The car is a 1981 300SD. After the car has been highway driving for at least 15 minutes, and then parked, I'll turn the ignition key and get nothing, no crank or click. If I unplug the glow plug relay, the car fires right up on the first bump. When the car is cold with the glow plug relay plugged in, the car fires up on the first bump. This happened three times yesterday where the car would fire on the first bump after unplugging the glow plug relay.

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'81 300SD