BLIS & tapping into 4 prong trailer wiring

[NeilP]

If you plug a trailer light tester into the 4 prong plug, BLIS thinks there is a trailer plugged in. I know about setting the trailer length to 3 ft.

What I am wondering is how does BLIS sense something is plugged in when it's not visible to the sensors? Is it sensed in the plug or just a load on the electrical system?

I want to tap into the 4 prong plug wires to power a small LED DOT style light bar. It has 3 lamps that are illuminated when the taillights are on and has 8 additional LED lamps that function with the brake and turn signals. I temporarily connected the light bar to the existing 4 prong plug to test it. Everything works as expected including the trailer connected warning.

So what I am wondering is tapping into the wire harness before the trailer plug assembly going to mess with BLIS?

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[Big Blue]

First of all it is not BLIS that is detecting the trailer, it is the truck that is detecting the trailer. It is the trailer module sensing a load on the lights plugged into the trailer connector. I believe it is the brake circuit it detects, but it could be any or all. BLIS is just one of the systems having a trailer connected affects. It also affects/turns off rear parking sensors and rear cross traffic sensing.

So yes tapping into the harness ahead if the plug will have the same affect as plugging it in. This has been an ongoing issue with tailgate lightbars and hitch plug lights.

 

[OFC Ranger]

What he said, the electrical system detects extra load, pin-points where its coming from (in this case trailer harness), and that is the result you get.

There are some really nice sequential lights on the market, but the way Ford sets up their electrical system it is a total crapshoot to get them to work properly, even if you wire directly into a tail light harness.

 

[NeilP]

Thanks for the replies fellas. I am going to play around with this a bit more today & hopefully once it is installed, it works like it did on the first test run. I have this style light bar on my other pickup trucks and hope I can make it a permanent fixture on this one.

 

[Muscleford]

Thanks for the replies fellas. I am going to play around with this a bit more today & hopefully once it is installed, it works like it did on the first test run. I have this style light bar on my other pickup trucks and hope I can make it a permanent fixture on this one.

Let me know if you get it working and if the tailgate will clear it. Also where you got it.

Thanks

 

[NeilP]

Let me know if you get it working and if the tailgate will clear it. Also where you got it.

Thanks

I will let you know. Hoping to have time to tinker with it tomorrow, & if not Friday for sure. All signs indicate it will work & fit just fine.

I get them from iJDM Toy Store (red lens) or iJDM Toy Store (clear lens) If you do a quick search on Google you'll likely find a coupon good for $2 bucks off. Code HOTPIC may still work.

 

[NeilP]

LED light bar in action

 

[Toto]

Nice work NeilP!
Thank you for letting us know that you discovered a way to make it work properly.
Would you break the suspense and tell us the secret combination/connection/pinout?
Please, please, ?
Thank you

 

[NeilP]

Nice work NeilP!
Thank you for letting us know that you discovered a way to make it work properly.
Would you break the suspense and tell us the secret combination/connection/pinout?
Please, please, ?
Thank you

There's no secret to it.
Simple 4 wire connection. I had a 4 prong trailer plug extension in my collection. I cut it in half and soldered the male end (three exposed prongs)to the light bar wires (after I installed it on the plastic bumper cover, the plug won't fit through hole that you have to drill). The wires are color coded, brown, yellow, green & white.

After that, I removed the trailer plug from the bumper & exposed the wires that go to the 4 prong trailer plug. I stripped off about 1/4 inch insulation off of the right & left turn signal wires and the same off of the tail light wire (easy to double check which is which with your test light at the OEM connector). Soldered the female end of the harness to the exposed wires. Covered them with liquid electrical tape, then wrapped them in regular electrical tape & put them back in the OEM wire loom leaving the female plug hanging out. I connected the ground wire to a easily accessible bolt on the bumper (near the left side license plate light). Filled the holes on the connector with dielectric grease & connected the two ends.

About the only thing you can do wrong, (and I did) was to mount the light bar before checking the turn signals. I had it upside down so it was flashing opposite of vehicle turn signals.

 

[Big Blue]

There's no secret to it.
Simple 4 wire connection. I had a 4 prong trailer plug extension in my collection. I cut it in half and soldered the male end (three exposed prongs)to the light bar wires (after I installed it on the plastic bumper cover, the plug won't fit through hole that you have to drill). The wires are color coded, brown, yellow, green & white.

After that, I removed the trailer plug from the bumper & exposed the wires that go to the 4 prong trailer plug. I stripped off about 1/4 inch insulation off of the right & left turn signal wires and the same off of the tail light wire (easy to double check which is which with your test light at the OEM connector). Soldered the female end of the harness to the exposed wires. Covered them with liquid electrical tape, then wrapped them in regular electrical tape & put them back in the OEM wire loom leaving the female plug hanging out. I connected the ground wire to a easily accessible bolt on the bumper (near the left side license plate light). Filled the holes on the connector with dielectric grease & connected the two ends.

About the only thing you can do wrong, (and I did) was to mount the light bar before checking the turn signals. I had it upside down so it was flashing opposite of vehicle turn signals.

You basically just made a wye adaptor for the 4 pin trailer wiring. You will get the trailer connected warning everytime you start the truck. The truck will disable your rear parking sensors and your cross-traffic alert. If that works for you, have at it. I like my sensors working.

 

[NeilP]

You basically just made a wye adaptor for the 4 pin trailer wiring. You will get the trailer connected warning everytime you start the truck. The truck will disable your rear parking sensors and your cross-traffic alert. If that works for you, have at it. I like my sensors working.

I've not taken the Ranger out since I did the install, so you may be correct.....however, the manual says it will still work.

If the trailer is actually a bike rack or
cargo rack with electrical lighting, enter a
length of 3 ft (1 m). Cross Traffic Alert
remains on for trailers with a length of 3 ft
(1 m) or less

 

[Toto]

Thank you NeilP. For the picture and explanation. On my to-do list for these long winter nights.

 

[Trigganometry]

Only way I know of to get around the truck sensing extra load on the wiring is to use opto-relays. They use milliamperes of power to operate so no load will be detected. The newer solid state ones can handle a few amps of load on the switch side. I know they sell them as a turnkey units for just this purpose but might be pricey. You can breadboard your own for short money.

 

[Big Blue]

Only way I know of to get around the truck sensing extra load on the wiring is to use opto-relays. They use milliamperes of power to operate so no load will be detected. The newer solid state ones can handle a few amps of load on the switch side. I know they sell them as a turnkey units for just this purpose but might be pricey. You can breadboard your own for short money.

Only sucessful way I have heard anyone connecting tailgate lightbars into our trucks is using a 3 to 2 wire tailight converter. It should be a powered one to avoid adding load to the trucks tailight circuits, which are powered by the BCM. It is tied into the trucks taillights to avoid tripping the trailer detection.

 

[Muscleford]

Question for the more knowledgeable. I want to install the same light bar Neilp installed. I am planning to wire differently. I am planning to tap in at the tail lightto the respective wires.

Question is will I still have issues with BLISS and the truck thinking I am towing a trailer? I have a 2021 Lariat with the techknowledgy package.

Thanks for you help.

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