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Old 04-01-2010, 01:11 PM   #1
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Help with Brake Wear Sensors Please

While at the track, braking down from 100+ mph my Brake Wear idiot light came on. (And, has stayed on.) I recently changed and bled the brake fluid. The brake fluid level is good and the brakes continue to work fine. But I want to fix the idiot light situation.

Today, I double checked all the pads. They all have plenty of pad material left (over 1/4") and no pads are "sticking" to the rotors. So, I assume it must be either wiring or the dreaded Brake Wear Sensors.

I disconnected the two front sensors where they plug in to the electical wiring and measured them at between 0.7 and 1.4 ohms. This tells me that there is continuity (a connection/short) through each sensor.

Since the electrical diagram shows all the brake wear sensors being daisy chained together, I assume that if one (or more) is bad, I would get a reading of infinite ohms (an open). That way, if ANY of the sensors were "cut" (open), the system would NOT have a continuous circuit and would therefore turn on the idiot light.

Can anyone definitively confirm (or refute) my assumptions? OR, is there something else that I am not considering? I realize that I will still need to check the two rear sensors.

I am a little confused, because, I have read here that one can just disconnect the sensors and the idiot light will not come on. However, I have also read here that one can tie the sensors off (away from the pads) to eliminate the idiot light problem. I believe the latter but doubt the former. Does anyone know for certain?


Thanks in advance,
Mark

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Old 04-01-2010, 04:18 PM   #2
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Well, did you replace them. Once they make contact with the rotor, there history. Not sure if they are connected in some way the computer. But once you clear a fault, say bad spark plug creates a misfire. If you do not clear the code, it will clear itself, I think, after 10 starts. Or, if you did change them out, you could have a bad one.
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Old 04-01-2010, 05:25 PM   #3
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My understanding of how the system works is consistent with your write-up. Like the damnable Christmas lights, once you lose one "bulb", the whole strand goes dark.

I zip tied my sensors back out of the way when I did my first brake service, so they're still "good". However, my understanding is that you can clip the sensors anywhere after the connector and strip, twist, and solder the the two wires to close the loop. Disconnecting the sensors leaves the loop open, which necessitates the zip-tie solution a lot of folks use.

I also read that the brake wear sensor is not routed to the DME - just to the light on the instrument cluster - so there is no stored fault that has to clear. Once you take care of the open circuit, the light should go off.
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Old 04-01-2010, 05:44 PM   #4
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Thanks sd_b, Yes, there is no code thrown, only the dashboard light comes on.
Since both of my front sensors are not open circuits, I'll test the rear two tomorrow. I'll update this thread with what I find out.

Thanks,
Mark
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Old 04-02-2010, 10:10 AM   #5
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Well, I checked the back two brake wear sensors this morning. No joy. All four seem to be fine. But, the warning light is still on. I guess that means that there is a disconnected (or broken) wire somewhere in the harness.

Have any of you had this problem? I would love to be able to narrow it down a little. Anybody have any other suggestions?

Thanks,
Mark

p.s., It is a 1997.
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Old 04-02-2010, 12:08 PM   #6
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My understanding is that the light will come on if the sensor circuit is open (sensors removed) or if one is shorted to ground.

I replaced my pads last May and reused the old sensors. Two weeks later I was at the track and the brake light came on. I inspected all the pads and fluids and everything checked out A-ok but a failed master cylinder might be hard to spot. I didn't want a braking issue at 120+ so I ended my day early. When I got home I pulled the sensors out and found one had gotten really hot and was grounding out. Maybe I didn't have it inserted into the pad properly. I insulated all 4 sensors with some heat shrink tubing and zip tied them back per others instructions and the light is gone. I re-inspect my pads and sensors every time a wheel is off and all is well so far.

I hope this helps.
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Old 04-02-2010, 02:05 PM   #7
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Does anybody have any dead Brake Wear Sensors they could sell me cheap? A set of four would be ideal. They need to fit an early 986 (1997). I want to make a set of test/bypass plugs out of them.

Also, does anybody know if anything other than Brake Wear Sensor(s) and/or system wiring can cause the Brake Wear idiot light to come (and stay) on?

Thanks,
Mark
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Old 04-02-2010, 03:06 PM   #8
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