On my 91 300D, I knew I have one bad glow plug among 2-5 (because it had rough start symptoms but there was no fault revealed through the indicator...the FSM mentions this). So I measured the resistance of each plug. Plugs 1-4 were all about 0.5 ohm. But plug 5 is 9 ohms. So clearly that's my bad plug. What puzzles me is that I would have thought that a bad plug would have a much higher resistance value...like 50 ohms.

Can anyone help me better understand different resistance readings on bad plugs?

Thanks.

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 156k miles
06 E320 CDI "Rutherford", Black on Tan, 173k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 143k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 67k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

When I checked the plugs on my 603 a few weeks ago they were all .9ohm. I checked the resistance at the relay plug so you have to add the wire from the relay to the GP.

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CENSORED due to not family friendly words

In order for the heating element gets hot it has to have resistance to the current flow. And inside of the glow plug there is in fact a coil of wire surround by some sort of white powered.

The glow plugs work not unlike a old style light bulb. The Filiment in the light bulb is a heat resistant wire. If you notice the diameter of the wire is small and when the electrical current goes through the thin wire the resistance to the current flow causes the wire to get hot and in tha case of a light bulb it gives off liight also.

When I tested a good plug with an amp meter the resistance pegged the meter out at 60 amps (the meter only went up to 60 amps) and it starts to glow then as the element go fully hot the amperage needed to keep it hot drops to about 16 amps

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84 300D, 82 Volvo 244Gl Diesel

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84 300D, 82 Volvo 244Gl Diesel