projectSHO89: FIRST OF ALL LET ME SAY THANK YOU FOR THE HELP. I GREATLY APPRECIATE IT.

So I took a look at the brake pressure connector and it was dry. I did order a new sensor a while ago, so since I had it lying around I replaced it. Mostly because the potential fire issue and the new one having a fused wire that comes with it. As you expected, that wasn't the problem. I still have no brake lights.

I was able to confirm that I DO NOT have power to the brake pedal switch either. I assume the power is going from the brake switch to the multi-function switch on the steering column?
Also, fuse #13 keeps blowing.
While poking around a little I did find a wire with some corrosion and a cut in it that is exposing the wires. I don't know what the wire is for.....probably something unrelated. I attached a photo though.

So if I don't have power to the brake pedal, where should I be checking for power next? It would have to be an issue "upstream" from there, but I don't know where that is.

Just a recap of what I know/have found for sure.
- There is no power for the brakes at the rearmost connector of the truck (at the tail lights).
- There is no power at the brake pedal switch.
- I THINK that connectors (which did have some disconnected....one being the brake light wire LG/Red....) are now firmly connected.
- The brake pressure switch is new and was not the issue.
- Fuse #13 in cab keeps blowing.

I looked at the circuit diagrams, and to be honest.....all i was able to figure out was that the wire for the brake lights is a LG w/red stripe.
Any advice for where to check for power next would be much appreciated.
The cut/slice in the wire is just below where i'm pointing at it with the screwdriver. It's a red wire with a blue stripe. Sorry it's hard to see

 

If F13 is blown, you will have no power to the brake light switch and will never have brake lights.

Does the fuse blow immediately upon insertion or only after pressing the brake pedal?

The coloring on wires tends to fade over the years, sometimes leading to misidentification of wires. You may need to open up harness coverings to see unexposed wire insulation colors.

 

projectSHO89:
Good to know about the fuse. The fuse does not blow immediately when replaced. I will check it again today after having driven it yesterday. I'm guessing it will be blown. Should I attempt to apply power to the wire at the multifunction switch? I only ask because if the lights were to work when doing so that would mean everything after that point in the circuit is fine.

In the electrical circuit, what would be the flow of power in the circuit?

Ie. Battery --> PCM? --> Brake Pedal switch? --> in cab fuse panel? ---> multifunction switch?--> etc?

I'm not sure the order. That's what i'm trying to figure out so that I can back track test the circuit. That is to say testing for power at the rear tail lights all the way up to the battery. Hopefully that makes sense? Thanks again for the help.

 

The brake light fuse will blow if there is a short circuit before the light bulb. The bulb is the resistance that reduces the current. After you insert a new fuse watch it closely while you press the brake pedal. If it blows then you have a short circuit.

Do you have a trailer light plug in the back? If they get dirty they can short circuit. People often leave them uncovered and exposed to the elements back there where they collect dirt and road salts. I had that happen on an old Pathfinder.

 

Sounds good. I'll have to check and keep a closer eye on that. I know it doesn't blow right away upon installing a new one, but I'll have to see if it blows right after pressing the brake pedal. Thanks for the tip. I also didn't think about what you said with the trailer plug. I believe it looked pretty clean the last time I checked, and it is securely covered, but i'll take another look. If this were the case, would the fuse blow right away?

I need to work on my electrical skills. I can fix anything if it's mechanical, but suck at electrical. lol Thanks again.

 

The trailer tow circuits are isolated and separately fused from the F13 circuit. The trailer wiring harness cannot cause F13 to blow unless, perhaps, an electric trailer brake module is installed.

Yes, this is the next step in isolating the short.

Follow the red arrows in this annotated schematic. They are arranged in order of circuit operation.

Also, be sure to leave that brake pressure switch unplugged for the duration of testing. We don't need an avoidable complication.

 

My Pathfinder just used a separate harness that was inserted in to the main harness at the back. No relays or separate fuses involved.

The short circuit could be on the branch for the "trailer adapter circuits", according to your schematic.

OP would get ahead by learning how to use a meter. More effective, cheaper, and easier than watching fuses blow. If the short is not present when the truck is not moving then this might be an intermittent problem. That will make finding the short more difficult.

 

Yes. However, the "trailer side" as I noted, only goes to a pair of relays in the add-on relay box under the hood. Very limited (but not zero) risk of the short being there.

Agree with all of that. That said, learning to use a meter EFFECTIVELY is a skill that takes a good bit of development. In my case, I was in the Army working as a missile electronics tech and it was 2 or 3 years before I really, really learned how to use a meter.

 

So I checked what you were saying just a few minutes ago....
I installed a new #13 fuse and it was fine until I pressed the brake pedal. As soon as I pressed the brake pedal the fuse blew. So I should probably unwrap the wire loom, starting at the brake light working towards the front of the vehicle, and inspect all wires for damage. Basically anything exposed that could be causing a short? (I suppose I can eliminate any wires that do have power going to the brake lights and that always work.) It would be something after (downstream in the circuit) from the brake pedal switch correct? Thanks! At least i'm getting closer to figuring this out. Appreciate the help guys. Thank you for your patience.

P.S. Being an electronics tech on missiles sounds pretty badass! Thank you for the diagrams and making it a "for dummies" version. lol Honestly.

 

I went back over your posts. projectSHO89's advice is based on what the factory gave you. Maybe you could talk a bit more about the $15 trailer lights and how you "wired them in". Did you use the factory trailer light relay system or tap in to the existing harness? Wired in at the trailer or from the truck main harness?

Also, since you've had the bed off and built a new one I can imagine possible ways to damage wiring after miles of driving. How about a picture of this beast?

From above -

" I found some trailer lights for $15. I wired them in and they were flashing quickly....
I only mention it in case there is somehow a link between this and the brake lights not working. I don't believe there is though because the brake lights WERE working normally for a good while while having these current lights installed."

 

p.s. there are at least 5 connectors on the way from the switch to the lights. Find the ones that you can, disconnect them, and do the fuse test. When the fuse stops blowing you'll know that the problem is after that connector. I'd start with the one closest to the lights.

 

Before tearing anything apart, test the turns signals and hazards at the rear to see if they work or if one of those fuses blow. If neither of those two fuses are blown, the short is not "downstream" from the MFS. If blown, then it is at or upstream from the MFS and is limited only to the stop lamp circuit.

 

The turn signals and hazards work. The turn signals flash fast, but that is because of the new LED assemblies I talked about. They are not causing the lack of brake lights. I know that because the brake lights were working for hundreds of miles of driving while I had the LED lights installed.

So, you say if those work to look upstream of the multifunction switch? I can't read this diagrams for sh#t honestly, (they have references to check other pages etc) so can you dumb it down to a complete morons guide for figuring this out? I'm literally at the point where i'm just going to start unwrapping all of the wire loom around that wire starting at the back and working my way toward the front of the vehicle, looking for damage or a loss of power. I'm about to loose my Fing mind with this honestly. IF pressing the brake pedal is causing the fuse to blow immediately, wouldn't that mean that the issue is downstream of there? Is the multifunction switch upstream of the brake pedal switch?

Honestly....what would help me at this point....if you care to..... is like I said a complete morons guide.

Ie. Upstream end: battery to XXX to XXX to XXX to XXX to XXX etc. Check the XXX points for power. Just saying. I appreciate all the help, but i'm just getting myself more confused here and am extremely frustrated.

I will try disconnecting the connectors along the way starting at the rear as BareBones suggested and pressing the brake pedal. I don't care if it costs me a few $ in blown fuses.

Thanks guys. Sorry for the frustrations. I have another issues with a squeaking belt that is driving me insane as well. It should be something easy to diagnose and fix, but it isn't. I even started a thread about it. Sounds stupid, but trust me it's something extremely weird and it's causing me to pull my hair out. I'm getting to the point where I don't even want to drive this damn truck anymore because it is just annoying the hell out of me.

 

It's definitely a complex circuit. Surprisingly messy. According to the diagram, (the D arrows) the multifunction switch receives power when the brake pedal is pressed. Not sure what the overall logic is but that's what the diagram shows.

But, overall, if you look at diagram 090-003 you can see that there are many, nine, "things", that receive power when the brake pedal is pressed. Look at the main connection point branching out in a star pattern. Probably for sensing braking, among other things.

Any one of those circuits could have a short to ground. Even though the trailer brake light circuit might be fused, there is a circuit to the "trailer electronic brake controller", for example.

At this point you might be best off to just do the things you understand. I would unplug all of the brake lights, including the overhead brake light over the bed and see if the fuse still blows. If it does then get in to the more complex devices, focussing on any that you migtht have messed with. If it doesn't blow then it's probably one of the things you unplugged.

p.s unfortunately you really need a Ford Wiring Diagram book to know where the connectors are. without it you'll just have to search physically.

 

The brake light signal has to go through the MFS because the same circuits downstream from it are the turn/hazard lights which are the same.
Correct. Lots of stuff needs to know when the pedal is pressed.
Only the third brake light needs to be disconnected. Leave the regular ones alone since they work as signals/hazards and are not the issue.