H3 Dies while driving, won't crank or start - Need help!

My 2007 H3 (manual, 105,000 miles) died while driving at 50 MPH. Engine died, dash lights come on as if key was turned to on from run.

I shut everything down, turned ignition to accessory, all lights came up normally. Turned to start and everything electrical is pulled down. I checked and re-seated the battery connections. I could find no blown fuses. Took battery in for testing and the tester was broke so they replaced with new battery since it was in free-replacement period.

Tried with new battery and same result. No solenoid click, all dash lights come on normally, no alerts. Go to start and everything goes black.

Passlock has been disabled when PCM was tuned. The only alert is a TPM light for a TPM sensor whose battery died 2 months ago.

I could sure use some good suggestions!

Broom connector under the under-hood fuse panel? Known to come loose especially if it's ever been previously disconnected. Just a wild guess.

What makes this more curious to me is how everything goes black when you go to start - and I assume you mean turn the key to crank the starter. It makes me wonder if there's a short somewhere so that all of the juice is being focused through that short.

Thanks for the clue alrock, I'll look into that. Yes, when I turn key to start, all dash, interior and HVAC goes down, but engine does not crank. When I release key back to run, everything comes back to normal.

Update: Checked the fuse box connector and reseated fuses and relay. No ODB codes show. Yesterday morning it suddenly cranked and started. Started successfully several times and I took it for a test drive. This morning it does not crank and does the same dead dash when I go to start.

When I turn key to ON, I can hear the fuel pump humming.

When it doesn't crank, does the starter get power?
How is that power controlled? (Probably a relay?) - Find that relay and test to see if there is signal on the control side of the relay (is the relay being commanded on) and whether there is power on the load side of the relay. If the relay isn't being commanded on, then look into what signal is supposed to turn the relay on? Where does that signal come from? Try tracing back towards the "Source" of that signal. -See what that yields?

Thanks LagunaH1 - Great advice! My H3 was sitting outside the auto shop having baffled a half dozen "GM trained" mechanics. I used your methodology along with the circuit diagram and I first eliminated suspected grounding issues by measuring positive to left body and positive to engine. Both measured 12.6 volts. I then walked through the circuit with my VOM starting with the Run, Crank activation voltage. That was good, but when I moved to the next relay I had nothing. I went back and swapped the rear defogger relay with the run, crank relay. I found voltage on the next downstream relay. The engine cranked, started and appears to be fixed.

LagunaH1 said:

When it doesn't crank, does the starter get power?

Click to expand...

That's the question in my head as well. I wonder if the cable to the starter has failed, perhaps become corroded over time. I had that happen on an old Chevy 350. Again, I don't know electronics that well so I'm shooting in the dark, but just trying to apply some logic.

MtnDog said:

Thanks LagunaH1 - Great advice! My H3 was sitting outside the auto shop having baffled a half dozen "GM trained" mechanics. I used your methodology along with the circuit diagram and I first eliminated suspected grounding issues by measuring positive to left body and positive to engine. Both measured 12.6 volts. I then walked through the circuit with my VOM starting with the Run, Crank activation voltage. That was good, but when I moved to the next relay I had nothing. I went back and swapped the rear defogger relay with the run, crank relay. I found voltage on the next downstream relay. The engine cranked, started and appears to be fixed.

Click to expand...

So, sounds like a relay had gone bad? Glad you found the issue!

Nice!!!