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one of gods prototypes
 
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brake pad wear indicator light question......

my brake pad wear light on my 85' carrera came on yesterday after a short drive in the rain. when i first start the car it stays out but as soon as the car moves it comes on. the sensors for each wheel are disconnected and zip tied (done by previous owner).
is there anything else in this curcuit that would make this light come on? brake performance has not been effected.
any help appreciated
thanks
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Old 07-28-2002, 09:43 AM
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KTL KTL is offline
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Nope, the only wiring which lights that warning light is the sensor wiring. Check how they're tied back and make sure a connection hasn't been broken.

When my rear pads were worn to where they'd cause the sensor light to illuminate, the light would come and go. Didn't always stay on until the pads were worn enough to keep the light illuminated. Not sure how that happens since the cause of the light going on is a break in the circuit. How does the break temporarily "fix" itself? Dunno.
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Last edited by KTL; 07-28-2002 at 02:56 PM..
Old 07-28-2002, 02:52 PM
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KTL--my understanding of that worn brake pad circuit is that the wear sensor is "looking"
for a ground which it gets when the pad is worn down enough to allow the sensor to make contact with the disc. Doesn't the emergency brake utilize the same light? Might check that the E-brake is all the way down.
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Old 07-28-2002, 03:34 PM
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KTL KTL is offline
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Sounds good to me.

E-Brake is a separate circuit. The light is in the same light cluster (e-brake/brake warning light, pad wear indicator, fasten seat belts), but on a separate circuit.
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Old 07-28-2002, 03:48 PM
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thanks guys
again......this board rocks

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Old 07-28-2002, 05:18 PM
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Actually, the pad sensors are in a loop. KTL was right on the money. When a pad wears enough for the sensor to wear through the contact, the loop is broken, the light comes on and the sensor needs to be replaced. Check to make sure each connector has been jumpered if you are not using them. Any one of them open will cause the loop to be broken and the light to come on. The E-brake light works differently.

Tim
Old 07-28-2002, 05:50 PM
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thanks...i'm going to get in there and check the "loops" this week.
porsche only put these sensors on so owners who don't get dirty would know when to replace the brakes........
my.02
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Old 07-28-2002, 06:00 PM
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Tim Polzin--re: brake wear indicators--I beg to differ. This is what Bentleys manual has to say about it.

"With the ignition on and the engine stopped, the pad warning indicator light is activated via the oil pressure light circuit. The light comes on if:
* There is a short to ground anywhere in the warning circuit.
* A brake pad is worn, causing the sensor to make contact with the brake rotor while braking.
* The wire is damaged and touches ground.
The indicator light then stays on until the ignition is turned off, even if there is no longer a ground contact. If the ignition is turned off and back on, the light stays off until a ground short turns it back on again. As I said, the sensor is "looking" for a ground. It also states that replacement of the warning sensors can be avoided by replacing the brake pads ASAP.
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Old 07-29-2002, 11:36 PM
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Hi George.

If you look at the wiring diagram in the Bentleys manual, you'll see that the circuit follows through in a loop from the warning light circuitry serially through each wheel sensor then finally back again. If the loop to is broken, the light comes on. As an example, if you were to unplug a sensor at one of the wheels, the light goes on. The sensor in each wheel is in effect, nothing more than a block with a small wire inbedded in it. When the brake pads wear, eventually, the sensor pad will wear through to the point of breaking the wire, loosing loop continuity.

I'll have to admit, I did not read the paragraph you quoted in Bentley. As you quoted, what Bentley says really does not make much sense to me. Having replaced the entire rear brake sensor harness from the tunnel to each wheel on my Carrera, their wiring diagram is as described.

Tim
Old 07-30-2002, 06:45 AM
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Tim--the write-up I quoted from re: the brake wear sensors is on pg.460-2 in Bentleys '84 to '89 manual. What pg. is the circuit on that you described? I tried disconnecting one wheel's sensor and didn't get a light and would like to review the wiring diagram you mentioned. Thanks.



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Old 07-30-2002, 07:32 AM
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Hi George. Got to love this forum. Learn new stuff every day. By the way, did I tell you what a great choice of cars you have. I also have an 86 Targa. What colour is yours, mine is Iris Blue.

Anyway, look at the bottom of page 970-79 in Bentley's.

Reading through Bentley's here is what I think is happening. The circuit has two triggers. Once triggered, the light stays on till the ignition is turned off.

1) If the serial loop through all sensors does not have continuity.
2) If the line is grounded.

I was not aware of trigger # 2. That would also explain why KTL experienced an intermittent warning light if the wear of the pads was great enough to cause a grounding.

I know that on mine, if I did not complete the loop for the rear wheels, the light would always come on without grounding. This was also reported on post:

Brake wear indicator light

Tim
Old 07-30-2002, 08:15 AM
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Tim--I interpret the diagram differently than you do. The sensors are tied together in a loop so that any one of them touching a disc (ground) will complete the circuit. As the write-up states: If the ignition is turned off and then back on, the light stays off until a GROUND SHORT TURNS IT BACK ON AGAIN. Now, what do you make of the sensors shown on page 970-81?? The symbols look like they're some sort of choke or inductance! Interesting!
My 86 Targa is Prussian Blue (Dark midnite blue). Shows dust and light soil up a bit quicker than the lighter colors but she she sure cleans up and details beautifully! Have had it since '93--bought it with 19.9K miles and it's been my daily driver ever since. GREAT car! What problems have you experienced with yours? Let me know and I'll relate my experiences. Enjoy!


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Old 07-30-2002, 12:28 PM
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We've certainly hijacked the thread George. Anyway, interesting to me.

I actually think we are talking the same thing. The only thing I don't understand is why your light does not come on when you disconnect one of the connectors. Perhaps someone else can chime in here and confirm the operation of theirs. Mine goes on if any of the sensors are disconnected.

Anyway, here is how I think it works. One of the two leads going to the three lighty thingy (techical terms lay people may not understand) supplies power through a limiting resistor through the four sensor loop and back to the second lead. The second of the two leads is kept high through the four sensors resulting in the light staying off. If it is brought to ground, (or open) the second line goes low and the light comes on. The tie to the oil light just tests the lamp to make sure it works. It goes out when the engine starts anyway.

When the pads wear to the point of causing the sensor to contact the disk, voila, the line is grounded and the light comes on and may be somewhat intermittent. When the pads wear further, the connection in the pad opens and the light stays on all the time.

I actually have a spare three lighty thingy and if I get really bored some time, will take it apart to confirm operation.

Tim
Old 07-30-2002, 05:30 PM
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Quote:
Originally posted by Jorgeman
.......It also states that replacement of the warning sensors can be avoided by replacing the brake pads ASAP.

and, to avoid replacing the engine, fill up the gas tank when its emplty

and, if you dont use the brakes.... you wont need to replace the pads
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Old 07-30-2002, 08:44 PM
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