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Old 06-26-2019, 11:20 AM
JHZR2's Avatar
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Join Date: Jan 2002
Location: New Jersey
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Intermittent Brake Lining Warning Light

My 82 300cd has an intermittent brake lining warning light.

I just had a partially seized caliper, so I rebuilt both front calipers, replaced the rotors, pads and sensors, and used dielectric grease on the clean/dry sensor connection to the harness

So no itís not worn pads, no itís not bad sensor leads. Itís something else.

And itís the yellow lining light, not the red brake light.

Iíve gone through this:

W123 dash brake warning lights, a review

Wires look good, connection points look ok.

Makes me wonder if itís actually the cluster.

In the link above, it looks to me that the charging light and the lining light are isolated by a resistor. I assume thatís to allow for redundancy in the alternator excite circuit. Could that be a clue? Charging system works fine on this car.

Anyone have any other experiences troubleshooting this light?

I assume perhaps disconnecting both harnesses Underwood would help isolate side/location?

Thanks!
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Current Diesels:
1981 240D (73K)
1982 300CD (162k)
1991 350SD (113k)

Past Diesels:
1983 300D (228K)
1985 300D (233K)
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Old 06-26-2019, 11:28 AM
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Has the cluster been removed lately? If yes, is there a possibility that the bulbs for the brake pad warning light and the charging light were switched when plugged back in ? It is possible for the cluster to have a bad ground which is fixed by attaching a new ground wire (brown) from the back of the cluster to the steering column support where the other brown ground wires are attached.
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Old 06-26-2019, 12:49 PM
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Quote:
Originally Posted by BWhitmore View Post
Has the cluster been removed lately? If yes, is there a possibility that the bulbs for the brake pad warning light and the charging light were switched when plugged back in ? It is possible for the cluster to have a bad ground which is fixed by attaching a new ground wire (brown) from the back of the cluster to the steering column support where the other brown ground wires are attached.
No it has not been removed. Even if it was and they were switched, the alternator charging light should only ever be on when starting the car and then go out. The charging system works fine.

I’m sure the cluster could use an extra ground. I’ve not seen a w123 car that hasn’t benefitted. That said, the rationale why an extra ground, when the function of the brake light is to turn on when the circuit is shorted to ground, is foreign to me. Since the wiring harness and sensors are all in tact and operating fine, how another ground would improve it, unless there’s some other stray current in the cluster that needs to get directed to ground, is not clear.

There is a metal part of the brake wear sensor harness, which is supposed to be touching the mounting point on the caliper. I haven’t tested but think that’s electrically continuous to the light circuit. The connection just needs to be made via the sensor. So for the light to be coming on, some ground path needs to be intermittently being made, as I think voltage from fuse 10 is continuously there to the sensors.
__________________
Current Diesels:
1981 240D (73K)
1982 300CD (162k)
1991 350SD (113k)

Past Diesels:
1983 300D (228K)
1985 300D (233K)

Last edited by JHZR2; 06-26-2019 at 01:04 PM.
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Old 06-26-2019, 04:21 PM
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Join Date: May 2016
Location: Texas Hill Country
Posts: 4,657
The brake wear indicator is pretty simple. Basically a fuse passing power through a light bulb to the wear bars that ground against the brake rotor when they wear through. Make sure the wear bars still have their paint/coating on them. Inspect the wiring to make sure it hasn't rubbed through and is grounding against the chassis somewhere. There's not much more that will make that light glow.
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'11 Honda Accord EX - "The Daily" 64K
'83 500SL Euro - "The Money Pit" 116K
'91 350SD - "The Diseasel Jr." 168K
'86 300SDL - "The Diseasel" 189K (Totaled 1/31/19)
The Diseasel Thread - Everything You Didn't Know You Wanted To Know
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