Unfortunately the insane engineers have seen fit to over-complicate the simple, reliable starter system with an unnecessary computer. I will do my best to help you figure this out. All testing must be done while the problem is occurring.
Start by going to the starter relay socket. Remove the relay, then use a test light to check the voltages there. You should find twelve volts on one terminal, (30) all the time. Next, turn the ignition switch to "run", and shift into "park". Now you should find twelve volts on a second terminal, (85). If it is missing, shift to "neutral" and try again. If you only have it in neutral, suspect the transmission range switch.
If both voltages are there, move the test light's ground clip to the battery's positive post. Probe the two remaining terminals in the socket. The test light should light up on one of them, (87), indicating a good ground path through the starter solenoid coil.
This diagram is for Chrysler relays, but I see the terminal number designations are the same on the wiring diagram for your vehicle. These are different than for older GM vehicles, so I hope they will be right. Do not panic if you find terminals 85 and 86 switched around. I cannot remember if my drawing is for the socket or for the relay when it is flipped over to look at the terminals.
Thursday, December 22nd, 2016 AT 3:55 PM