Rear Brake lights stuck on all the time

hopefully not rodents

 

If you pull one of the connectors off the brake light switch do the lights go out?
You can use and ohm meter across the switch, while both leads disconnected,
and press/release the switch and it should zero out the "open".

Seems like the switch is shorted causing the lights to stay on.

[QUOTE="Hlfxprius, post: 3159728, member: 164017"
I removed and tested the green colored switch attached to top of brake peddle (I assume this is the brake light switch?) and it tests okay.

If I remove the fuse under the steering wheel for the stop lights the lights go out.
[/QUOTE]

 

Boy that is a weird one? I have never hear of this across many cars of assorted make and model's?

But the brake switch is an electro mechanical part and it could be gummed up? I'd start with pulling it out and letting it hang in free air and then just press it off and on. And have someone in back, tell you if they go off and on that way?

 

Start with bulbs and work your way back through the whole circuit till you figure it out... The right rear tail light is where the red and slightly larger diameter brake light wire is...

 

The 2010 circuit is pretty simple, and the light assemblies return to body ground. The power to light them comes from forward, through the STOP fuse, the switch on the pedal, and a relay. The stop light wiring is blue.



If the lights stay on when you have the switch disconnected, my next three guesses would all be that stop light relay, which is the only place power can arrive on the wiring back to the stop lights (unless some wiring is actually damaged and bridged to something else powered, which can happen, but I would definitely look at the relay first).

 

I had a feeling you'd be asking that. It's called out as A28, here:

 

You do come up with some good ones occasionally.
A bad bulb will cause it to light when it should not......

 

Well, the only bulbs involved are all LEDs in Gen 3. Not much to check there anyway.

They are shared as the taillights too. There are a couple blocking diodes back there to make sure the taillight power doesn't backfeed the stoplight circuit and vice versa. If one of those diodes failed shorted, you could get a situation where turning the taillights on would look like stoplights. But I don't think that's the OP's situation.

That stoplight relay actually has a six-position connector. It's got two more wires that aren't shown in #7 (the ABS diagram and the Cruise Control diagram do show them).

Those connect to the brake ECU, allowing it to trigger the stoplight relay when you're using the hill-hold feature and your foot's off the pedal, or if you have the pre-collision or radar cruise and it puts the brakes on.

So if the relay turns out ok, it could be the signal from the brake ECU. There's also an active test where you can flip that signal on and off in Techstream.

 

The red one, right? Look at the diagram. There is constant power to the relay on pin 6, straight from the STOP fuse. It's the relay's job to pass that power along to OUT (pin 1, the green wire) when the lights should be on, and not when they shouldn't.

It should be activated either by voltage from the pedal switch, arriving at pin 5 (blue wire), or by a signal from the brake ECU when the hill-hold feature is on (or when you have PCS and it is braking). There are two wires from the relay to the brake ECU, the yellow one in position 9, and the beige one in position 4. I think the yellow one is the ECU output for turning the relay on, but I'm not sure, and I haven't found the beige one explained in the manual. Maybe @Elektroingenieur knows something? I also don't know whether that output is active low or active high. The way I'm reading the manual suggests it's pulled up by the relay, and the ECU pulls it down to light the lights. Note that's opposite of how the pedal switch input works. Seems like an interesting "relay".

Have you checked for brake system codes at all? I would expect a C1380 (or blink code 64, on the VSC light for some reason) with INF 762, maybe. C1249 is also a code pertaining to the stop light relay (it blinks 49 on the ((!)) light). The sections for both of those codes in the repair manual have useful ideas for testing the circuit.

 

The ABS, TRAC, and VSC pages in the System Circuit section of the EWD show that, at connector A28, terminal 4 is designated ESSI, connected with a beige wire to the EXO terminal of the Skid Control ECU, and terminal 9 is designated ACC, connected with a yellow wire to the STPO terminal of the Skid Control ECU.

The Repair Manual (more info) gives some information about the STP0/ACC line (note “0” not “O,” which differs from the EWD) in the information for diagnostic trouble codes C1280/64 and C1249/49. STP0 is described as a “stop light control relay drive output” from the Skid Control ECU, but as you noted, the “ON” and “OFF” states aren’t defined. The Terminals of ECU page regrettably just adds to the confusion, describing STP0 as “Stop light control relay output — Always — 11 to 14 V,” which seems unlikely to be true.

I didn’t find any information about the EXO/ESS1 line (note “1” not “I,” which also differs from the EWD) in the U.S. edition of the Repair Manual, but the European edition (RM12A6E) explains that it’s found only on vehicles with “Emergency Brake Signal,” and the Terminals of ECU page there says that the EXO terminal on the Skid Control ECU goes from 11 to 14 V with the brake pedal depressed (stop light switch ON) to below 1.5 V with the brake pedal released (stop light switch OFF).

 

I spotted a note in that part of the manual saying that voltages couldn't be measured with the harness connected to the ECU because of the waterproof connector design, so the voltage measurement on STP0 would be the voltage on the harness terminal while disconnected from the ECU ... meaning the relay probably does "always" pull it up to 11 to 14 volts, leaving me to guess that the ECU pulls it down to activate the lights.

 

that doesn't sound good, but there might be an extended warranty

 

That's peculiar, and I wonder if a typo is involved; there isn't an INF code 760 listed in the manual. There's a 761 and a 762.

In the Gen 3, the brake ECU and master cylinder are integrated into the same assembly (which is sometimes called the master cylinder, or two or three other names). And the brake ECU is able to trigger the brake light relay, and that ECU also monitors the brake light circuit to see if it's behaving; that is, when the ECU is using its STP0 output to trigger the brake lights, it wants to see the that brake lights (STP2) are ON. Whenever it isn't asserting STP0 to trigger the brake lights, it wants to see STP2 match whatever the pedal switch (STP) is doing. Otherwise, it reports C1380.

So it isn't outlandish to suspect the ECU (and therefore the "master cylinder" assembly, because that's the only way the ECU comes) could be at fault.

On the other hand, the ECU is sitting there saying "C1380 - hey the brake lights don't match the ON/OFF state I think they should be in", and under the circumstances that seems like a fairly sane thing for it to be saying, so I might not decide it's off its rocker just yet.

I kind of hope the tech is looking at the STP0 voltage (pin 3 at the ECU, pin 9 at the relay). If that's a signal that's low-going to trigger the relay, perhaps it's inadvertently grounded somewhere. It is in that top row of brake ECU pins, where some other issues have also been reported ... I'm starting to wonder whether that connector there isn't quite as weatherproof as they intended.

If there's a short to ground there at that pin, probably replacing the whole assembly is the only thing the dealer would do about it, but I wouldn't be surprised if a simpler solution could be found.

Although there is a warranty extension that applies to the same assembly, in cases where its hydraulic bits develop a leak, that's a far cry from this situation.