About a week or so ago it happend to Me; I turned out My Battery was shot.

My Battery showed 12.5 Volts. With the Volt Meter connected to the Battery I had My Wife attempt to crank the Engine. I got the Solenoid Click and My Battery Voltage dove down to 2 Volts.
So that is an easy test.

I figured it was unlikely that the Voltage drop was due to a short because that sort of load on a short circuit is going to burn something.

If you Solenoid clicks and your Voltage remains the same or close you have another issue somewhere.

I installed an extra Battery I had and have had no more issues.

The definative test would be to charge the Battery and have it load tested.

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84 300D, 82 Volvo 244Gl Diesel