Headlight / dimmer switch keeps popping fuse..

homelessduck

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When the switch is pulled out the first click it immediately blows the fuse. Headlights still work but no running lights. I replaced the switch , traced wires , checked tail lights , disconnected the power side of the running board lights and the visor lights , disconnected the trans temp gauge and the ones in the pillar pod which are tied in... Any suggestions??
 

Big Bart

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I am not totally up to speed on the wiring in our trucks but I would look also at -

1) Assuming you replaced the fuse that blew.
2) The dash light circuit.
3) Your trailering wiring and any splices into your running light wiring.
4) Your running light bulbs and sockets. One or more could be shorting out.
5) I recall there is some running light relay for trailer lights on some of the years on these trucks.

Let us know how it goes.
 

homelessduck

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This is kind of out in left field, but have you tried replacing your headlight switch?
Yep , I replaced it the other day. I'm basically disconnecting everything that I know is on that circuit one by one , popping a fuse in and seeing if it pops lol. I have a pretty big pile of blown fuses LOL
 

Noiseydiesel

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Your not going to like me. . .
Remove ALL of your exterior lamps. Remove your dome light. Remove your dome light.
Now pull on the HL switch. Fuse still blow?
NO? Put your bulbs back in after inspecting each one. One at a time.
 

homelessduck

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Your not going to like me. . .
Remove ALL of your exterior lamps. Remove your dome light. Remove your dome light.
Now pull on the HL switch. Fuse still blow?
NO? Put your bulbs back in after inspecting each one. One at a time.
That's exactly what I planned to do next , I just need to pick up some more fuses LOL
 

Philip1

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One thing you could do instead of constantly replacing fuses is to put a light in the fuse slot instead. If the short is still grounding the light will stay on, of the short is removed from ground the light will go out.
 

homelessduck

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I've been trying to figure out this issue off and on and have made very little progress. I've gone through about 40 fuses LOL. I pulled all of the bulbs , inspected every socket , verified the headlight switch is good , disconnected the running board and visor lights,checked and unplugged the trailer harness.. The only thing I have found is the brown wire on the switch plug ( Pin R ) is the one that goes to exterior lights , and it is grounded and popping the fuse. So I'm pretty sure it's grounded somewhere between that pin and the exterior lights. But I have no clue how to narrow it down any further , aside from tearing the whole truck and wiring harness apart to look for shorts. Any suggestions??
 

chillman88

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Do you have clearance marker lights? I worked on a truck that they had shorted out and melted the harness in the roof. Could try unplugging that harness?
 

homelessduck

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Do you have clearance marker lights? I worked on a truck that they had shorted out and melted the harness in the roof. Could try unplugging that harness?
It doesn't but it has a a visor with lights. I disconnected it and it still popped the fuse.
 

aggiediesel01

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Do you have the Factory EVTM for the truck? That manual will show you all the splices and connectors and such that are supposed to be in the circuit from the factory. It also generally tells you where the harnesses route to figure out where the circuits may be found. If you don't have it, you can download the '94 version from my google drive folder. The chassis service manual has additional component testing and pinpoint tests that might be useful as well, Section 17 in that manual.

 
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homelessduck

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Do you have the Factory EVTM for the truck? That manual will show you all the splices and connectors and such that are supposed to be in the circuit from the factory. It also generally tells you where the harnesses route to figure out where the circuits may be found. If you don't have it, you can download the '94 version from my google drive folder. The chassis service manual has additional component testing and pinpoint tests that might be useful as well, Section 17 in that manual.

Thank you!! That helps a lot! I have been trying to understand the wiring diagrams but I'm not very good at electrical stuff lol. The headlight switch pin B2 is constant 12v and when the switch is pulled out one click for running lights it transfers power to the pin R which is brown and runs behind the dash and in to the main harness at the firewall, then out to the running lights. This brown wire is grounded , but shouldn't be right? When I ohm test it with the negative lead grounded the meter reads 0.I unhooked the main wiring harness connector on the firewall and did some testing on the brown wire. From the switch to the firewall plug on the inside it is 0 ohms. The brown wire pin from the inside firewall connector to the outside firewall connector ( with it disconnected ) is 78ohms. I'm not sure what this means , if anything. I don't understand why both sides are grounded , unless there is a short on both sides which seams unlikely. :dunno
The brown running light wire is number 14 in the diagrams.
 

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