Post number 3 is the fastest way to determine if it is the Neutral Safety Switch (Automtic Transmission only).

Similar to post 2 clean all of the battery cable contact points to included the mentioned ground strap.

Hook up the Volt meter to the Battery and have someone try to start and see how far the Battery voltage drops. If voltage plungers it is likely your battery is shoot.

I drove, stopped, started and stopped to eat. Got in the car and nothing. The Battery showed 12.50+ volts.
When I was trying to do the test as in post 3 I noticed a had littl spark when I applied the wire and no cranking.

I took the jumper wire and applied it to the terminals and just a tiny spark. I determined from that while I had voltage I had minimal amperage.

I walked acrosse the street and bought another Battery and put it in and started right up. So it was the Batter that was shot.

__________________
84 300D, 82 Volvo 244Gl Diesel