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Discussion Starter #1 (Edited)
'2013 Ford F750 Brake Lights Not Working

Ok so I have a 2013 Ford F-750 with a Cummins Diesel. The issue is that when the brakes are applied, the brake lights do not illuminate! Now here's the kicker, I work for a Truck Fleet and we DO NOT have any schematics or wiring diagrams so we are working blind on this. We have tested all the wiring from the lights all the way to the relays and fuses and have continuity. We even applied power directly to the lights themselves and they work. All other lights are functioning to include the parking lights. We check the switch at the pedal and even swapped it for shi+s and giggles and still nothing. We tested the relays and checked the fused and everything seems fine. ANY HELP would be greatly appreciated...I'm at the end of my nerves with this issue. TIA.
 

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This may not be much help, but when I'm doing electrical diagnostics, I try to break the circuit into pieces to try to isolate a segment at fault. I would start at the brake switch. I would check for 12 volts on one leg. If you don't have power to the switch, then I'd go looking for why. If you do, then I'd jumper the connector and see if the brake lights came on. If they do, then the switch (or it's adjustment) is at fault. If they don't, then the problem is between the connector and the switch. Let's say they didn't come on. Then, I'd find the connector near the rear bumper that the frame wire connects to where it splits to the two sides. Check for power there when the brake switch is applied. Yes - problem is further downstream. No - problem is between the switch and that connector. By halving your area of interest each test, you'll eventually get it narrowed down to a specific spot.
 

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Still not having any luck with this issue.. I checked the cable that goes to the brake light switch and none of the connectors have any voltage. What I need to know is where does that cable get voltage from, passenger side junction box or outside driver side junction box. Better yet, if anyone has any wiring diagrams that would help even more.. THANKS
 

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Oh, before you go nuts, do us both a favor and exercise the hazard switch on the top of the steering column. Turn them on and off a dozen times and wiggle the button a little. Then see what works. The hazard part of the multifunction switch tends to gather dust and God knows what other crap that causes the contacts to foul. The stop lamp circuit runs through the multifunction switch. If doing that corrects the stop lamp concern you are probably good to return the truck to service. If the concern returns then I would replace the multifunction switch. Read on...

Also, forget about relays. Anything labeled "Stop Lamp Relay" is not for the lights. The relays are for the Caterpillar or Cummins ECM and simply isolate the modules from the lamp circuit and one other for the trailer tow brake light circuit.

The simple run down is Fuse F1-17 in the BJB (under the hood) supplies power directly to the brake pedal switch at the LG/RD wire. When the pedal is applied power is supplied into the multifunction switch at the LG wire and out from the multifunction switch on the LG/OG wire for the left rear and OG/LB wire for the right rear.

HTH
 

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@Ford_Doc: We have already put a brand new MF Switch and still nothing. I checked the cable that runs to the brake light switch at the pedal for voltage and I'm getting 0 volts at all four connectors. I have a Green, Tan, and two stripped black wire on that cable. All the fuses at the junction box are good.
 

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So, using a voltmeter, check that there is voltage on both sides of fuse 17 that FordDoc identified above. If there is, then you've identified the segment with the issue. Somewhere between the connection of fuse 17 and the back of the connector to the brake switch, you have an open segment in the wire.
 

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Okay this is silly, I attached the full Turn/Stop/Hazard schematic for you. Hope this helps.

FYI for the cost of the parts you already threw at this truck your company could have purchased a years subscription to the Motorcraft website with access to all the vehicle documentation available.


https://www.motorcraftservice.com/Home/Index
 

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Discussion Starter #12
Okay this is silly, I attached the full Turn/Stop/Hazard schematic for you. Hope this helps.

FYI for the cost of the parts you already threw at this truck your company could have purchased a years subscription to the Motorcraft website with access to all the vehicle documentation available.


https://www.motorcraftservice.com/Home/Index
Brother, youre preaching to the choir... I've told my boss a million times... they just dont want to spend the money on stuff like that...lmao
 

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Discussion Starter #13
Okay this is silly, I attached the full Turn/Stop/Hazard schematic for you. Hope this helps.

FYI for the cost of the parts you already threw at this truck your company could have purchased a years subscription to the Motorcraft website with access to all the vehicle documentation available.


https://www.motorcraftservice.com/Home/Index
I'm in talks with my Boss about getting access to the motorcraft website. Not sure how this will go but hopefully it will go well. @ford_doctor, thanks for the wiring diagrams, but I dont know if you noticed or not, but you sent me diagrams for a 2002 not a 2012. Thanks though.
 

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Could swear it read 2002... now the title reads 2012 and your now edited first post saus 2013. Which is it? I can get you the right diagram... jeese! I know my eyes are beginning to go but I know I read 2002 with my glasses on. LOL... no bother.
 

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See what happened was... I fat fingered it and put 2012 by accident, apparently I did it twice, but it 100% most definitely is a 2013 Ford F-750 With a Cummins... my bad.. lol Sorry bout that @ford_doctor
 

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Okay - the 2013 system is a different system.

Normal Operation — Hydraulic Brakes

Voltage is supplied from Battery Junction Box (BJB) fuse F5 (5A) to the dual brake warning switch. Voltage is supplied from the Battery Junction Box (BJB) fuse 7 (15A) to the ABS event relay switch and coil. When the brakes pedal is pressed the dual brake warning switch inputs a voltage signal to the hydraulic brakes module. The ground side of the ABS event relay is controlled by the hydraulic brakes module. When the ABS event relay is energized voltage is routed to the multifunction switch and the body builder stoplamp relay coil.

Air Brakes

Voltage is supplied from the BJB to the stoplamp switch(es). When the brake pedal is applied, the stoplamp switch(es) routes voltage to the multifunction switch and the body builder stoplamp relay coil.

All Brake Systems

When the brake pedal is applied, the multifunction switch routes the voltage to the LH rear lamp (both LH and RH circuits), and to the LH and RH body builder turn relay coils. Voltage for the RH rear lamp is then routed out of the LH rear lamp to the RH rear lamp. Ground for the RH rear lamp is routed into the LH rear lamp. The ground circuit is spliced internally to the LH rear lamp.

The BJB supplies voltage to the body builder stoplamp relay switch side through. When the body builder stoplamp relay is energized, voltage is routed to the body builder connector.

The body builder turn relays (switch side) are supplied voltage at all times. When the LH or RH body builder turn relay is energized, voltage is routed to the body builder connector.
 

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Update: So the brake light issue has been fixed! I ended up getting a 72 hour www.motorcraftservice.com subscription just to see how helpful it would be, and by-golly it was amazing, everything that I needed was in there!! Turns on a connector close to the BJB had a very small wire that was corroded, I repaired the circuit and all is well now! EXCEPT, now my back up lights and the warning beeper are not working, I honestly don't know if they ever worked simply because when we got the truck we never had to put it in reverse, until I was backing it out of my bay and that's when I noticed that issue. So here's the problem, I have the schematic and the workshop manual for this problem, I have encountered that even with the VIN inputed into the motorcraft website, there are slight differences with the truck im working on as opposed to the truck in the workshop book. The workshop manual tells me to check for voltage at the Vehicle Interface Module circuit 295. Now the plug its asking me to check for isn't on this transmission! So yeah now I'm stuck again...FML!!
 

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Odd that this is coming up on the active threads page now...

With regard to "stuff not being the same" look more carefully at the schematics. Each section may contain different diagrams depending on the engine or transmission you are working with. Also, those boxed arrows with a letter inside and page/section numbers next to them send you to a different section in the book. Also, yes, there can be production changes that are not reflected in the manuals due to timing. If you are absolutely sure that something does not look the same, try looking in the previous or next model year book. I have run into this a few times especially with the medium duty trucks.
 
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