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ABS light on after LED swap


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Putting this in AVM since this is modification-related, let me know if I got the wrong forum.

So, some time back, I went the way of the LED with my taillights and my front turn signals. Also swapped out my hazard and turn signal flashers with no-load flashers so they would actually work. Not long after I did this, my ABS light came on and has been on ever since. I remember reading in a thread (that I can't find anymore) that the ABS computer will throw a fault if something is really off about the brake light circuit. I reviewed the wiring diagram, and I see this B-12 resistor inline just after the brake switch.
Has anyone here who's swapped out the brake/taillight bulbs out with LEDs kept their ABS? What needs to be done to the brake lock-up circuit? Is it as simple as replacing this with a higher-ohm resistor?:
AFqMNwY.png

I know I could just delete the ABS, but I'm kinda trying to keep things original for the moment, and I'd hate to toss out an otherwise functional ABS system. Any help is appreciated.

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21 hours ago, tux said:

I didn't need to do anything when I swapped in LED's.

I had my ABS light on before the swap, however, when I took apart the abs solenoids and cleaned out/regreased them, the light went off.

Yeah I was kinda afraid it might have been convenient timing. The brake fluid does look old, I'll try flushing it to see if that helps. If it comes down to it, how difficult is it to regrease those solenoids? Didn't immediately see a write up on it.

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  • 4 months later...

It's been a minute, but I thought I'd revisit this. Hadn't gotten around to rebuilding the solenoids, but I did flush the hell out of the fluid when I swapped in Mikie's brake fluid reservoirs which unfortunately made no difference.
However, I did 100% confirm this issue is caused by the LED brake lights by performing the following:

  1. Replaced the 6 LEDs in the taillight housings with the standard bulbs
  2. Foot on brake, turned key to ON and listened/heard the power-on self test of the ABS solenoids
  3. Started engine, then foot off brake
  4. No ABS light
  5. Swapped the bulbs with LEDs
  6. Performed steps #2 & #3
  7. Got ABS light after roughly 10 seconds of run time

Did the above a second time to triple-confirm it was the LEDs, so I'd like to try getting at that B-12 resistor. Has anyone tried replacing this resistor before? Can it be reached with the gauge pod off, or do I have to go through the bottom of the dash? This line art I found isn't great (page 8-36).
LDTDUss.jpg

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  • 5 months later...

Alright, I figured out how to make this work with my LEDs. Long story short; per the diagram, I replaced the white w/green stripe wire coming out of the resistor with a short to ground, and all seems to be functional again. Here's how I did it:
B1VUNk0.png
In case the picture doesn't show, the B-12 resistor is behind the kick panel next to the dead pedal, taped to the wire harness. It's a simple 2-pin connector, Green wire on one pin and a white/green stripe wire on the other. The kick panel is held in place by 2 screws going through the relay cluster.

G8aAMuU.jpg
And this is me connecting the green wire from it's connector to the resistor, but simply shorting the other pin of the resistor to ground.


Explanation:

I spent a lot of time figuring out how this works. Below is what I found, I don't believe the repair manual goes over any of this:
LcbzCWV.png

  1. On key-forward, the ABS computer puts about 5 volts to the circuit highlighted.
  2. The manual doesn't illustrate this, but somewhere on this WG wire, it splices into the WG wire connecting the hazard switch and the turn signal switch illustrated on page 8-188 of the electrical diagram, the stop light circuit. Splicing here means the only path to ground is through the brake light bulbs. This allows the control unit to see if any of the brake light bulbs are functioning. If they are, there should be very little voltage available at the control unit. If ALL of the brake light bulbs are out, there would be no path to ground, thus all 5 volts will be available at the computer, indicating a "malfunction" in the brake light circuit. Keyword here is ALL; if just one bulb is functional, that completes the circuit, and all of the provided voltage would be used by this bulb. I verified one incandescent bulb is enough to convince the ABS computer the circuit is good in my testing.
    • This becomes a problem with the modern LED replacements I added. Obviously, LEDs require WAY less power to operate, but if we put unrestricted battery voltage to bare LEDs, they would burn out very quickly. So, to make these replacements as drop-in as they can be, manufacturers have built-in resistors with these LEDs. These added resistors dramatically drop the flow of current through the circuit to protect the LEDs. In our (and I presume many other older) cars, this has the consequence of dramatically increasing the available voltage at the ABS computer, making it think the circuit is "open".
  3. This is my fix logically illustrated. Instead of this circuit going through the bulbs, I instead just shorted it to ground AFTER the resistor. This drops the available voltage at the ABS computer roughly as it would with an incandescent, so it thinks everything is good.

Once I did this, I verified my anti-lock light stayed off, and I test drove my Quest afterwards. The system seems to be working, as I could not lock up the rears for the life of me, but I'll be double checking this the next time I get it in the air.

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