2002 Mustang Tail Light Wiring

shock4878

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To keep a long story short my tail lights went out and I managed to completely kill the circuit trying to fix them. When the tail lights first went out my turn signals, cruise control, and hazard lights all continued to work.

When I first noticed they were out I replaced the brake light switch on the brake pedal and then noticed that my hazards, cruise control, and turn signals all stopped working (internet says they're all on the same circuit). The brake lights did not start working again and now neither does the hazards, cruise control, and turn signals. So that led me to believe it was a wiring problem and I have been scratching my head for days trying to figure out what to even check.

First and foremost: I'm getting 9.7V on the brake light switch when it's in the neutral position (not pressed) and 0.0V when it is pressed (key is turned on for power). Is that correct? I think it is, but I could be wrong.
I took the steering wheel cover off and have started messing with the multi-function switch (turn signal switch) on the steering column. I have the diagram below to go on but am unsure how to really interpret it so maybe somebody can help?
Multi-switch diagram I'm unsure of how to really test it in the various positions considering I was plugging the meter into the ports that go into the switch, I think it would be really tough to test it plugged in. So directly from the pigtail that goes into the switch with the key turned I have the following voltages:
  • 44 - 6.7V
  • 385 - 6.1V
  • 3 - 0V
  • 2 - 0V
  • 511 - 0V
  • 5 - 0V
  • 9 - 0V
I'm thinking maybe my problem lies here. I tried using a working switch out of a 95 Mustang GT but I believe they're different switches (even though they look identical). The 95GT multi-function switch turns the windshield wipers on/off but no differences in wiper speed and no lights/hazards. The 2002 multi-function switch has varying speeds on the windshield wipers but no lights/hazards. Should I replace the multi-function switch or the wiring to it? Or would that even be a problem?

I've also checked the fuses in the fuse box below the dash and haven't noticed anything blown that is necessary for function. I'm going off this diagram under dash fuses the fuses I notice are 11 (reverse lights), 17 maybe?, 33 (brake pedal switch), and 41 (multifunction switch).

Looks like 33 and 41 are the most important and both of those fuses aren't blown.

As a side note, the brake lights will illuminate when I turn the headlights on and I think that maybe the brake lights worked for awhile and the third brake light didn't.

Thank you so much to anybody who has the expertise and kindness to help.
 
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wmburns

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For me to be able to help you I will need us to refer to the fuse numbers as called out in the following diagrams.

1999-2004 MY fuse panel schedule:
https://www.allfordmustangs.com/for...r-swap-wont-fire-please-help.html#post2669271

Confirm there is power in and out of fuse F2.18, F2.13, and F2.33. Use a test light that will "load" the circuit. Use a KNOWN good ground. So we are going to test both sides of the fuse to confirm power. Even better. Back up the test with a VOM to verify that full battery voltage is seen at the fuses.

Next test for power in and out of fuse F2.41 and F2.35 with the brake pedal depressed. Again test both sides of the fuse using a test light.

Where to go next depends upon the test results.

Note, if all of the fuses are confirmed OK, then likely your problem is either:
  • bad flasher module
  • bad flasher module ground G204.
  • wiring fault.
Note, If this were my car I would CONFIRM that the tail lights are correctly grounded and the ground is good and able to carry current. Because if the tail lights are not grounded then the brakes lights will back feed through the parking lights. This could also cause some strange voltage readings.

Here's some information on how to perform voltage drop testing.

Howto perform charging system voltage drop test
https://www.stangnet.com/mustang-fo...perform-charging-system-voltage-drop-test.56/
 
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wmburns

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What about F2.41 and F2.35 with the brake pedal depressed?

Did you do anything to "load" the circuit or just use a VOM?

Is the voltage at the battery also 11.5 volts?

What about confirming the tail light ground?
 

shock4878

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I haven't checked F2.41 or F2.35 yet, but I will in the morning. I got frustrated with it and pulled the turn signal relay and decided I'd replace it too.

Also, I am a complete beginner at electrical work and just recently learned how to use a VOM. But I used it to check. I believe the tail light grounds are okay, would they be bad if I could still turn the lights on when I pull the headlight switch? That's really what is boggling my mind the most right now is why I can't make the brake lights work but I can still get the tail lights to illuminate.

Also, yes. The voltage from the battery is 11.5 and that is only because I have left the key turned for a long period trying to test the circuit. I know the charging system is efficient because I recently replaced both the battery and the alternator.

I really appreciate your help. I can't find a local mechanic that will look at it for me for under $500 and the Ford dealership said it would take several diagnostic hours to discover the problem before they could repair it and that could cost anywhere from $300 to $1000 and the car really isn't worth $1000 brake light repairs. I'm going to get a new multi-function switch when I get the turn signal relay. I just didn't want it to be something stupid that I'm missing.

Also, something I just discovered, 2 or the 3 fuses you asked me to check were blown. I replaced them and still no lights but that is an interesting discovery. Does that mean it's not grounded properly somewhere?
 

wmburns

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Also, I am a complete beginner at electrical work and just recently learned how to use a VOM. But I used it to check. I believe the tail light grounds are okay, would they be bad if I could still turn the lights on when I pull the headlight switch? That's really what is boggling my mind the most right now is why I can't make the brake lights work but I can still get the tail lights to illuminate.
Well......trying to figure out how best to answer this. Just wondering if you can think of a reason why I would mention to check for something if there's no way for it to cause a problem.

The other way to answer your question is to repeat what was mentioned in the first place. That IF the tail light ground is un-grounded or "floating", then the brake lights will back feed through the parking lights. To understand WHY this happens one just has to look at how the lights are designed. That is TWO filaments of different brightness sharing a common terminal (ground). Depending on the quality (lack of quality) the lights may actually light but not bright enough to be easily seen.

Fuses do not just "blow" for no reason. It's best (if possible) to find the reason. Unless the fuse blew because of your trouble shooting. Remember there are tons of "always on" circuits in today cars (the brake lights are always hot).

OBTW, the blown fuses might explain the low voltage readings at the mult-function switch reported in the opening post. Which is why we start trouble shooting by covering the basics.

I'm very good with electrical trouble shooting. I have a full set of Ford Factory wiring diagrams to refer to. I'm confident that if you do (and post) the requested test results I can help you to trouble shoot this issue.
 
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shock4878

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Sorry it's taken me so long to get back to this, I actually took the bar exam last week and have finally gotten back around to figuring out my problem. I ordered a new flasher relay and multifunction switch, replaced both and still nothing.

I have also checked and replaced all fuses. I feel like I'm over complicating this.

So, I will follow any and all instruction you can kindly give me. At this point I'm completely out of ideas having replaced everything except the wiring.
 

wmburns

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I made several requests for test results and other information. Start over from the top. Review the information given. Do the tests. Post the results.
 
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shock4878

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I did the test on the battery and it seems to be in working condition. I replaced both it and the alternator less than 6 months ago. The battery was making 11.5V.

As far as grounding the tail lights, how would I do that? Would it matter that the lights still illuminate when the headlights are turned on or does that circuit share no similarities with the brake light circuit?

Also, I will test both F2.41 and F2.35 with the brake pedal depressed and report back shortly.
 

shock4878

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Okay, scratch that. Battery is making 12V.

Not pressed to pressed - F2.35 - 3V to 11V and F2.41 - 3V to 11.7/11.8 it fluctuated. But would eventually hang at 11.7.
 

wmburns

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Not pressed to pressed - F2.35 - 3V to 11V and F2.41 - 3V to 11.7/11.8 it fluctuated. But would eventually hang at 11.7.
Are the parking lights on or off? Where did you ground the VOM?

The fact that the F2.41 brake pedal depressed of almost 12 volts indicates that the brake pedal switch is working. This also means the fuses that feed the circuit are good.

The fact that the F2.41 open voltage of 3 volts indicates there is "back feeding" of the circuit occurring. What we don't know is how much current is back feeding because the VOM does not "load" the circuit (like a test light does). Nor do we know where the back feed is coming from.

What could could "cause" back feeding in the brake circuit? An open or weak trunk ground is one "possible" cause. Another possibility is a wiring fault or poor connection inside the multi-function switch. More tests will be needed.

What we need to know is IF the brake power is making it back to the rear. Do the high mounted 3rd brake lights work?

What are the voltages at the 3rd high mounted brake light with the brakes off. With the brakes pressed?

IMO you may be well served to double check any work done in the trunk. Check the connections. Look for pinched, kinked, or over heated wires.
 
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shock4878

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Buddy, lemme just tell you what a trip I've had this evening. I got so pissed off that everything was getting power and none of my fuses were blown that I completely took one of my tail lights out and started testing it's voltages, that was the culprit. Some of the wiring to my driver side taillight was fried and the connection wasn't touching anymore. I ran a wire from it to the brake pedal and voila, working brake lights.

I sincerely appreciate your help, you've helped me rule out everything under the sun when it's came to fixing this problem. You weren't kidding when you said you were confident we could troubleshoot this. You are my hero.
 

shock4878

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I think maybe I bypassed the wrong wire and don't have turn signals or hazards now. However, I do have cruise control and all brake lights.
 

wmburns

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The power for the brake lights splits with some passing THROUGH the multi-function switch. By running a wire from the brake position switch directly to the rear brake light assembly you have BYPASSED the multi-function switch.

My recommendation is to REPAIR the wiring damage back to stock factory.
 

wmburns

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There's a fairly complex set of wires to/from the multi-function switch. If interested in getting a full copy of the Ford wiring diagrams I may be able to help. PM if interested.

Previous post indicated that you found wiring harness damage. Why do you want to re-wire a car instead of fixing obvious wire damage? It seems to me that anyone capable of running a new wire from the front to back of a car can cut out part of a damaged wire and splice in a new run.
 
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tessatrolo

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I am hoping that someone will see this but I recently got a 97 cobra and I am having a similar problem, I recently got new headlights and after I installed them the tail lights dont work. Every other light works but the rear parking lights. Ive checked all the fuses including #5 multiple times, I took out the headlight switch and it works fine but I noticed that three of the wires in the connector dont get any power. (interior lights, dash lights, headlights, flash to pass, brake lights, third brake light, side markers all work but the parking light for the rear) Please help!!
 
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