'94 C1500 blowing #9 fuse, even with lights unplugged

Discussion in 'General GM Discussions' started by haromaster87, Jan 11, 2017.

  1. haromaster87

    haromaster87 New Member

    Posts:
    6
    Likes Received:
    0
    Joined:
    Jan 11, 2017
    Hello all,


    I'm new to the forum, but this looks like a great place! I'm afraid I must jump right into an issue. I recently got a 1994 GMC C1500 with single sealed beam headlights, and I'm trying to track down a pesky electrical issue. I noticed early on my tail lights, front parking lights, and gauge lights weren't working. Checking the fuses, I found the #9 25A Park lights fuse was blown. Threw a new one in there and it blew as soon as I turned the parking light switch. I'm not new at working with 12v electronics at all, but I must admit this is my first time tracking a random short and I'm not terribly familiar with the wiring on these trucks yet.

    So, here's the troubleshooting I've done:

    Unplugged the tail light assemblies as a whole, unplugged the license plate lights, and removed the front bulbs, no change here. Fuse still blows instantly. Seems like that rules out any of the parts in the rear light assembly.

    I got under and traced the tail light light wiring all the way up and could not find any points where the wire loom was worn through or even pinched. I know there can be a short under the loom, just haven't gotten that far yet. It should be noted, the brake lights still work fine, it's just the tail lights not working.

    After that, I started looking for other ways to narrow it down. I was trying to rule out the front parking light section of the harness and I found the big bulk head connector on the firewall, that's kind of below/to the side of the booster. I see the front lights obviously go to this connector, and it looks like another couple of harnesses go off to the back. I'm hoping/assuming one of these harnesses is the rear light harness.

    Anyway, I unbolted the big connector and threw a new fuse in there, and it still blew instantly when I flipped just the parking light switch. I should also mention that the headlights work fine, even before I did all of this.

    If I'm seeing it correctly, it seems like that connector should pretty much rule out a short in any of the long wiring to the front or rear lights? Meaning I'm basically looking at either the switch or some issue with the wiring behind the dash?

    Like I said, I'm new to these trucks, so any confirmation of my suspicions or any other ideas are much appreciated! I've searched all over google, but can't find anything that quite lines up with my circumstances.

    Thanks!

    Tony
     
  2. John Clark

    John Clark GMFS Member

    Posts:
    221
    Likes Received:
    19
    Joined:
    Feb 1, 2011
    The best thing to do is go to a wiring diagram. Below is the wiring diagram for the light switch system. Sounds like it's only on the parking light side. There are still lots of things it could be but I would use the diagram to disconnect everything from the switch and see if the fuse still blows. If not, then add back the circuits one by one until the fuse blows. I would look closely at S210 (Sorry, I don't know where it is) and see if you can disconnect everything there and then add it back one by one.

    http://www.revbase.com/BBBMotor/Wd/DownloadPdf?id=42746

    I got the diagrams from www.bbbind.com/free_tsb.html
     
  3. haromaster87

    haromaster87 New Member

    Posts:
    6
    Likes Received:
    0
    Joined:
    Jan 11, 2017
    Thanks for the wiring diagram! Very helpful indeed. So, I decided to try and test the switch theory by bypassing it and jumping the leads. The connector for the head light/parking light switch has a brown wire, then a red, a yellow and an orange cable.

    The brown tested properly as a ground. The red showed 12v constant. The yellow had nothing, so I used a wire to jump the red to yellow. This turned on the headlights only, which checked out. It didn't attempt to turn on the parking lights as the fuse didn't blow.

    I then checked the orange wire and found that it has a constant 12v, like the red wire. Is this normal? I can't figure it out off of the diagram and I wonder if it might be a clue to my issue.
     
  4. John Clark

    John Clark GMFS Member

    Posts:
    221
    Likes Received:
    19
    Joined:
    Feb 1, 2011
    Well, looking at the diagram the orange wire is your power feed for the parking lights from the fuse so, yes, that says it's hot all the time. The red looks like it is the power feed from the fuse for the headlights (terminal C on the headlight switch--looks like it's not marked on the diagram, though) and the orange is the power feed for the parking lights. Isn't the parking light circuit the circuit you're having the issue with, not the head lights?

    The brown shouldn't test as a direct short to ground. If it is that's the source of the short and the reason the fuse is blowing. The brown is the power feed from the switch to your loads (the lights in this case) and there should be some resistance between it and ground. If you jump the orange wire to the brown wire and the fuse still blows then you know the problem is not in the switch and is in one of the circuits to the lights.

    If it were me, I'd disconnect all the wires at S210 (should be a splice somewhere,) turn on the parking lights and see if the fuse still blows. If it does, I'd know it's the switch or wiring between the switch and the splice point. If it doesn't blow, I'd reconnect each wire at S210 one by one until I found the one that blows the fuse.
     
  5. haromaster87

    haromaster87 New Member

    Posts:
    6
    Likes Received:
    0
    Joined:
    Jan 11, 2017
    Hey I wanted to say thanks for that post. That cleared everything up for me and definitely put me on the right path. So I've gotten under the dash and traced the brown wire from the switch all the way into the lume where it goes to the fuse panel.

    I didn't find any issues there, but there's 2 other brown wires that look like they were soldered in that run away from the fuse panel, up somewhere in the dash. I think I remember reading about a wire going to the light on the 12v cigarette lighter. gonna pull that out tonight and see what I find.
     
  6. haromaster87

    haromaster87 New Member

    Posts:
    6
    Likes Received:
    0
    Joined:
    Jan 11, 2017
    So, I pride myself on usually being able to figure out an electrical issue once I know where it's at, but this has me stumped. I found that I did indeed have a short somewhere in the wiring from the main body harness connector to the tail lights. I had isolated the under dash brown wire that feeds back to the lights and found it to be grounding out. When I unplugged the main body connector, it didn't ground out, so I started working my way back.

    So long story short, I find what I believe is an issue with the rear most harness, that connects the two rear lights and license plate lights to the harness that runs along the frame to the engine bay. I first discovered that when I unplugged this harness from the frame harness, the brown didn't test as a ground at the interior.

    After different steps, I found that I can have that rear most harness plugged into the frame harness, and as long as both tail lights and LP lights are unplugged, it doesn't ground out. As soon as I plug in either tail light to either side, or the LP lights, the brown wire tests as a ground at the interior.

    To rule out faulty tail light circuits, I even took some speaker wire and plugged one side into the brown wire plug on the rear end of the frame length harness, and bolted the other wire to the frame for ground. I connected the switch in the interior and turned on the parking lights and was able to illuminate the tail lights on either light assembly by putting the correct wires to the correct pins. That told me that the brown wire from the switch all the way back to that rear connector must be good.

    Since I made it work without the rear most harness, and I had already gone over every inch of that harness and fixed all of the little spots I found, I assumed there was a deeper issue with the rear most harness. I picked up a nice uncut one today from the pick and pull. Much to my confusion, same issue. Brown wire tests fine even with rear most harness plugged in. Plug in any one light from it, brown wire grounds out. Any ideas?
     
  7. John Clark

    John Clark GMFS Member

    Posts:
    221
    Likes Received:
    19
    Joined:
    Feb 1, 2011
    So, with a light circuit you have to remember that light bulbs have very little resistance. That's why they're on high amperage circuits (i.e. 20A fuse in this case.) If you put your ohm meter across a light bulb it will show maybe 0.5 ohms or so, depending on bulb type. Touch your test leads together and get a resistance reading of your leads and meter (usually 0.1 or 0.2 ohms.) Now subtract that reading from the reading on the bulb and you'll get your actual bulb resistance. Some meters even have a function to zero the ohms when you touch the leads to remove the lead resistance from your reading. If you're testing your harness resistance and it's showing around 0.5 - 0.7 ohms then you're reading the resistance of the bulb(s) and it's not necessarily a short to ground. If you're getting 0.1 or 0.2 (the same as when your meter leads are shorted) then you're getting a short to ground.

    That's why I suggested removing each sub harness first, put a good fuse in, and then plug each subharness in until you find which harness is blowing the fuse. That will narrow it down to the harness with the problem and then you just find the short. You might go through a couple of fuses but it's easy to follow the logic that way.
     
    Last edited: Jan 22, 2017
  8. John Clark

    John Clark GMFS Member

    Posts:
    221
    Likes Received:
    19
    Joined:
    Feb 1, 2011
    Additionally, in your first post you said you were initially getting the short even with the tail lights unplugged and the LP light bulbs removed so obviously that's not where your problem lies. Did you find a short along the frame harness or the old rear harness? I'm guessing you may have fixed your problem and you're just looking at the bulb resistance and thinking that's a ground. Have you tried it with a new fuse?
     
  9. haromaster87

    haromaster87 New Member

    Posts:
    6
    Likes Received:
    0
    Joined:
    Jan 11, 2017
    Well, I come back here humbled in your advice. So I had located an under dash short and that was sorted already. However, I got too hung up on the wire testing as a ground and never took into account that maybe that's normal on a light circuit. Even after reading these posts, I continued to chase down some issue that I was sure existed. At some point it occurred to me that I had never bothered to simply test the lights. And wouldn't you know they work just fine. I don't want to talk about how much time I wasted with that. Guess I'll just chalk it up to a good lesson learned. Thank you again for the advice and help with the issue. I'm glad I got to learn a bunch about the wiring on these trucks.
     
    Last edited: Jan 22, 2017
  10. John Clark

    John Clark GMFS Member

    Posts:
    221
    Likes Received:
    19
    Joined:
    Feb 1, 2011
    Hey, if you've got the problem fixed then you're doing better than most.

    What was shorted?
     

Share This Page