Read Brake lights stuck on all the time

Discussion in 'Gen 3 Prius Care, Maintenance & Troubleshooting' started by Hlfxprius, Jun 13, 2021.

  1. Hlfxprius

    Hlfxprius Junior Member

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    2010 Prius the rear brake lights (On both sides and on hatch) are on all the time (even when car is off). Also at the same time this happened the power steering light on the dash started flickering. Everything else on the car functions and starts and runs fine.

    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.

    I've put my 12v battery on a charger for a few hours just to make sure that's not the issue. It reads 12.6 volts. However I suppose it's still possible its a battery problem. The battery is over 5 years old.

    Other thoughts (Maybe a connector is corroded or something is shorted somewhere?)

    Does anyone have any other ideas?
     
  2. bisco

    bisco cookie crumbler

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    hopefully not rodents
     
  3. ASRDogman

    ASRDogman Senior Member

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    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]
     
  4. Samuel Williams Jr

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    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?
     
  5. PriusCamper

    PriusCamper Senior Member

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    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...
     
  6. Hlfxprius

    Hlfxprius Junior Member

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    [/QUOTE]
    When I pull the connector off the brake light switch the brake lights still stay on.

    If I leave the connector plugged in and manually press the switch, the car will turn on when the switch is not depressed, if I depress the switch and try to start the car I get error on dash about brake pedel not being depressed. So to me appears switch is working as it's suppose to.
     
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  7. ChapmanF

    ChapmanF Senior Member

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    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.

    stop.png

    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).
     
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  8. Hlfxprius

    Hlfxprius Junior Member

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    Thanks so much, do you know where the relay is located?
     
  9. Hlfxprius

    Hlfxprius Junior Member

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    I'm sure this is a possibility.
     
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  10. ChapmanF

    ChapmanF Senior Member

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

    A28.png
     
  11. sam spade 2

    sam spade 2 Senior Member

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

    ChapmanF Senior Member

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    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.
     
  13. Hlfxprius

    Hlfxprius Junior Member

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    Thanks. I removed the relay and the brake lights go out. I check the connector that plugs into the relay and there is a constant 12v on one of the pins. I assume this is were it's coming from.

    I'm getting more lights on the dash that are acting strange now and flickering intermittently.

    For a few hours today everything went back to normal and worked fine. Then brake lights came on again and symptoms came back.

    I removed the 12v battery and temporarily replaced it with a brand new deep cycle 12v 90 amp hour battery and same symptoms occur so I put prius battery back. The original battery reads 12.6v with car turned off so I assume battery is good.

    Thinking some type of short or ecu failure ???
     
  14. ChapmanF

    ChapmanF Senior Member

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    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.
     
    #14 ChapmanF, Jun 15, 2021
    Last edited: Jun 15, 2021
  15. Elektroingenieur

    Elektroingenieur Senior Member

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    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).
     
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  16. ChapmanF

    ChapmanF Senior Member

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    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.
     
  17. Hlfxprius

    Hlfxprius Junior Member

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    I ended up taking the car to a toyota dealer today. They spent 1 hr on the car and couldn't quite figure it out, they needed more time so it will be going back beginning of next week.

    So far this is what they have said.

    Codes; C1380 dtc760

    (Now I also have a right front wheel sensor failure that happened last month that has nothing to do with the brake light issue)

    The tech said he is learning towards it being an issue with master cylinder, but he needs another hour or so to do further troubleshooting. He did not indiciate to me why he believes that to be the case.
     
  18. bisco

    bisco cookie crumbler

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

    ChapmanF Senior Member

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    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.
     
  20. Hlfxprius

    Hlfxprius Junior Member

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    Hmm I wondering if the same issue could be causing my front right speed sensor to not report any speed. I replaced it but I still wasn't getting a reading from that wheel in techstream, so I assumed it must be the hub assembly, just havn't got around to swapping that out. Maybe it's all related to wiring corrosion. When I think back now, the ABS problem (Speed sensor) started the day of a snow storm that had super wet and slushy snow. I had a 2 hour drive in the snow that day and when I got home our street wasn't plowed and I came up the road pretty fast so I would make it up the hill, I hit a few bumps of slush, as I pulled into my driveway that's when the ABS code came in for the speed sensor. A couple nights after that a friend was over and was looking out the window and noticed my brake lights randomly coming on and off for a few minutes when the car was off, I thought nothing of it at the time as it went away until now.
     
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