To be clear, it's circuit 15 (hot when ignition is in start or run) and the pin is #1 in the GP relay assembly connector. There is basically a direction connection from the battery (circuit 30) to the ignition switch so no fuse, and through the switch in Run/Start to make circuit 15. If you pull the small plug in the glow plug relay assembly you should see 12V at pin 1 when in the key is in the Run position, and pin 2 should be open. (Use pin 4 of the connector for ground to make these checks if you want to really verify the circuit. If you get nothing, then see what happens when you switch the ground wire of the meter to the chassis or engine block.) If you want to check it continuity wise, disconnect the positive post on the battery, and check continuity from the + connection (not the battery post, that's out of the circuit now) to pin 1 of the connector with the key in Run. Turn the key to start and you should see a connection from the + connector to pin 2 of the GP relay assembly connector. If you check continuity from pin 3 to ground that will tell you the dash lamp is probably okay. Pin four of the connector is ground.

If you have a bad connection somewhere, it may not show up with continuity checks, but only when voltage is applied and the circuits are connected normally. That's because with no current in the circuit there are no voltage drops. So if you are looking at pin 2 of the GP relay assembly connector in the run position and are seeing no voltage, either you have a bad connection between the battery and the GP relay assembly, or no connection to the relay assembly. If you had a bad ground through the connector and everything else was fine you would still see 12V.

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mjk

'84 300SD 119KMi (Liesl der Diesel)
'84 300D 326KMi when the oil left (former parts car)
'82 300SD 253KMi (new parts car)