GMC Troubleshooting Questions? Ask a Mechanic for Answers
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I would say that you likely have a starter beginning to fail. Obviously it would be best to diagnose when it is not working
Do you have a test light?
Can you check for 12 volts at the small wire on starter when it is in start position? Also do you have a piece of wire?
No not at the positive terminal the S terminal which is the small wire.
If you have 12 volts at the S terminal I would like you to attach the jumper from the starter case to the frame and see if it starts then, It is common for these starters to have a bad ground. keeping them from starting. If it starts with the ground wire I would get a large 4 or 6 gauge wire and attach it to the starter and frame.
If you have 12 volts at the S terminal and the ground doesn't help the you need a new starter.
OK, Then we need to figure out why. Can you try moving the shifter slowly while trying to start? There should be a fuse for the starter. have you checked that?
Ok, There is a starter relay in the relay center under back of cab. You should have 12 volts at 2 of the 4 terminals and ground at the other when cranking. Can you verify this?
Let me know what you need assistance with.