Well, the problem was the glow plugs! I had them replaced less than a year ago, but glow plug #3 was completely shorted out - showed continuity between the outer casing and the leads. No wonder the relay was tripping, performing its function correctly. Either my 2 full minutes of manual engagement or a slow death seems to have overheated one of the resistor wires (plugs 4 -> 5) and it oxidized it heavily. The excess current also burned out glow plug #5, which is still intact but colored white and showing 0.65 ohms (the good ones show 0.22 ohms). I put the shorted plug in hole #1 and the nearly-burned out plug in #2 and the car fired right up. I suppose 3.5 working glow plugs is sufficient.

So much for Bosch Made in India glow plugs. Will order the Germany ones now.

Also, after tinking under the dash, I seemed to have messed up the gauges. The fuel gauge, water temp gauge, and brake light are now in-op. The oil pressure and battery lights still perform correctly. I see two possibilities on this issue:
1. I pulled out a wire or shorted something while handling the relay.
2. The high current of the shorted glow plugs possibly melted through and shorted out the gauge wires when I manually connected the relay yesterday for 2 minutes.

This issue is reason to make a strong case to not convert to a simple manual switch for the relay. Or if you do do it manually, make sure to use a fuse. I don't know if Kent Bergmas's conversion kit has a fuse on it, but the original system performs just fine if the parts or good, and it's not overly complicated. Any manual non-fused conversion kit is simply dangerous. If plug #5 had shorted, rather than plug #3, it would have been a direct route from the relay to ground through the wiring harness, and this wire would have certainly melted apart and taken other things with it. The W115 was engineered well; this case gives me further reason to question any re-engineering of original functions.

If anyone has tips on getting the gauges working again, please chime in!

Take care,
Henry C.

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1976 Mercedes-Benz 300D, Colorado Beige / Mahogany
www.hmclever.com