Easiest is to go right to the starter relay under the hood. Remove it and test all for terminals in the socket with a voltmeter or test light. One terminal will have 12 volts all the time. A second one will have 12 volts only with the ignition switch in the "crank" position.
For the remaining two terminals, move the ground lead of your meter or light the battery positive post. Now you're testing for a good ground circuit. Probe the last two terminals. One will show continuity to ground all the time through the starter solenoid and motor. The last one will also show continuity but it will go open circuit when you shift into "reverse" or "drive". That's the neutral safety switch circuit.
In some GM circuits the neutral safety switch is in series with the ignition switch. If that's how your car is wired, the two ground terminals will show continuity to ground all the time, and the switched 12 volts from the ignition switch will go away when not in "park" or neutral". Those four tests will tell you which part of the circuit has the problem.
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Wednesday, June 27th, 2012 AT 5:21 PM