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JFN1
JFN1, ASE Certified Technician
Category: GMC
Satisfied Customers: 4752
Experience:  GM Master Technician
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My 2004 gmc Sierra LB7 duramax won't start. And it shows the

Customer Question

My 2004 gmc Sierra LB7 duramax won't start. And it shows the security light on the dash. But nothing else also I have no blinker lights
Submitted: 2 years ago.
Category: GMC
Expert:  JFN1 replied 2 years ago.

Do you have a good quality scan tool by any chance?

Customer: replied 2 years ago.
I have my edge monitor that reads trouble codes
Expert:  JFN1 replied 2 years ago.

OK, does it communicate with anything besides the ECM? I'd be looking to see if the BCM is communicating?

Customer: replied 2 years ago.
I could not tell you that. I don't think it does
Expert:  JFN1 replied 2 years ago.

I don't think it does, either, but I'm not as familiar with that device as you probably are.

Nonetheless, let's try to pick out some things to look at. Did the turn signals stop working at exactly the same time that the engine failed to start? And you have no other gauges working on the dash either, is that right?

Customer: replied 2 years ago.
the turn signals stopped at the same time and the battery indicator turns on and the gauges work but the glow plug indicator and everything else do not turn on
Expert:  JFN1 replied 2 years ago.

OK. Do you have a test light?

Customer: replied 2 years ago.
Expert:  JFN1 replied 2 years ago.

Turn the key on, then use the test light to touch the tops of all of the fuses, both under the hood and under the dash. I'm looking to see if you have large groups of fuses that don't light up. Make a note of which ones aren't hot, and let me know what they are.

Customer: replied 2 years ago.
Etc\ ecm fuse, ign E, help rt lt hi and low, horn, trl park rr park and lr park, int park, crank, starter, I can't reach the bigger fuses under the hood my test light probe is too big. But those are all not getting juice under the hood
Customer: replied 2 years ago.
Excuse me the starter is getting juice
Expert:  JFN1 replied 2 years ago.

OK. Is the maxi fuse 40A labeled IGN B hot on both sides of the fuse?

Customer: replied 2 years ago.
No just the engine side
Expert:  JFN1 replied 2 years ago.

OK, that fuse is open. Replace that maxi fuse.

Customer: replied 2 years ago.
OK I replaced it.
Expert:  JFN1 replied 2 years ago.

Are the circuits working now, or does the truck now start?

Customer: replied 2 years ago.
Still won't start
Expert:  JFN1 replied 2 years ago.

Alright, did that fuse open again?

Customer: replied 2 years ago.
it is
Expert:  JFN1 replied 2 years ago.

OK, there's a short in that circuit. Let's go to the schematic.

Does the fuse blow as soon as you replace it, or when you turn the key on?

Customer: replied 2 years ago.
Which one the ign b? I don't think it does blow it still looks good
Expert:  JFN1 replied 2 years ago.

I asked you if that fuse was open again, and you said yes. So you have power on both side of that fuse now, correct?

Customer: replied 2 years ago.
Sorry I misunderstood no there is still only power to the engine side
Expert:  JFN1 replied 2 years ago.

Maybe we're missing something in the communication here.

With the fuse installed, touch the test light to the tiny metal tab on either side of this fuse. Both side of this fuse should be hot when it's installed. If it's only hot on one side, that would be because the fuse is open.

Customer: replied 2 years ago.
Oh OK gotcha. Hot on both sides of that fuse now with it installed
Expert:  JFN1 replied 2 years ago.

OK, good. Follow that schematic, you can see that this fuse feeds a red wire, which turns to red/white, and goes to the ignition switch. With the key ON, there should be power then coming down the brown wire at the ignition switch. Can you access the ignition switch wires inside the steering column and test to see if you have power on these circuits?

Customer: replied 2 years ago.
I can. Let me take the steering column cover off and I'll get back to you
Expert:  JFN1 replied 2 years ago.

OK

Customer: replied 2 years ago.
Did not mean to request phone call haha
Expert:  JFN1 replied 2 years ago.

:)

Customer: replied 2 years ago.
OK so I got the column exposed and its the brown wire going into the ignition switch supposed to have power with key on correct. And I'm not getting power to the ignition at all
Expert:  JFN1 replied 2 years ago.

The brown wire is the wire that comes out of the ignition switch, and it's fed by the red/white wire. So, if you have power at red/white, the key is ON, and there is no power on brown, your ignition switch is no good and needs to be replaced.