1997 Suburban - Rear Defogger Circuit Fault

Discussion in 'Audio + Electronics' started by cjmspartans, Jan 10, 2020.

  1. cjmspartans

    cjmspartans Newbie

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    Hello everyone! I've got a bit of an issue that has me scratching my head. My 97' K2500 Burb blew the 30A "RR DEFOG" fuse under the hood, so I've been without the rear defogger for a few months. Otherwise, the truck has been running great aside from a interior door handle snapping again :rolleyes:

    What I (stupidly) did yesterday was replace the blown 30A RR DEFOG fuse with a good 30A fuse, just to see if the issue would reproduce its self, and oh boy did it! Turned the truck on, turned on the rear defogger (orange light came on... yay?)... and smoke started coming out from under the dash. Fantastic. Absolutely fantastic. Immediately removed the key and the 30A fuse. When I tried to remove the fuse, it was interesting because the plastic just melted and the fuse never popped. What this tells me is we have high resistance in the circuit, but it's not pulling 30+ amps to pop the fuse. For what it's worth, the rear passenger glass was warm but the drivers side glass was cold in the end. The Burb has barn doors, if that matters.

    My big question here is, does anybody have wiring diagrams that they could share for this circuit? I've tried searching but my searching skills might not be that great. The truck still runs and everything else (that I can tell) works fine. No more smoke is coming from the dash after the fuse was removed. I need to test that these now crispy wires are indeed not energized with a test light or multimeter before I drive it around.

    Additionally, does anyone have any insight into these rear defoggers? Is this type of event common if they fail? I don't have the slightest idea how these things work. I'd like to avoid bringing it to a shop because that sounds like a big bill, but I want to make sure that the wiring is at least safe. If I can safely drive it around with no rear defogger, that's fine. But I want to make sure there isn't any additional damage. I can already tell that the dash will need to come apart so I can check the wiring going back from the HVAC controls.

    Here's some pictures for you guys:

    [​IMG]
    The 30A fuse when I pulled it out of the fusebox. Yummy!

    [​IMG]
    The destroyed purple wire that I assume runs to the back of the truck. Crispy!

    [​IMG]
    The now crispy red wire coming out of the dash wiring harness that connects to the rear defogger switch on the HVAC controls. This is definitely going to be labor intensive or expensive. Or both. And yes, the copper is showing in those holes on the insulation. I'm assuming that damage was done when it started smoking.

    What did I learn? When a fuse pops, it's for a reason. This was bad on my part but the problem likely would have occurred even if I brought it to a shop for diagnosis.

    Very sad day for me. But hey, things happen. Hopefully someone here can provide some insight. Many thanks guys!
     
  2. Eveready

    Eveready I'm Awesome

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    First thing would be to try directly connecting the defogger (with the circuit cut from it) to a 12 V line from the battery and see if it works properly. If so then there is a short somewhere on the other end of the circuit probably near the fuse block. If it doesn't work or shows a short then likely the defogger itself caused the problem.

    My bet is a bad switch shorting to ground somewhere.
     
  3. cjmspartans

    cjmspartans Newbie

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    I'll look into doing that. Any idea if I can just test the defogger element for a specific amount of resistance? Granted that would be documented somewhere, of course.

    I've been trying to dig up schematics to give me an idea of where else to look. I've only found this, and I don't like what I am seeing:
    [​IMG]

    Looks like the orange wire that is now a little crispy (and has exposed copper near the HVAC controls) is going to the battery. I'll electrical tape the exposed wire for the time being to prevent a short, although I'm nearly certain that wire is fused.

    What would really be useful is a diagram showing how this integrates with the convenience center under the dash. I find it interesting that the purple wire under there is heat damaged, yet the purple wire on the dash harness that plugs into the HVAC controls seems fine. As the purple wire is damaged at the convenience center, that makes me worry that there's a issue behind the firewall.
     
    Last edited: Jan 10, 2020
  4. east302

    east302 I'm Awesome

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    This is for a 98, maybe it will help...

    [​IMG]

    Ground G200 is on the metal bar near the parking brake...

    [​IMG]

    I couldn’t find a diagram for the ground G400 location for cargo doors, but it shouldn’t be hard to spot if you need it.

    [​IMG]
     
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  5. east302

    east302 I'm Awesome

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    And in case you need them, here are the connectors at the back of the hvac control panel...

    [​IMG][​IMG]
     
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  6. cjmspartans

    cjmspartans Newbie

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    Thank you so much east302! These are exactly what I was looking for. I don't normally look at wiring diagrams so I want to clarify a few things ;)

    What exactly is the convenience center for? Seems silly to have that purple wire for the rear defogger element to connect to the back of it, just to come out the front. Is it as easy as replacing the wire going into it and out of it? I believe this is the box under the dash that has the door buzzer in it.

    Secondly, S223 between the HVAC control module and the convenience center is a switch, right? If I am reading the diagram correctly, is this saying it goes to a connector? Looks like the diagram continues onto another page there.

    Edit: S223 obviously isn't a switch. Just another wire in the circuit between the two. Yeesh haha
     
    Last edited: Jan 11, 2020
  7. east302

    east302 I'm Awesome

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    S223 is a splice, though I’m having trouble finding it in on alldata. Scans from the factory manuals are here...


    https://www.gmt400.com/threads/88-95-service-manuals.43575/

    Attached are the diagrams from the service manual (versus alldata which is what I posted earlier since the resolution is a little better). In the FSM it’s called splice 425 instead of 223, but the wire gauge and circuit number 293 are the same. Either way, it looks like it goes to the heated outside mirrors if yours came with that option.

    [​IMG][​IMG]

    As for the convenience center, yes it’s the thing with the door chime on it. I think it was actually meant for the convenience of assembling the trucks. There are three or four small connectors and one big one that go through the firewall into the cab from the engine side and are hardwired to this ‘center’. From there, it goes to the interior components so I see it as a coupling or junction box of sorts.

    When you pull the dash, you can separate the harnesses on the engine side, unscrew the convenience center and pull the whole thing into the cab. This allows you to pull the dash while leaving the wiring harness and fuse box installed inside the dash.

    That’s a long-winded way to say that my guess is that it probably made installation easier in the factory.
     
    Last edited: Jan 11, 2020
  8. tinfoil_hat

    tinfoil_hat I'm Awesome

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    Maybe a dumb question... Did you try cleaning all the contacts in the door frame? You will have a lot of extra resistance if the current has to jump across corroded connections.
     
  9. cjmspartans

    cjmspartans Newbie

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    I did clean those connections before I turned on the defogger. When I opened the doors yesterday and looked at the contacts, I noticed that the plastic around the bottom contact was melted. Passenger side was a-okay.
     
  10. tinfoil_hat

    tinfoil_hat I'm Awesome

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    Well thats not a good sign.
     

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