Strange Connections Killed the Mercedes-Benz

I must have been in shock after the Loma Prieta earthquake of '89, because my wife and I went out and bought a (very) secondhand 1973 Mercedes-Benz 280 sedan. It was an enjoyable car to drive, especially on long trips, until it died for no apparent reason at a stoplight when coming to idle.

I got it re-started, using plenty of throttle, and I found I could keep the engine running by never letting it drop to idle. We got home okay, and sure enough, the engine died when we came to a stop in the driveway. While my wife went inside to get out of the cold (it was nearly winter by then), I stayed outside to troubleshoot the problem.

Quickly, I found a blown fuse that apparently powered the solenoid that fed fuel to the idle circuit. I replaced the fuse and the Merc ran fine. I drove it around the block without incident. When I returned, my wife emerged from the house to announce that we needed to run an errand. She settles into the seat and we take off, only to have the engine die at the stop sign at the end of the street.

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Back into the driveway, new fuse, everything fine again. I set a voltmeter on the cowl where I could see it through the windshield to monitor the voltage on the again-replaced fuse, start it up and proceed to wiggle wiring under the dash and under the hood in an effort to flush out an intermittent short.

No joy. Idles like a champ. I stare dejectedly at the meter, open the passenger door, and plop myself down in the right seat to think.

The engine dies.

Upon further investigation, I find a seat-occupancy switch under the passenger seat that was part of the then-mandatory seat-belt interlock system. The insulation on the wire exiting the switch was frayed and making contact with the seat frame when someone sat in the seat. More importantly, the interlock circuit shared a fuse with the idle solenoid. A few seconds with wire cutters and electrical tape fixed the problem for good.

Sometimes when solving a problem, it's good to sit and think about it. Then again, sometimes it's good to just sit.

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Doug Pocius is a senior engineer at Soraa Inc.