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Old 09-23-2012, 09:54 PM
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Default #18 fuse is blowing when I hit the brakes....no brake lights.

Ok, so got home tonight, and went to back up the driveway and noticed I had no brake lights. Running lights are working fine. Checked the manual...and sure enough the #18 fuse is blown. I replace it, and the soon as I hit the brakes, the brakes light up for a millisecond, and the fuse pops again.

Help.... I absolutely SUCK with electrical issues.....I already looked under the dash and couldn't find anything abnormal. Really need some help/troubleshooting assistance.

Thanks in advance, fellas.

From the Manual:
Quote:
#18 (20A*) Turn lamps/Brake on-off switch (high)
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Old 09-23-2012, 10:22 PM
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I should add the 3rd brake does not work, blinkers do not work, (front or rear) dash-turn indicators do not work, and 4-ways do not work.

FML.
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Old 09-23-2012, 10:33 PM
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#18? Isn't that the fuel bowl heater circuit? If so, there's your help.
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Old 09-23-2012, 10:41 PM
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Quote:
Originally Posted by TyCorr View Post
#18? Isn't that the fuel bowl heater circuit? If so, there's your help.
No, it's the brakes and turn signals. Per the manual:
Quote:
#18 (20A*) Turn lamps/Brake on-off switch (high)
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Old 09-24-2012, 10:35 AM
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Look for a wire shorting to ground. Probably back by the brake lights.
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Old 09-24-2012, 12:26 PM
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see if you backed in your trailer plug
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Old 09-24-2012, 01:04 PM
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Quote:
Originally Posted by Arisley View Post
Look for a wire shorting to ground. Probably back by the brake lights.
Spent a LOT of time doing that. Found nothing but rust and grime.

Quote:
Originally Posted by mudwizer View Post
see if you backed in your trailer plug
Nothing there, and I haven't since I owned the truck.

Update:

I think it is fixed. I got a tip on another forum to check the relay under the hood on the driver's fender and switch the 2. (Electronic Wiring Manual says one is for A/C, the other is for "Flasher Relay" which does not appear in the normal manual.)

Switched them...brake lights/blinkers work again, and no popped fuse! Woot!
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Old 09-25-2012, 08:33 PM
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Now you need to go and pick up the proper relay so your AC works when you need it.
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Old 09-25-2012, 08:58 PM
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That's strange. I would've guessed a short to ground around the back of the truck.
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Old 09-25-2012, 09:09 PM
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Next thing I would do is getting a wiring diagram and test for resistance in the wires..
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Old 09-26-2012, 03:21 AM
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Quote:
Originally Posted by Powerstrokeracing View Post
Next thing I would do is getting a wiring diagram and test for resistance in the wires..
Lol he doesnt know how to do that.
He HATES electrical problems.
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Old 09-26-2012, 06:31 AM
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Wow. Relays fail in the strangest ways. I always have a couple of spare relays in my truck. At the age these things are getting, you never know when one is going to die.
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Old 09-26-2012, 06:48 AM
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Quote:
Originally Posted by PatriotHauling View Post
Lol he doesnt know how to do that.
He HATES electrical problems.
^Yup.
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  #14  
Old 09-26-2012, 07:42 AM
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I'd say maybe a shorted brake light switch.. I don't have diagrams for an 02, maybe I can find online..

try this link
Michigan Electronic Library

use login M123456789012
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Last edited by JLDickmon; 09-26-2012 at 07:48 AM.
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Old 09-26-2012, 02:49 PM
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Quote:
Originally Posted by JLDickmon View Post
I'd say maybe a shorted brake light switch.. I don't have diagrams for an 02, maybe I can find online..

try this link
Michigan Electronic Library

use login M123456789012
I figured it out...it was a relay. (See post #7) Thanks for the link, anyway.

Also, there is a link under the tech files with a PDF that has the electrons diagrams (ALL of them) for an 02 250/350/EX. VERY useful, only thing I wish it did better was show a diagram of WHERE on the truck the relays/fuses are.
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Old 09-26-2012, 08:00 PM
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Quote:
Originally Posted by ToMang07 View Post
I figured it out...it was a relay. (See post #7) Thanks for the link, anyway.

Also, there is a link under the tech files with a PDF that has the electrons diagrams (ALL of them) for an 02 250/350/EX. VERY useful, only thing I wish it did better was show a diagram of WHERE on the truck the relays/fuses are.
It's called a "component locator" and there's one in the link I provided..
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Old 09-26-2012, 09:34 PM
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Quote:
Originally Posted by JLDickmon View Post
It's called a "component locator" and there's one in the link I provided..
I see/understand that. I was merely commenting on the on I used, which did not have a locator.

And again, thanks for the link!
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  #18  
Old 08-27-2015, 03:58 PM
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Same problem trail work fine on 2 other ford trucks keeps blowing fuse #18 .
  #19  
Old 09-01-2015, 11:11 AM
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Quote:
Originally Posted by viper59 View Post
Same problem trail work fine on 2 other ford trucks keeps blowing fuse #18 .
Mine ended up being the brake pedal switch in cab under the dash. It takes all of 5 minutes to change.
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  #20  
Old 06-22-2020, 03:46 PM
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Talking My fuse#18 issue

Thank you all above for the input and things you found. I thought it would be appropriate to share my experience tracking this down on my 2003 Excursion.

Every single time I touched the brakes... *POW*, #18 blew. Started looking for obvious stuff, but nothing.

Disconnected trailer plug etc. via the connector behind the license plate. Tested *POW*

Moved up to the mid-frame area on the drivers side. Disconnected the back end lights from there. *POW* Good thing I have a 20 pack of 20 AMP fuses.

Disconnected those same cables from under the hood right in front of the firewall. *POW*

Disconnected the brake switch. Did not blow, but no brake lights of course. Manually jumped pin 4 and 5 of the brake switch connector. Found a schematic online and those are the contacts bridged by the switch on the 20 amp circuit. *POW* on the manual jumper. Something elsewhere...

Pulled all the relays, etc from the two block areas under the hood just in front of the driver. *POW* The issue is getting in a smaller and smaller area.

Pulled the entire fuse block / power distribution block lose under the dash. 4 bolts 10mm heads. Got a good look behind. Saw nothing. Ready to give up. Tried one last fuse (still have 10 or so left)... maybe it was MY last fuse. :-) Tried the brake, no blow! What?!?

Added relays, etc. back. Tested. *No blow*

Added connectors under hood. *No blow*

Added mid frame connectors. *No blow*

Added connector behind license plate *No blow*

Mounted fuse block / PDB *No blow*

Everything is back together and working. Driven 150+ miles since and it is still working. Not the outcome I exactly wanted as I wonder when (not if) it will pop up again. Hopefully the extra details of the things I did will help someone else down the line with diagnostics.

2003 Excursion 6.0L Limited
2000 Excursion 7.3L Limited
1971 Suburban dead 350 :-(
  #21  
Old 06-23-2020, 07:39 AM
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Probably one of the bolts holding in tthe fuse box was touching a wire in the brake circuit. Over time it wore through just a tiny bit. Hard to see. When you pulled the block you moved the wire and when reinstalled, it was no longer toughing the bolt.
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  #22  
Old 06-23-2020, 07:41 AM
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Aaack! I hate mysteriously self repairing issues!
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