Electrical 2003 Mustang GT - Brake and Blink Fuses Blow

Froggman

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Apr 4, 2020
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I have a 2003 GT Convertible and randomly it blows 3 fused all at once. I can't pinpoint the situation when it blows. It seems totally random during driving. I try and replicate it in my garage by hitting the brakes, using the turn signals. Holding the brakes, and hitting the turn signals etc etc and cant get it to blow

Fuse 13 - Electronic Flasher Module (im assuming this is Fuse 1 on diagram)
Fuse 33 - Brake Pedal position switch
Fuse 41 - Multifunction Switch. High Mounted Stop Lamps

Things I have done so far:
  1. Replaced the Multifunction Switch
  2. Replaced the Brake Switch
  3. Inspected all bulbs - no sagging etc
  4. While turn signals are on, wiggled the wires and sockets of each bulb
I got a Haynes manual and pulled up the circuit diagram but it appears slightly different with the brake switch sharing a fuse with the hazzard side of the multi.

Im not great and wiring and stuff, but does anyone see base on diagram of something else i should focus on that would share all 3?

Could the Electronic Flasher be shorted causing the 3 fuses to blow.

Any help is appreciated.

Shawn
 

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Froggman

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So, it looks like I have found a better schematic.

I have looked at all the downstream wiring and end points.

Since all 3 fuses are blowing at the same time, could the problem be the HOT side of the circuit?
13, 33 and 41 are HOT all the time.

If there was an issue on that side of the circuit grounding, i would assume the other fuse protecting them would blow.
 

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wmburns

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Could the Electronic Flasher be shorted causing the 3 fuses to blow.
IMO not likely. Should be one of the last things looked at.
Since all 3 fuses are blowing at the same time, could the problem be the HOT side of the circuit?
13, 33 and 41 are HOT all the time.
NO! The problem has to be down stream of fuse F2.41 based upon your reported symptoms. Since it also blows UPSTREAM fused, likely this is a dead short to ground.

If you want to know for certain, then replace all of the fused except fuse F2.41. Do the other fuses blow?

If this were my car, this is how I would attack the problem. Take out fuse F2.41. Get yourself an incandescent automotive test light that will "load" the circuit. With the test light pig tail on battery positive, confirm that the test light lights when the probe is touched to a good ground.

Remove fuse F2.41. Put the test light probe on the LOAD side of fuse F2.41. Does the test light light? Does it light BRIGHT? If so this means there's a path to ground.

If the light is bright, start disconnecting loads trying to see IF the brightness of the light changes. Start first with the brake lights. Start from far away and work towards the front.

Also try removing the multi-function switch to see if this causes the test light to change.
 

Froggman

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Apr 4, 2020
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Thanks for the advice.

Once I get some time away from the family i will run through your test advice.

Intermittent problems are such a pain in the ars to diagnose. I wish it would blow on demand...then at least I could narrow down an ares.

I will post what i find.
 
Mar 21, 2019
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If it is intermittent it is likely a wire chafed bare. The flasher can't short internally to cause a fuse to blow as it does not convert power to movement or light, it is simply a switch. Pull the lights in the back and check for ground at each fuse. It sounds like the harness is being rubbed bare on its way to the rear lights. If you don't remove the lights they will show ground, but by removing them it should have an open loop.

The common endpoint is the rear lights. (brake light from brake switch, flasher for turn signals, and multi switch for rear signals as well)
 

Froggman

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If it is intermittent it is likely a wire chafed bare. The flasher can't short internally to cause a fuse to blow as it does not convert power to movement or light, it is simply a switch. Pull the lights in the back and check for ground at each fuse. It sounds like the harness is being rubbed bare on its way to the rear lights. If you don't remove the lights they will show ground, but by removing them it should have an open loop.

The common endpoint is the rear lights. (brake light from brake switch, flasher for turn signals, and multi switch for rear signals as well)
I finally got some time to work on the car.

I pulled the the high-mount brake light and both rear left and right tail lights from the system.

I hooked a test led direct to the positive of the battery and ran the test light on the negative side of f.41. The light goes on, so there is a path to ground through C202B into the multi-switch (im assuming). When I hit the 4ways, the bulb goes out since I'm breaking the connection in right at the source of C202B with engaging the 4ways (At least that's how I'm reading the diagram)

When i engage the right turn signal, light goes out (opening the circuit), but when i hit the left turn signal, light stays on. I am assuming my issue is in my left rear circuit? of could it in fact be the front turn signal?

I did take the bulbs out of the front and it did the same thing.

I went a step further and put the fuse back into F.41, I then took my multi-meter and ran the negative terminal direct to ground and tested across the fuse. 0 Volts steady. I hit the brake pedal and get the 12Volts expected.

However, I get a variable few milivolts when i hit the right turn signal and 4ways. (1-3 +/- milivolts)

Thoughts?

Thanks!
 

Froggman

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I thought i would post what i found, I believe I fixed the issue.

I continued to keep testing and I disconnected the harness from the kick plate on the left to take any dash wiring out of the picture. I tested the right brake wire line to the connector pin and had the ground issue.

So then I started unwrapping the harness until I hit the drivers door were it goes over the back seat and found this!

I pulled the wires apart and my ground issue was gone.

I took a few pics, not sure how it happened, but I'm just glad I found it. I will put everything back together and drive it for a while and hope the issue has been resolved.

Thanks for the advice everyone on how to test for the issue.
 

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