taillights not working properly?

[speedracer99]

Can anyone tell me on a 2020 mustang (or any 18+ mustang) what, if any, changes would make the taillights not work properly? I have no turn, run, or brake lights. If I turn the hazards on only the front flash. if I turn the park lights on the rears are brake light bright and turn it to headlights on then nothing. Have changed the taillights to known working ones, swapped the taillight module (bcm "b") in the trunk, and checked the 2 fuses that in the bcm fuse diagram control the rear bcm "b" module. Wondering what changes to say the ipc enable/disable table or any of the other could cause this or what else to test . Thx!

Sponsored

 

[Torched10]

Can anyone tell me on a 2020 mustang (or any 18+ mustang) what, if any, changes would make the taillights not work properly? I have no turn, run, or brake lights. If I turn the hazards on only the front flash. if I turn the park lights on the rears are brake light bright and turn it to headlights on then nothing. Have changed the taillights to known working ones, swapped the taillight module (bcm "b") in the trunk, and checked the 2 fuses that in the bcm fuse diagram control the rear bcm "b" module. Wondering what changes to say the ipc enable/disable table or any of the other could cause this or what else to test . Thx!

Try the old disconnect battery trick.check baytrr yvoltage it may be too low and car is shredding the load to reduce power drain
Battery shoul.d be12.3 alternator over 13.5.auto zone will checj charging system for free.

 

[SheepDog]

Can anyone tell me on a 2020 mustang (or any 18+ mustang) what, if any, changes would make the taillights not work properly? I have no turn, run, or brake lights. If I turn the hazards on only the front flash. if I turn the park lights on the rears are brake light bright and turn it to headlights on then nothing. Have changed the taillights to known working ones, swapped the taillight module (bcm "b") in the trunk, and checked the 2 fuses that in the bcm fuse diagram control the rear bcm "b" module. Wondering what changes to say the ipc enable/disable table or any of the other could cause this or what else to test . Thx!

Maybe a bad brake light switch, headlight switch, or combination switch? I don't have a wiring diagram for that system

Any mods on the car or repair work done recently?

Have you been screwing around with FORscan?

 

[Mike Pfeifer]

Did you do something recently after which you noticed the behavior?

 

[speedracer99]

So some background. 2018 gt pp1 flood car we call the u-boat (was to the roof). completely stripped to tub and donor 2020 gt pp1 car parts used (full electrical, motor and manual trans). Car is a dedicated road race car (no street use) running oem ecu as per rules. brake light switch, headlight switch all new. I did go into forscan and disable (not in the as built menus) in the pic menu some things like diff temp, parking sensors, and some other stuff that wasn't pertinent to the build (using all aftermarket gauges w/ 5v outputs and soon data dash) but don't remember all I changed. Ive never used forscan before but read enough (so I thought lol) but forgot to backup initial. restored as built on 2nd attempt but didn't change back the enable/disable stuff (is there a way to restore that?). I had never checked the taillights after the swap but they worked in the donor car. and I have known working set in the car. Just trying to fig out where else to look either in forscan or elsewhere before I bypass the taillight wires and rig it from the brake switch. no turn signals or anything like that being used (stalks are removed). Only need running lights (for rain) and brake. Ultimately will depin the gem harness and not upset the can bus circuits to get what I want when I have time and not hurrying to get it on track to test before its first race in march.

 

[ORRadtech]

If you didn't replace the wiring harness then my guess is internal corrosion somewhere in it.

 

[Mike Pfeifer]

Well, if they light up at all then the wiring is good between the lights and module and the lights themselves are good. That leaves logic, either by software or hardware (module internal), and inputs. It looks like you already have come to this conclusion based on your initial post. Not sure if you can check for brake light input to the module to make sure the module is seeing what you think it is (pid), but I would start there. Maybe something is mixed up there, like turning the lights on causes a signal at the brake light input for some reason. If that’s all ok, then it sounds like restoring the car to factory is what you need to do, at least as a diagnostic step. I have not tried it myself, but from what I understand, restoring the as built data should do that. I don’t know why it didn’t for you. Unless it’s something like a vin mismatch due to the donor car vs the actual car. Not sure if that’s a thing on the fords. I will definitely follow this thread to see where you end up with this.

 

[Boyd]

2021 schematic book:

Park lights do not use BCM-B, only BCM straight to tail lights. Different output / wiring for each side.

Brake lights also come thru BCM before the BCM-B.

I'd suspect BCM.

 

[speedracer99]

tomorrow im going to scan a 2016 and a 2022 ipc with forscan to see what should be enabled/disabled on the cars and compare to what I have currently and see. Will post results soon

 

[Jaymar]

If I recall, the European taillight reassigns those leads to different functions hence the light housings aren't plug and play. Is there a chance you've configured the BCM to use the output mapped for those units?

 

[speedracer99]

If I recall, the European taillight reassigns those leads to different functions hence the light housings aren't plug and play. Is there a chance you've configured the BCM to use the output mapped for those units?

I have no clue on how to even begin to do that so prob not lol

 

[Jaymar]

I have no clue on how to even begin to do that so prob not lol

That's what you were changing with Forescan, the modules like the BCM and such are generic boxes that cover all versions and sometimes multiple models of cars. The programming is just the configuration of the module to that cars features.

 

[boostmark]

Start simple. Check the grounds in the trunk first.

 

[speedracer99]

Start simple. Check the grounds in the trunk first.

Im an idiot lol. So I cut the taillight grounds from the main ground tab in the trunk to clean up the wiring in prep for my depin-fest on the whole harness (grounded those in each corner of the car) and forgot to reground the trunk bcm "b" unit. as soon as I read your post I cursed a bit as ive been chasing this for like 4 days. Yea, was that simple. Thanks!

 

[Cobra Jet]

Im an idiot lol. So I cut the taillight grounds from the main ground tab in the trunk to clean up the wiring in prep for my depin-fest on the whole harness (grounded those in each corner of the car) and forgot to reground the trunk bcm "b" unit. as soon as I read your post I cursed a bit as ive been chasing this for like 4 days. Yea, was that simple. Thanks!

Sometimes it’s something so simple that takes the most time to sort out… LOL…. Well good to hear you’re back to operational again!

Sponsored