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Hey guys question, bought a truck off of a neighbor. 1999 7.3 f350 drw. It didn’t have any brake or turn signals when I went to use them when I bought it. Odd. But changed the relay up under the dash thinking it was that and nope, no hazards, no brake lights, no turn signal, the wipers work, and all that. As soon as I put the 20a fuse into the block it was instantly pop them. Why is that? Thinking the whole switch assembly? Any help would be appreciated
 

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Could be the switch, or a chafed wire that's grounding out. Unplug the switch and see if the fuse blows then (or use a multimeter to keep from blowing fuses). If it doesn't show a short with the switch unplugged, it's either the switch or a wire downstream of the switch. If it shows a short, it's between the fuse panel and the switch.

Good luck
 

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Discussion Starter #3
Could be the switch, or a chafed wire that's grounding out. Unplug the switch and see if the fuse blows then (or use a multimeter to keep from blowing fuses). If it doesn't show a short with the switch unplugged, it's either the switch or a wire downstream of the switch. If it shows a short, it's between the fuse panel and the switch.

Good luck


replaced the switch and same problem 😑
 

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I'd start by crawling under the truck and looking at any and all wire harnesses that you can see. Pay attention to areas like around the rear bumper and perhaps the front of the bed and see if there are any wires hanging around or drooping down.

A problem with looking for shorts with a meter is that unless you pull all the light bulbs you will see what looks like a short due to the filament of the bulbs.
 

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Discussion Starter #5
I'd start by crawling under the truck and looking at any and all wire harnesses that you can see. Pay attention to areas like around the rear bumper and perhaps the front of the bed and see if there are any wires hanging around or drooping down.

A problem with looking for shorts with a meter is that unless you pull all the light bulbs you will see what looks like a short due to the filament of the bulbs.

Didn’t see anything but the previous owner beat this thing half to death. I’m kind of thinking of just cutting it up and making a whole new harness it would be easier
 

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Making harness are not for the faint of heart. There is a lot more involved than just a few wires.

It may be time to find or get access to a EVTM manual for the truck. Then start disconnecting the bulkhead connectors to determine if the problem is in the cab or outside somewhere.

I'd even talk to the guy that your purchased it from and ask if he did any extra wiring on the truck.
 

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Discussion Starter #7
Making harness are not for the faint of heart. There is a lot more involved than just a few wires.

It may be time to find or get access to a EVTM manual for the truck. Then start disconnecting the bulkhead connectors to determine if the problem is in the cab or outside somewhere.

I'd even talk to the guy that your purchased it from and ask if he did any extra wiring on the truck.
He just put the flat bed on it and the 7 way plug extension to pull a flat bed. I’ve been working with VANAIR and Muncie at work the last few months making our own harnesses for the god awful set up on the new 2020 f series so I think I can swing it as long as I can my time
I didn’t see any breaks or anything all the way up.
 

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Did you check with the switch disconnected? You said it blows the fuse as soon as you put one in, so it has to be shorted either between the fuse and switch or in the switch. Disconnect the switch and stick a fuse in. If it blows, it confirms the problem with wiring between the fuse block and switch.

A problem with looking for shorts with a meter is that unless you pull all the light bulbs you will see what looks like a short due to the filament of the bulbs.
That's true to some extent. It will show very low resistance with the bulbs in place (may not be much difference that a true short). But a good comment. With the switch in the off position, it should only affect checking the downstream circuits.

But removing and checking ALL the bulbs is usually a good starting point for any lighting circuit issue. A bad bulb is a cheap fix.
 

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Discussion Starter #9
Did you check with the switch disconnected? You said it blows the fuse as soon as you put one in, so it has to be shorted either between the fuse and switch or in the switch. Disconnect the switch and stick a fuse in. If it blows, it confirms the problem with wiring between the fuse block and switch.


That's true to some extent. It will show very low resistance with the bulbs in place (may not be much difference that a true short). But a good comment. With the switch in the off position, it should only affect checking the downstream circuits.

But removing and checking ALL the bulbs is usually a good starting point for any lighting circuit issue. A bad bulb is a cheap fix.



I tried with it unplugged and it didn’t pop, put a new switch and plugged it in, instantly blew
 

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Next would be the bulkhead connector where those wires pass through the firewall.

Sorry, I can't help you on which one it would be
 

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Tell me what fuse is blowing and I can do some research for you. If you have to, take a picture of the fuse block.
The one that feeds the brake pedal position switch is #13. Fuse #1 feeds the flasher relay. Both are 20A.
From the fuse block it there is a light green/red stripe wire going to the brake pedal switch and a red/white stripe wire to the flasher relay (pin 85 on the relay.). Those wires should be hot at all times. On the opposite side of the brake pedal switch (light green wire) there should be no voltage unless the pedal is pushed.
 

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Discussion Starter #12
Tell me what fuse is blowing and I can do some research for you. If you have to, take a picture of the fuse block.
The one that feeds the brake pedal position switch is #13. Fuse #1 feeds the flasher relay. Both are 20A.
From the fuse block it there is a light green/red stripe wire going to the brake pedal switch and a red/white stripe wire to the flasher relay (pin 85 on the relay.). Those wires should be hot at all times. On the opposite side of the brake pedal switch (light green wire) there should be no voltage unless the pedal is pushed.

it keeps blowing #1 and #12 on the fuse panel under the dash in the cab
 

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it keeps blowing #1 and #12 on the fuse panel under the dash in the cab
Fuse #1 feeds the coil on the flasher relay, and nothing else. Maybe your relay is toast, but you said you already changed that. Look for a short in the wire that feeds the flasher relay. It's a red/white wire running from the fuse panel to the relay block.
Fuse #12 feeds the horn relay (thru the switched contacts) to the horn. That should only blow if you have a shorted horn wire and you mash the horn on the steering wheel. There is a yellow/Lt blue jumper wire going from the fuse panel back to the fuse panel between pins 11 and 29 on connector 242A (one of the two connectors on the back of the fuse panel). Possibly that wire is shorted if the fuse blows immediately on replacement.
 
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