When I turn the key you can still hear the glow plugs warm up etc.........then when you turn the switch.....nothing!

In no particular order:

• Solenoid on starter is bad. If it clunks (the solenoid) but no starter motor running, the contacts have failed. They are repairable.
• Neutral safety swtich is bad. Can be bypassed easily.
• Wires to neutral safety switch are bad/disconnected. Reconnect/repair.
• Starter relay is bad. Test by substitution--common relay.
• Blown fuse.
• Ignition switch is bad. Rare problem, but can happen.

Peter

I just finished putting in another GM starter and all it does it click. What gives, can a bad starter run down my batteries? Or is something wrong with the new starter?

Yes, a bad starter could have run down your batteries.
Yes, there could be a problem with the new starter.
An incorrectly installed starter (wrong shims) may cause this.

But...

A bad connection could also be the fault. If all that happens is the solenoid is clicking, then you need to debug:

1. When it clicks, do the lights dim (say the dome light) fully or almost off? (bad battery connection, or dead batteries)

2. If the (dome) lights don't dim/go off, the battery cables at teh battery are probably OK. But at the starter, the positive may be bad.

3. With the bottom splash shield off, check clearance between spur gear and ring gear, is it correct?

Peter

First... when you hook it up be sure to check the connections and that they are clean... then line it up with your eye and make sure that your "new" starter will properly engage with the flywheel, if it looks askew, then it won't engage properly or at all... and that needs to be fixed...

Next, get under there and have someone try and crank it while you watch it... see if its a starter problem (i.e. bendix doing nothing) or if its an alignment problem (i.e. bendix kicking the gear out and then - due to misalignment - gets stuck at the flywheel)....

Is this a Hummer starter? or just a replacement GM starter? It sounds like it might not be the OEM Hummer starter which would mean that you need to do some homework and make sure that everything is hooked up properly... make sure the power is to the power, ground is to the ground and the "switch" is to the switched terminal....

You can bench test it easy enough... take some jumper cables, hook them up to the positive / negative terminals of the starter and one of the batteries... then take a wire and clamp it to the positive side and touch the "switch" spot of the starter and see if it turns....

Scott