Update. I went in and changed the 47 ohm resistor, which read 50 ohms... to a 68 ohm. This is a good match for the "dim" mode of the glow plug lamp.

Reason I went into the glow relay was checking the whole glow circuit out... was getting rough starts, and no glow indicator when key was in glow position.

The relay checked out fine, 80AMP fuse fine..

Glow plug #6 was found to be bad. Failed open (no continuity). When this particular plug goes bad, the relay senses it and shows the fault as "no glow lamp". It's designed to do this. But with this resistor mod, there's actually a "dim" glow lamp which says power is being applied to all plugs. There's just no "full bright" period. Only dim.

Found it is possible to change glow plug #6 on om603 with minimal yanking of things off the engine to get at it. I took off the engine lift mounted at the rear top of the engine, it straddles the air intake manifold on hole 6. With that out of the way, a flex head ratchet and deep 10mm takes off the glow plug wire, then a deep 12mm takes the plug out. Can just move the ratchet about 20 degrees, 1 or 2 clicks at a time, slow going...

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Cheers!
Scott McPhee

1987 300D