Courtesy Lamp/Tail Light Fuse Keeps Blowing - Cannot Figure Out Why - Need Help!

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huskerfan

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Hey guys - i'm having an electrical issue that i need help on. My courtesy lamp/tail light fuse keeps blowing whenever i turn on my headlights. This is weird because i cannot pinpoint any reason or event as to why it would have happened. Was all working fine until the weather got cold so maybe that's why? Not sure. I've searched and read many many forums and traced out all of the electrical circuits for it, but still no dice. Maybe Doghead could help since he's the resident electrical expert on this stuff? Here's the issue i'm having:

Noticed that my instrument cluster lights were not working, checked fuse and it is fine, but have no voltage on either side of the fuse. So then i checked all of the fuses and noticed that the courtesy light/tail light fuse was blown, so i replaced it. But then when i pulled the headlight knob out, it blew again. So i replaced it again, and i am able to rotate the knob with the headlights off and get the under dash bulb to come on, but when i pull the knob out to turn on the head lights, the fuse blows again. So the fuse blows regardless of whether or not i have the dash light on or off when i pull out the knob to turn on the headlights. I have voltage at the courtesy light fuse on the right contact, but not the left contact (i know the feed comes into the fuse at the right contact, but didn't check the left contact until after the fuse blew, so got no voltage at the left contact).

I have voltage at both contacts on the headlight fuse.

I thought i had narrowed it down to the headlight knob, so i pulled off the wiring harness and pulled out the knob switch assembly and didn't see any corrosion or anything. Took it apart and deciphered how all the prongs get their power and correlated the prongs to the corresponding service lighting wiring circuit diagrams on here, and it all looked fine. Cleaned up what little dirt and stuff i could see, even though there wasn't really anything, and reinstalled it and replaced the courtesy light 20A fuse, pulled on the headlights and the fuse blew yet again.

Any ideas?
 

porkysplace

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mid- michigan
Hey guys - i'm having an electrical issue that i need help on. My courtesy lamp/tail light fuse keeps blowing whenever i turn on my headlights. This is weird because i cannot pinpoint any reason or event as to why it would have happened. Was all working fine until the weather got cold so maybe that's why? Not sure. I've searched and read many many forums and traced out all of the electrical circuits for it, but still no dice. Maybe Doghead could help since he's the resident electrical expert on this stuff? Here's the issue i'm having:

Noticed that my instrument cluster lights were not working, checked fuse and it is fine, but have no voltage on either side of the fuse. So then i checked all of the fuses and noticed that the courtesy light/tail light fuse was blown, so i replaced it. But then when i pulled the headlight knob out, it blew again. So i replaced it again, and i am able to rotate the knob with the headlights off and get the under dash bulb to come on, but when i pull the knob out to turn on the head lights, the fuse blows again. So the fuse blows regardless of whether or not i have the dash light on or off when i pull out the knob to turn on the headlights. I have voltage at the courtesy light fuse on the right contact, but not the left contact (i know the feed comes into the fuse at the right contact, but didn't check the left contact until after the fuse blew, so got no voltage at the left contact).

I have voltage at both contacts on the headlight fuse.

I thought i had narrowed it down to the headlight knob, so i pulled off the wiring harness and pulled out the knob switch assembly and didn't see any corrosion or anything. Took it apart and deciphered how all the prongs get their power and correlated the prongs to the corresponding service lighting wiring circuit diagrams on here, and it all looked fine. Cleaned up what little dirt and stuff i could see, even though there wasn't really anything, and reinstalled it and replaced the courtesy light 20A fuse, pulled on the headlights and the fuse blew yet again.

Any ideas?
Start here : [h=1]CUCV Helpful Threads[/h]
Then go to here and read through these:

HEADLIGHTS
CUCV Headlight Circuit
http://www.steelsoldiers.com/showthr...dlight-Circuit

CUCV Melting Headlight Fuse and Horn Circuit fix
http://www.steelsoldiers.com/showthr...t-fuse-melting

LMC Headlight Wiring Harness upgrade
http://www.steelsoldiers.com/showthr...ht-circuit-mod
 

cucvrus

Well-known member
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Location
Jonestown Pennsylvania
Sounds like a dead short. If it blows the fuse instantly. Hard thing to diagnose over the internet. Could be anywhere in the effected circuit. Has ice built up on wiring and torn it loose? I would say check you front park turn sockets and your tail lamps. Good Luck. Report back. Be Safe. Get a short finder. they do work. I borrowed one once. Also a test light is helpful.
 

huskerfan

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Lincoln, NE
Shouldn't really have had any ice build up as i keep the truck in the garage. I checked all the other lights and they all work. The flashers and turn signals all work. May be of interest, but the rear drivers flasher and turn signal was not working when i dove into this issue, so i tackled that first, and pulled out the rear drivers trail light, didn't see any bulbs burned out, so left it hanging there and tried it again and viola it worked. Reinstalled it on the truck and tested it again and it still works. Not sure why or what was causing that, but as of now all the turn signals and flashers work, if that is of any relevance.
 

cucvrus

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Jonestown Pennsylvania
Does the 30 amp green fuse on the right side of the fuse box about 1/2 way up look melted, corroded or distorted in any way? I would say the 3 prong plug on the tail lamp harness has issues. Pull that light again and remove the short tail lamp harness and thoroughly inspect it. Good Luck.
 

nyoffroad

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Rochester NY
Salt water dosn't just cause rust it also is conductive. I doubt thats all of your problem but could contrubute to it. I've worked on similier problems and most time found the offending wire in the back where someone else hooked up a trailer plug or probed the wire and broke the insulation.
 

huskerfan

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Lincoln, NE
CUCVRUS - I don't think it was originally. Actually i know it wasn't. After the courtesy lamp fuse broke, i replaced it with a 20A and it blew again. Then i went to a 25A fuse, same issue. Then i went to a 30A fuse, and the 30A headlight fuse blew instead (i know this isn't smart practice, but i was reading an outdated TM i think at the time and was all sorts of confused, so i just thought i would give it a try to see if it would get me anywhere). So i have replaced the 30A headlight fuse i know for a fact, and it is still good. You got me thinking over lunch break though and i was able to finally in my head understand it all.......i think. Hear me out and let me know what you think:

Since the horn still works and is jumpered from the right contact of the courtesy lamp fuse, that means that all is good "upstream" of the courtesy lamp fuse since power comes in to the courtesy lamp fuse at its right contact. So now we follow the power out from the left side of the courtesy lamp fuse and it goes to the courtesy lamp bulb. I did not check the bulb yet, but if it is oversized, then it would draw more current, which would blow the fuse and also prevent the tail lights from working. BUT the fuse blows when i pull the headlight knob out, which it shouldn't, even if the bulb is oversized because you have to rotate the headlight knob all the way counter clockwise to the end and then a bit more to engage the contacts and complete that circuit. So, could current be back feeding from the rear driver's tail light that i had an issue with and blowing the courtesy light fuse when i pull the headlight knob out to turn them on?
 

huskerfan

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Lincoln, NE
Well guys, i figured out my issue and got it all fixed. My theory was partially correct. I'll explain for anyone that may encounter this issue in the future since it took me a while to understand how this issue was occurring, but now that i understand it, it makes the wiring diagrams and the wiring in the truck easier to comprehend.

I had no rear drivers tail light or signal light, nor would my dash lights illuminate when i turned the knob. My under dash courtesy light was also not coming on. When i replaced the fuse for the courtesy light, it would blow whenever i would pull the headlight knob out, however, if i left the headlight knob in (lights off), but rotated it all the way counter clockwise, the light would come on and the fuse would not blow.

So i pulled out the rear drivers tail light and found that the brown wire for the brake and turn signal had worn through and were shorting out. After looking at the wiring diagrams, it makes sense now what was happening (i think - correct me if i'm wrong).

When i would pull the head light knob all the way out, power was flowing from the head light switch to the tail lights via the brown wire indicated on the wiring diagram from pin #4 on the head light switch. Since the wires were short circuited at the rear drivers tail light, this caused an excessive amount of current to flow.

The under dash courtesy light receives power from the 20A courtesy light fuse on the fuse box via the orange wire indicated on the diagram. This fuse receives power from pin #1 on the head light switch via the orange and black wire indicated on the diagram.

Here's where the problem of the fuse blowing lies (i believe). Once the power goes through the under dash courtesy light, it returns to the head light switch via the white wire indicated on the diagram. From reading the TM, this light always has power - it illuminates when you rotate the knob all the way counter clockwise by grounding out on the headlight switch.

So, since there was a short circuit at the tail light, excessive current was flowing from the headlight switch, and the headlight switch is fed from the orange and black wire coming from the service light switch, indicated on the diagram, into pin #1, AND excessive power was flowing through the under dash courtesy light fuse, which is only a 20A fuse, so that's why it kept blowing whenever i would pull out the head light knob as the headlight fuse is a 30A fuse, and feeds the right contact of the 10A stoplight fuse, which goes to the service light switch.

As for the dash lights not illuminating and why the 5A dash light fuse did not blow, i am guessing that since it is only a 5A fuse, the electricity was flowing through the path of least resistance (the short circuited tail light wire) and not flowing to the dash light fuse.

I attached a couple pictures for reference. Hope this helps somebody out! And if i made a mistake anywhere, please let me know!

CUCV Colored Wiring Diagrams - Tail Light Short Circuit Page 001.jpg CUCV Colored Wiring Diagrams - Tail Light Short Circuit Page 002.jpg
 

cucvrus

Well-known member
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Jonestown Pennsylvania
I thought I mentioned that. post # 5

I would say the 3 prong plug on the tail lamp harness has issues. Pull that light again and remove the short tail lamp harness and thoroughly inspect it. Good Luck.

Anyways. No need to do headlamp harness updating and all that other stuff. Just clean and maintain the stock system in it's entirety and it will last for years to come. I mean it has lasted this long so it will last just as much again. Have yourself a Merry little Christmas. From now on light troubles will be miles away. Good Luck. Be SAFE.
 

huskerfan

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Lincoln, NE
CUCVRUS - you were definitely on the right track pointing me to the area to inspect - the wires had worn away closer to the tail light though - looks like they were both rubbing on the edge of the metal where they go up through the hole into the taillight receptacle. about 5 inches of the harness was actually coiled up and zip tied behind the receiver hitch, so i clipped those zip ties to give me more length to work with and so that the wires wont rub on the metal edge anymore, just need to zip tie the excess up now.

Thanks for your input on this and you have yourself a Merry Christmas as well!
 
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