires get very hot to the touch. Motor can start from cold (14 C day) after just 4 or 5 seconds of glow this way.

I have the wiring diagram, and the system seems quite simple - power is supplied from the battery when the key is in the run position to the gorilla knob, the gorilla knob in the glow position provides power, through the indicator filament, directly to the first glow plug, through all the plugs to the ground strap and ground to the engine block. As far as I can tell that system is in place, wires are in good shape, and no shorts.

Anyone have any ideas or hints on what the problem could be? Advice would be greatly appreciated and I'm stumped here for more than a week (and blown 2 filaments).

PS. this is the older "loop" style glow plugs, in series with each other.