1. Attachments are working again! Check out this thread for more details and to report any other bugs.

Short in taillights/brake light causing ABS light?

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by c4, Nov 24, 2005.

  1. c4

    c4 Active Member

    Joined:
    Mar 29, 2005
    607
    51
    0
    Here's one for you Prius technicans out there...

    Is it possible that a short in the taillight/brake light circuit could cause the ABS light in the combination meter to come on (and regenerative braking to be shut off, and no codes generated on any of the ECUs)???

    Here's the scenario- I got a package of LED bulb replacements yesterday and installed them, and everything seemed to be OK, but since it was the last thing in the day, I never did a really exhaustive test.. I depressed the brake pedal adn watched the reflection of the lights on my garage door to make sure they turned on, and then I turned on the tail lights and went out and made sure they all turned on, but I never tested the brake lights and the tail lights at the same time.. The one symptom I did notice was that the high-mounted light turned on when the taillights turned on, but I dismissed it, thinking that this must have been normal behaviour and I just didn't notice it before..

    This morning, I drove to work and everything was fine; I drove to lunch and everything was fine, but this evening, when I was ready to go home, I got in the car, turned on the headlights and started the car, and the ABS light came on. I said "Wth?" and immediately checked the codes with my Miniscanner- no codes, so I went out and checked all the fuses- all still good. I had to meet friends for dinner, so I decided to try driving and see what happened- the car drove fine, but there was no regenerative brakes (the normal regeneration and coast regen were limited to about 18 amps- ie the system was using only its mechanical brakes and basically behaving as if the car had hit a bump and regen was shut off).. When I came out from dinner, I started the car with the lights off, and the ABS light was off, but turned on as soon as I turned on the lights, but I didn't clue into the fact that it was light related until later; I thought that this might have been a sign of a loose ABS sensor connection that was possibly temperature related (today was the first day we got -10C temperatures). Anyways, I drove home and immediately looked under the car to check the ABS sensor connections- nothing wrong, but the ABS indicator was still lit.. It was then, that I happened to look out the passenger side mirror and noticed that the brake light did not brighten when I had the pedal depressed (it was on because I had left the taillight switch on).. This is when I clued in that perhaps the taillight replacements may have had something to do with it.. I pulled the bulb in question, and it appeared that because the wires contacts on the plastic wedge base were rather flimsy, that the inner contact (taillight) shorted against the outer contact (brake light)... Once I fixed this, the high-mount light went out and the taillights started working properly, and lo and behold, the ABS light also went out..

    Looking at the schematic, the stop light switch is connected to the STP input on the brake ECU, so my conclusion is that due to the short in the bulb connector, turning on the taillights caused a voltage to appear on the STP input even when the brakes were not depressed, which triggered the ABS warning light.. It also explained why the ABS light did not come on until after I turned on the taillights..

    I guess the moral of this story is that if anyone else out there is considering replacing their bulbs with LED replacements, make sure that the pins are properly aligned!!!