Brake lights flashing in extreme cold

If you still have the oem tail light lamps, installing those and returning wiring harness to oem would rule out the likely cause if it's related to the new tail light/harness. A 2021 truck with a faulty relay seems unlikely.

 

I don't even think it's the relay because that would control the 3rd brake light as well.

But yes, easy to rule out the culprit by returning to stock.

 

Without knowing if the ABS module (has HOT/Battery source fuses ) provides juice to circuit while vehicle is off, the only source for juice to STOP circuit is the fuse. Rear of vehicle circuits are also OFF when vehicle is OFF (REVERSE, TURN, TAIL) so where is the juice coming from?

 

The brake lights don't work when vehicle is off and brake pedal is pressed?

 

Yes, they do.

 

Weird...
As @caribe makaira suggested...
I'd disconnect the brake light switch. If it stops; remove, inspect and/or adjust. Might have some kind of fluid contamination exacerbated by the cold.

 

Thank you. I was too dense to realize the obviousness of what you are suggesting.

Thank you.

So I disconnected the brake light switch. Confirmed it was correct by depressing the brake pedal and the lights did not come on fully. However, the flashing continued with it disconnected. I reconnected it and the brake lights come on fully when the pedal is depressed.

Again thank you both.

 

So if brake switch disconnected and flickering still occurs, ABS system might be misbehaving.

 

On a truck that new, I would start by removing the added-on aftermarket bulb hardware and return it to stock.

 

Thank you for getting my brain working. I looked at all the computer stuff and had no clue where to start.

I don't have ABS due to A-TRAC in the Off Road, but it of course has a similar system connected to the reservoir. I walked through and disconnected the three harness I saw going to it (only one at a time, not all three at once). This did not result in a change. You can see all three in this picture. I'm guessing this indicates that this part of the system is not the problem.


I then pulled the relay (which I should have done this moreing to save the drain on the battery). This of course stopped it, but does not tell me much as to why the circuit was closed. I'm having a beer and letting the relay thaw. I will try poping it back in before going to bed. If it does not flash with the relay being warm I know that is the issue. If it still flashes, well my theory on a frosen relay is probably bunk.

As a side not, all of Toyotas wire harness connectors remain flexible at -26 F. Makes me happy as I was worried about breaking plastic parts messing with frozen connectors.

 

Well, got the relay nice and warm, but as soon as I put it back in the flashing started.

 

Can you hear or feel the relay chattering when installed?

 

Are you running LED bulbs or incandescent? With the brake switch disconnected you still have the flashing but it is dimmer? And the relay stops it completely? Does the third brake lamp flash as well? If it is LED, the amperage required for them to flash is tiny, it could be a voltage leak that the LED's flash with but an Incandescent would scrub off. I would remove the brake lamp bulbs and see where the voltage is coming from. To me it sounds like the brake switch is being used as a ground for whatever is happening, from the dimming description. You may be looking at the wrong side of the circuit. This might be feedback from the running lamp circuit.

 

No, the flash dim with or without the break switch connected (no change). When the break is depressed (and switch is connected) they come on solid and at full brightness.

The stock (incandescent) 3rd brake light is also flashing.

The relay doesn't make any noise/vibration. As they are flashing at a out 1/10th or less of full intensity, if it is the relay it is not fully activating. A slight depression of the brake pedal immediately produces full brightness (and solid).

 

I'm not positive it's the relay. You're just opening the circuit when you remove the relay. That just confirms the issue is at or before the relay.

 

I still vote for putting the OE units back in and see what happens.

If they still flicker, let your warranty fix it.

If they don't flicker..........

Change Management says always start with what was last touched when something breaks

 

I agree.

 

What are the Tail/Side Markers doing in all this? Wonder how they are sent voltage...and if it's feeding back into the stop circuit because of some wiring mishap in the new lights. Or it's the Tail/Side Marker circuit that is the actual problem. OEM appears to be a combo lamp...don't know what an led assembly does with those 2 circuits.

 

Nothing (as is appropriate). Turning on the tail lights results in the tail lights being appropriately turned on without flashing. However, the 3rd brake light (both on the cab and on the bed cap) continut to flash without any change. No change with blinkers or reverse lights either.

Literally every other system I have used or looked at is behaving appropriately. The colder it is, the higher the flashing frequency is. As ambient temp warms, or as the truck warms up from running, the frequency decreases, and stops at a certain point.

 

LED output is temperature sensitive... Although it's not the problem you describe, it could be tangential to it. I think swapping back to stock tailights would be worth your while.