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Break Wear Light On


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The Brake Wear light came on a while ago. The pad thickness was not thin enough to create this problem but I changed the pads and sensor cables on all wheels thinking this will turn off the light, but the light still on. Is there something else I can do before going to the dealer this light is driving me nuts.

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It's a pretty simple circuit. All the sensors are wired in series - which means any break in a wire will cause the light to go on. If you have an ohm meter you should be able to trace it back one wheel at a time until you find the high resistance.

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I can get an ohm tester but what exactly do I measure if all four cables are new. Or of what do I take the measurement form the sensor inside the pad or inside the plug area?I am not clear how to test resistance.

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Are you certain you don;t just have a bulb out?

The cicuit is a closed loop. So any break in it will show up as a fault. This includes the brake pads and bulbs. Use your ohm meter set on low resistance. You should always get a low reading. If the meter on the portion of the cuircuit shows high resistance or open circuit then you have located the fault. Next you need to trace it further back to find out where it's open.

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I can get an ohm tester but what exactly do I measure if all four cables are new. Or of what do I take the measurement form the sensor inside the pad or inside the plug area?I am not clear how to test resistance.

New cables does not mean that one of them does not have a flaw or a connector that was crimped on the insulation instead of the wire. If they worked before then there is even more reason to suspect one of the new ones.
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Ok, I measured each new sensor and got a 0.1 ohm reading. So i guess the new cables are ok. Now for the weird part I measured ohms inside the receptacle where the cable plugs in and got 0.3 ohms on the back wheels and 348.0 on both the front wheel sensor plugs. I traced the cables on the front and they go in a bundle with other wires inside the cabin under the dashboard. We move the cables around to identify them but they are together with many more. Is there a relay or module that can be damage, where do I go from here?

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  • 3 weeks later...
Ok, I measured each new sensor and got a 0.1 ohm reading. So i guess the new cables are ok. Now for the weird part I measured ohms inside the receptacle where the cable plugs in and got 0.3 ohms on the back wheels and 348.0 on both the front wheel sensor plugs. I traced the cables on the front and they go in a bundle with other wires inside the cabin under the dashboard. We move the cables around to identify them but they are together with many more. Is there a relay or module that can be damage, where do I go from here?

You not only need to be concerned whether the "loop" is OPEN at any point, but also whether it is GROUNDED at any point. The loop starts out from the instrument cluster where approx. +11 volts (measured to ground) is passed around the loop and then back into the cluster. If the voltage doesn't make it back because of an open circuit, OR the voltage is dropped due to a short to ground, the BRAKE WEAR caution light illuminates. The loop's sequence, starting out from the source of voltage in the cluster is: RIGHT-FRONT, RIGHT-REAR, LEFT-REAR, LEFT-FRONT, and then back into the cluster.

Keep these things in mind when troubleshooting: First, make sure there's NO voltage present in the circuit when you make resistance measurements, or you will get erroneous resistance readings, second, be sure you aren't measuring resistance back through the cluster.

I suggest you start troubleshooting by disconnecting the RIGHT-FRONT sensor. That way, you can verify you're getting the +11 volts from the instrument cluster and you'll also partially isolate the rest of the loop for additional troubleshooting. With the ignition switch in position 2, +11 volts should be present on the forward male contact in the RIGHT-FRONT connector. If you aren't getting the voltage, that would indicate your problem isn't in the sensor loop. If the voltage is there, turn off the ignition and continue troubleshooting the rest of the loop.

Measure the resistance between the RIGHT-FRONT sensor's leads (female contacts in the connector) to verify it's less than an ohm. Also, check that neither lead shows resistance TO GROUND. Leave the sensor disconnected.

I would then disconnect the LEFT-FRONT sensor and check its resistances. The remaining portion of the loop is now completely isolated from the cluster. You can make a quick check of the other two sensors without having to disconnect them. At the aft male contact on the RIGHT-FRONT connector, check to see there's no resistance to ground. In addition, the resistance from there to the forward male contact in the LEFT-FRONT connector should be an ohm or two, at the most. If BOTH these checks are good, the other two sensors and their wire harnesses should be OK. If the resistance on either check is not correct, you'll have to disconnect at least one other sensor to localize the problem.

It's most likely you'll find one sensor shows continuity though itself, but also happens to measure low resistance to ground.

NOTE: The male pin configuration and the loop sequence I mentioned above, is what I have verified on my 1999 996. I imagine it would apply for other MY 996s, but I cannot be sure.

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You could also remove the idiot light bulb from the instrument cluster.

It's one of those not too important idiot lights because you can just check your brake pads visually like all the other cars that don't come with an idiot light for brake pads...

I'm an electical engineer, so I like to have everything working the way it should, but I'm also a good-enough-is-best engineer...

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:cheers:

Dear Sandy, and Chaps,

Thanks for you comprehensive info on testing the loop, I found only 4.8V at the FR disk! And on further investigation found that the cable had cut though where it passes under the plastic wheel arch liner.

Soldered it up and WOW it works.

I have attached some pictures.

Regards and Thanks

William

UK London

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