Can't get starter to crank over with start switch
 

This is pretty basic but I must be missing something in my old age. The engine is a ZZ3 350 that was in a 1985 Corvette. I put the engine in my jet boat and just hooked up all the wiring but when I turn the key to start, I get nothing. I put the voltmeter between the start wire and the start terminal on the solenoid and when I turn the key I get 12.5 volts but no action. I tried a new solenoid and got the same thing. I took the starter with the old solenoid(I rotated the power lead post so it was sure to make good contact) to Autozone and they put it on the machine and everything checked out fine. The first time was sluggish but after that it sounded like normal. I put it back on the engine and get the same thing. I put a screwdriver across the power lead post(which has the battery cable connected to it) on the solenoid and the start post on the solenoid and it engaged and cranked over the engine. I hooked a jumper wire from the positive post on the battery to the start post on the solenoid and got nothing. I would think that the full battery power (12.5 volts) going to the start post should engage the solenoid and starter. So what am I missing here? :scratch:

Edit: I don't have the engine grounded to the aluminum hull, I'm wondering if I need to.
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My first thought was ground

Is the boat fiberglass ? If so what are you grounding it too?

Ground

Boat is aluminum so I'm going to try grounding the engine to the hull tomorrow.

If you bypass the solenoid and put 12 volts directly to the starter and it starts, problem is with the solenoid. Either bad solenoid or small wire going to solenoid is not energizing.Chassis ground should not have nothing to do with starting the engine. The battery ground going to the block is all you need to have. Check that your ignition switch is grounded.

The ground problem could be between the block and the starter, that's why I didn't say chassis ground, it's unlikely, but it could be between the starter and the solenoid

I've recently read that you should also put a ground wire to the alternator case (or have a ground lug put inside, I forget which) to eliminate electrical gremlins

I recently had a problem in my suburban. The GPS DD module would stop working for no reason and when DD replaced it under warranty the replacement did the same thing. All the dash components have the same ground. Sometimes I would come out in the morning and the dash screens would be on the default screen instead of where I left it. Turned out to be the ground connection on the battery. Had no problems starting it or anything else wrong. Chasing ground issues can be a bitch

I think it's either the ignition switch or the wiring from the switch to the (S)tart post on the solenoid. I connected a battery cable from the battery to the (S)tart post and it cranks over. When I turn the ignition switch, I see the voltage through the multimeter starts around 8-9 volts and builds to 12.5 but it doesn't engage the solenoid. When I let go of the key, I can hear a clunk which I think is the solenoid plunger dropping back. I'll try a new wire from the switch to the solenoid to see if that works, then maybe a relay to get full power from the battery. I tried a ground from the block to the boat hull, but that didn't make any difference. I tried another battery and got the same results.

Update: It's the yellow wire, I jumpered a new wire from the switch to the solenoid and it cranks over fine. I tried cleaning up the ends and the wire harness connection for the yellow wire and still no good. It lets power through but it's not clean enough to engage the solenoid.