Hello, All. I have a 2016 F350 for which I built a work bed. I bought the truck used with 5,800 miles on it from a dealership (FWIW). I am having problems with the driver's side (DS) brake/turn lights. Prior to removing the factory box bed I had no problems with the lights. I had a mechanic remove the bed pending receipt of a factory-made work bed which never materialized (the distributor quit calling me back and never delivered the bed). The truck sat for months and then was driven for a few months before I finally built and installed a work bed. The fuse has been checked several times and, to underscore that it is working, the DS front turn signal is functioning with the fuse installed, but doesn't function with the fuse removed; the fuse is good.

I bought a CM (they make aftermarket work beds) wiring harness so I wouldn't have to cut and splice the factory wiring on the truck. The driver's side stop/turn lamps on the rear do not work and have not since the bed was installed. The bed has all LED lights except the tag lights. Today I tore into this to try to get the driver's side stop/turn lights working. First, I disconnected the CM wiring from the truck's factory Molex and then I used a 12v DC power source to provide power to the various lighting wires in the CM harness. All the lights worked flawlessly on the bed (DS, PS, Running, and Backup). I then used a DMM to test for voltage from the Ford factory harness. I only checked the power being supplied for the stop/turn lamps, both DS and PS. PS sits idle at about 0.0 volts, as expected. I had my son hit the brake pedal and the voltage went to 12.5v. Perfect. He let off the brake pedal and the voltage dropped to about 7 volts. WTH?! After about 20 seconds or so, the voltage dropped to 0.0v. Okayyyy. I checked the DS wire and it was at 0.0v, brakes on it went to about the same 7 volts, brakes off it stayed at about 7 volts, then, after about 20 seconds, finally back to 0.0v. Really? Okay, it's a Horrible Freight DMM with a low battery, that must be it. I got another HF DMM I had, not with a low battery, and checked the wires again. Same results only that the idle/float voltage read about 5v. Alright, enough monkey business, I broke out the Extech DMM (about a $400 meter). Same #$%%# thing! One side (PS) goes from 0.0v to 12.5v then idles at about 5-8 volts for about 20 seconds then goes back to 0.0v, the lights work fine. The other side (DS) does it all the same except there's no spike to 12.5v to activate the LED's while the brake pedal is depressed.

If it was a break in the wiring harness somewhere I would expect to have 0.0v at the Molex where I connect/checked. I'm baffled.

So, I'm here to ask if anybody has an idea of what the heck is going on and will, please, shed some light on this for me.

Thank you.

--HC

 

I know it's a tall order and hard really to do without damaging the insulation on the wires, but you must check the voltages with the lights plugged in. Maybe you have enough room in the back of the plug to back probe the connector while the wires are plugged in. You must check these circuits with the load connected.

 

Okay, I tested the wires while connected. I used a T pin to probe the conductor in question, each one of them. The PS (functioning side) goes from about 0.0 volts to about 12.5v when the brakes are applied and then hovers around 3.6v after the pedal is released. The DS (non-functioning side) was still floating at the 3.6v area when I switched over to test that wire. It does not spike/move/jump to the 12v range when the pedal is depressed. I skipped both "maybe flaky" DMMs and used the Extech. For some reason, the truck is not sending HI (12v) to the circuit on the DS. I've got good connectivity as I see similar voltage on both sides at idle (float voltage).

Just for grins, I connected the pin for the PS to the DS lights on the bed and activated that circuit. The DS lights on the bed lit up as they should. There is no flaw I can find in the lights/wiring to the DS. It seems simply that the truck isn't sending HI (12v) to the DS light circuit.

I assume that the float voltage residual power is to retard the cooling of the elements found in traditional incandescent bulbs (like found on the factory lighting) to extend their life. Regardless, the DS stop/turn circuit is not being sent 12v to activate.

Still stumped on that one.

Thanks for your reply and time.

--HC

 

I did a little more looking, your lights are controlled by the BCM or Body Control Module. The power doesn't go up through the steering column and the turnsignal switch like the old days. The turnsignal switch and the brake switch just send a signal to a module near the instrument cluster, this is all low current signals. The module in the instrument cluster takes the voltage input from the switches, and then turns this into data and it is sent to the BCM over a two wire data communication network. The BCM receives this information on the data lines, interprets it, and then electronically switches the power to the rear light wiring. Since it's very easy to burn out the electronic switching devices in the BCM, this may be your problem. I am thinking the small voltage you are seeing at idle is the voltage that leaks through the electronic switch, which is just a large transistor on the circuit board

I don't have very good diagrams and locations for your year truck.

 

Thanks for the reply. I stopped by a dealership today where an acquaintance of mine works in the body shop. I told him what was going on and he walked me over to the repair shop and we talked to the "go to" guy for working on the Fords (it's a small dealership in a small town so a) there are advantages to knowing someone in the service areas and b) the "go to" guy is one of maybe 4 mechanics at the whole place who works on the Fords).

He printed out wiring diagrams based on my VIN and basically the printouts show exactly what you described: the BCM sends voltage to the lights based on inputs (signals) from other modules and switches. A logical approach but one that brings the BCM into question; is it hosed? I asked him, then, about replacing the BCM and he said it requires a lot of programming, including the keys. He also indicated that the BCM is not cheap.

However, he also indicated that the BCM can shut down a circuit if it detects a short circuit. He claims that it will throw a code that, with an in-depth scanner, can be read and reset. This basically means "not the cheap OBD-II scanners we all can get easily from all over (including Horrible Freight). The feed wire to the Molex at the rear of the frame (the Molex I've connected to) which feeds the DS stop/turn had a break in the insulation (not the conductor). This could have caused a short circuit which, instead of popping the fuse, tripped the BCM shutdown. IF the BCM can detect a short circuit and shut it down. Sounds too good to be true. But desperation makes all kinds of snake oil sound plausible. Because, if this "BCM circuit shutdown" thing doesn't exist/fix the problem then my next solution is to run a wire from the trailer connector to energize the stop/turn on the bed and that is not ideal.

Thanks again for your reply and taking the time to look into this.

--HC

 

I do not know how far you want to get into this. Maybe you want to start your own Ford troubleshooting service in your small town, but there are some people that have broken through and reverse engineered the Ford software, and offer a scanner program that is almost equivalent to the Ford factory stuff. It's free and all you need is a laptop and you need to buy a cable for about $30. If you ended up swapping a module, you can do that if you upgrade to the pay version, they have several different time periods you can buy or you can buy a lifetime subscription for $50. Here's a link to it. https://forscan.org/home.html

If you think about it, it's awfully complex just to get some lights working on a truck, but that is the world we live in now.

 

Thanks for the reply. To follow up to offer conclusion to this (for current and future readers): the light is working now.

I scheduled an appointment with the dealership but it was going to be the following week. I took it to a mechanic who I've used for 30 years and who had a Snap-On computer and who could see me that same week. We weren't sure if he had the necessary software module to communicate with the Ford various non-emission modules (specifically the BCM) but he did. The BCM was throwing a fault/code indicating a short on the DS stop/turn. He cleared the codes with his computer and it fixed the problem.

What this means, and what sucks terribly, is that NOTHING was going to get that light to function correctly except a computer capable of communicating with the various manufacturer-specific modules. My understanding is that the emissions info/modules is standardized per EPA regulations (OBD, OBDII, and such which we've had for 25 or 30 years). What the manufacturers choose to do beyond that is up to them and not standardized across manufacturers. So, you can buy a $35 code reader at Horrible Freight, Autozone, whatever, and read and clear emissions codes, but you're SOL on ECM, BCM, and so forth. Further making this situation suck is that there is no information presented to the operator that there is anything amiss with the vehicle. The BCM throws a code and shuts a circuit off, but gives ZERO indication to the operator. One could change fuses and bulbs until they were broke, but nothing would get the light to work: only resetting the code in the BCM would get it to work.

So, for others to know: Yes, Virginia, the BCM *can* detect a short on a tail light circuit and shut the circuit off without giving any other sign or symptom (blown bulb, blown fuse, vehicle on fire) to indicate that there is a problem.

--HC

 

What you found out is entirely true. What is also sad, most of the new vehicles already have enough smarts in them that they could present the problem to the owner of the vehicle and let the owner reset it themselves. But this would be one less trip to the local dealer and we can't have that. Snap on and a few others are trying to backdoor engineer this stuff, doing a good job of it, but are having a hard time keeping up with the OEM's and their tricks. One thing you didn't ask your mechanic, or maybe you did, is how much that scanner he has cost him and how it costs him every year to keep it updated. It's not cheap.

 

Its true Snap On's scanner is as close to dealer level full access to the CAN-bus networks and they are expensive---probably very much cost prohibitive for anyone not in the automotive service business. FORScan is said to have a very high level of the same access and ability to control aspects of the CAN-bus network--and its mostly free.

A friend in the sevice business has purchased and currently using an Autel brand super scanner---equally expensive and requires yearly updates for a cost--he raves at its ability to diagnose and "fix" a lot of issues arising on his newer fleet vehicles.

We are indeed in the era where vehicles are rolling computers that require a higher level of tools and knowledge---shade tree mechanics might be the true endangered species.

 

I did ask him how much it cost: $8k. I did not ask what the annual updates cost him. For what it allows him to do, the price doesn't seem too bad. Probably keeps him in business. They say in SCUBA, "if you don't have a compressor, you don't have a shop". Probably the same for mechanic work these days, "if you don't have a code scanner/tool, you don't have a shop". Text is hard to convey emotion with: to be clear, I don't mean any of that to be argumentative or rude, in case it reads as such.

--HC

 

The above statement is partially true,the newer vehicles are rolling computers. And most normal people will take them to the dealer and get them fixed. There are a few smart people out there though who could fix their own vehicles if you gave them access to the tools to do it. It's not about having a sophisticated expensive scanner, most cell phones have more computing power than a scanner has. It's to do with software rights and access. It will all even out eventually. I do not believe Ford and other OEM's get it though. How if they made their cars more accessible, they would be even more popular. You have to admit the enthusiast drive a lot of the market for vehicles. I think they would be more "enthusiastic" if they had a vehicle they could tinker with.