Brake pedal turns on the front blinkers! (1 Viewer)

jbee

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Thanks for reading this. I've used the search function and learned a lot about brake lights.

4/72 FJ40. Stock as a rock. No KC lights, electric winch, radio, weird extra wiring.

When I push on the brake pedal, the front blinker lights get brighter; they act like brake lights. These are the lights on top of the fenders, left and right. Photos attached below.
No discernable change in tail lights (the ones that are supposed to be the brake lights) when I hit the brake pedal.

Here are some FAQs.
Flasher relay box thingy is new oem.
Hazards work. (rebuilt the switch) Switch is a *little* wonky; sometimes it needs a little wiggle to make the blinkers work.
Blinkers work, left and right, as do the blinker indicator lights on the dash. (new OEM blinker switch and arm.)
Headlights (bright and low) work, including floor switch, NEW oem headlight switch.
New brake switch (the one behind the brake pedal) OEM.
Running lights (the side corner ones) all work; orange in front and red in back. Cleaned the grounds and added star washers and dielectric grease.
Tail lights (new oem) all work.
The new brake light switch only plugs into the harness one way. I feel like I triple checked the part number and the new switch looks/acts like the old switch.
This truck has the dual master cylinder with the switches for the brake warning light.
Fuse box is new from @Racer65

Does this truck have a parking light brake switch? I mean the one that attaches to the dash behind the parking brake handle. I see them on some parts websites but don't see it on the wiring diagram.

thx,

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This is rare, but I've had it happen before so I thought I'd throw it out there. On a combination brake/turn bulb like in your tail lights, I have had a filament break loose and touch the other filament. So when I pressed the brake, it connected with the turn signal circuit and lit up the turn signals.
 
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My 40 does this too, always has. Been on the long list of trivial things to fix. All other lighting works as intended, it's been a mystery with no obvious cause. Outside of the wiring mess in the dash from previous owners, I haven't found a cause either.
 

jbee

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This is rare, but I've had it happen before so I thought I'd throw it out there. On a combination brake/turn bulb like in your tail lights, I have had a filament break loose and touch the other filament. So when I pressed the brake, it connected with the turn signal circuit and lit up the turn signals.
Thank you. I’ll check those bulbs!
 

jbee

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My 40 does this too, always has. Been on the long list of trivial things to fix. All other lighting works as intended, it's been a mystery with no obvious cause. Outside of the wiring mess in the dash from previous owners, I haven't found a cause either.
Thanks. I have not heard of it before either so this is good to know!
 
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Not sure if this helps but I had a similar issue when I first bought my 40. Ended up being the previous owners bad splicing job wiring in a pigtail for towing a trailer. Easy fix, I just removed his mess and taped up the connections. Never had the light issue again after that.
 

jbee

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Not sure if this helps but I had a similar issue when I first bought my 40. Ended up being the previous owners bad splicing job wiring in a pigtail for towing a trailer. Easy fix, I just removed his mess and taped up the connections. Never had the light issue again after that.
Thank you.
This one had a homespun trailer hitch on it when I bought it but when I looked at the wiring it looked pretty good, lots of green with white stripe, etc. I will climb under there and make sure it matches up to the wiring diagram. As this is a new problem I suspect maybe that wiring back there got bumped and messed something up.
 

pb4ugo

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When things start acting weird as you describe it's usually a ground issue. I'd 1st look around the rear lights. Your light fixtures are usually grounded by the bolts that hold it to the frame/body. Some could have ground wires too.
 
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If you haven't been messing with the wiring previous to this then it is likely that it is a ground problem.
Once had my dash lights started coming on when brakes were applied. Bad ground on the brake lights and it found a new path through the dash ground.
Electricity follows the path of least resistance.
Briefly worked at a gas station/garage when I was a kid.
Everyone thought I was an electrical genius because when someone came in with an electrical problem the first thing I did was check the grounds and nine times out of ten that was it.
 

jbee

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When things start acting weird as you describe it's usually a ground issue. I'd 1st look around the rear lights. Your light fixtures are usually grounded by the bolts that hold it to the frame/body. Some could have ground wires too.
Thanks for reading/replying. I agree with you on the grounds. I recently replaced all of the marker lights and tail lights with OEM and reset the grounds. I cleaned the connection points with sandpaper and wire brush and installed star washers and dielectric grease where I could. This includes the license plate light and reverse lamp.
They did get brighter with better grounds!
 
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The blinkers flash the rear brake bulb, so when you step on brakes you are getting current back thru the rear blinker feed, so when brakes on and blinker on the front bulbs might brighten depending on the current loss via connections, blinker switch, fuse connection so it is not all that confusing that when just blinker it is less bright and you notice the increase when brakes are pressed. I know the blinkers and brakes power thru the hazard switch which you said was wonky, maybe a voltage loss going on with the pass thru power to blinkers & brakes there.
 

jbee

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The blinkers flash the rear brake bulb, so when you step on brakes you are getting current back thru the rear blinker feed, so when brakes on and blinker on the front bulbs might brighten depending on the current loss via connections, blinker switch, fuse connection so it is not all that confusing that when just blinker it is less bright and you notice the increase when brakes are pressed. I know the blinkers and brakes power thru the hazard switch which you said was wonky, maybe a voltage loss going on with the pass thru power to blinkers & brakes there.
Thank you, I agree. I have another hazard switch in a box somewhere maybe I will clean that one up and install it and see if it makes a difference, I can tell that I’m close because everything else works!
 

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