LS1 starter pinion extends but doesn't spin

Question

Working on a motor swap project. Motor is 2002 LS1. Battery is less than 1yr old and was just tested at local Orielly. Starter is refurbished, bought and tested at the same shop.

4ga wire runs from battery to starter terminal. Starter is triggered by simple button on aftermarket switch panel. I verified that 12v goes to trigger when button is pressed. I pulled the spark plugs just in case the starter was a little weak and motor turns over easily with a wrench on crank pulley.

To rule out a poor ground, i added a 4ga wire from one of the starter bolts to the frame.

Answer 1

If you have verified the engine (LS1) spins (turns over) freely, then the problem is most likely the solenoid or the motor itself. With that, it could be:

1. There is a big flat copper washer at the back of the solenoid which makes contact between the main power source and the starter motor main lead. The lead is what pops out of the side of the starter motor and connect to the solenoid. If the copper washer is bad, it might not be transmitting power to the starter motor, which means it wouldn't spit ... yet the starter gear would still engage the ring gear on the flexplate/flywheel.
2. The brushes on the starter might be bad or worn out. If so, it won't allow the start motor itself to spin.

In either case, if you bench test the starter, you could put power directly to the starter motor connection which I mentioned comes out the side. If you have the starter body grounded then hit the lead with power to see if the motor will spin. If the motor spins, it's the solenoid. If the motor doesn't spin, it's the motor itself. The easiest way to provide power to this is by using a pair of jumper cables.

NOTE: If the starter orientation is just right on the vehicle, you can possibly use a screwdriver and jump between the two large terminals. This would be the same as the bench test I mentioned above, just without taking the starter down. If you go this route, be careful not to arc the screwdriver over to a ground source ... you'll see a lot of sparks if you do and you could possibly damage electronics in your vehicle.

Below is an image of the starter for your LS1. The blue arrow points to the post which provides power to the starter motor.

Answer 2

Turns out the issue was on the ground side. I relocated the battery inside the cab and ran a ground to one of the seat belt studs. That ground did not go to the frame and so I only had small 14ga wires providing incidental grounding between the body and frame.

I added a 4ga wire between the body ground location and the frame, now everything is behaving as expected.

I came up with the idea to check the stud I ground the battery too after a successful bench test of the starter and battery using a pair of spare wires from a grounding kit.