[cushjbc]

I'm trying to figure out what this item does on the rear of a 1979 450 SLC oil, temp, fuel gauge (instrument cluster). I think it has something to do with the brake wear indicator circuit.

For those of you that have a brake wear indicator, does that light illuminate when the ignition is turned on then extinguish after the engine is started (alternator running)?

 

[ryson]

I have a '79 450SLC and when my brake pads were low, the dash cluster light would primarily remain on while driving. Sometimes the light would turn off only to come back on later while braking as the sensor was getting triggered again.

 

[cushjbc]

I have a '79 450SLC and when my brake pads were low, the dash cluster light would primarily remain on while driving. Sometimes the light would turn off only to come back on later while braking as the sensor was getting triggered again.

Ryson,

Thanks for the reply. When you get a chance, can you see if your Brake Wear Indicator illuminates at ignition key position 1 (before you start the car)? ETM depicts a circuit that leads me to believe it was designed to do this but I'm not sure yet. There is a diode and resistor in between pin 8 of the triple gauge and a spade that I believe is supposed to be connected via .75mm blue wire to the bulb socket for that light. If I am right, you should have a battery light and a brake wear light when the key is turned to position 1 (Start / Run bus powered by battery only).

My theory - this circuit was designed as a bulb check for the Brake Wear Indicator.

 

[cushjbc]

@therling 's discussion in his relay thread prompted me to take a closer look at the relay looking object in post #1. There is a diagram on the side that sort of matches the ETM. I used it to test my theory and confirmed that this circuit is what I thought it was. Surprising MB would go to that much effort for a bulb check on the Brake Wear indicator. Maybe German automotive regulations caused this.

 

[jbparchitects]

Cush
On my 76 450SLC the brake wear indicator does indeed light up in pos 1 and extinguishes immediately the engine starts.

 

[cushjbc]

@jbparchitects ,

Thanks for the confirmation. I've been working on instrument clusters for a couple weeks. The circuit for the Brake Wear indicator is on one of my spares so I became curious about the relay. The battery light receives a ground at pin 8 of the triple gauge connector from 61/D+ at the voltage regulator when the alternator is not spinning. When the alternator is spinning, 61/D+ presents regulated 13+ volts at pin 8. The other terminal of the battery light bulb receives circuit 15 (start / run bus) power at pin 7 when the ignition is in key position 1 or 2. Voltage at both terminals is a groundless loop so the battery light extinguishes. The brake wear indicator taps pin 8 internally for the ground feature when the alternator is not spinning. The relay on the back of your triple gauge is in series with a diode and resistor. This circuit blocks regulated voltage to the brake wear indicator when the alternator is spinning - open circuit.

There is a spade terminal (seen behind the relay in post #1) that is the output of a grounded state of pin 8. It is joined by the brake wear sensor leads. When any of these leads presents ground to that spade, the brake wear indicator is grounded. When start / run bus power is present at the + terminal of the tachometer (protected by fuse 10 in 1979 ETM), the brake wear indicator bulb gets power. When both conditions are present, the bulb illuminates.

 

[ryson]

Sorry Cush. Finally had time to pick up the SLC to confirm the brake wear indicator light. As Jbp said, my SLC does the same thing. Briefly on in position 1 and disappears upon startup.