Brake wear idiot light on; sensors removed

[jazzbass]

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[dentist90]

Originally posted by arerrac
I'm at my desk so I can't remember off the top of my head, but doesn't that light also come on if/when the fluid level is low????

-W

I believe that function is integrated with the parking brake light. Also indicates failure at master cylinder. The brake wear light is a single function indicator.

Follow-up question: Just to clarify... if I want to check that my brake pad wear indicator is working, do I just attach an alligator clip to one of the leads to the pad sensors and ground it out? I guess the rotor itself is the normal ground source?

[EarlySport]

Originally posted by aj88cab
If you didn't remove the connectors, you can put a jumper across the terminals on the connectors at each wheel and the warning light should go out.

What a long post when the first guy to reply got it right...

[jazzbass]

Originally posted by Bill Verberg
3.2 Carrera use a one wire system that seys the light off when any part of the system is connected to ground.

Later cars have a 2 wire system that energises the light when there is a break in the circuit. Just the opposite of the way the 3.2s work.

Wrong. The pad sensor system in a Carrera is NOT a one wire system. I saw this thread earlier today but wanted to check one more thing to verify before I commented. I also checked the archives and found there is a ton of misinformation about this, so I think it's time to put this to rest once and for all:

1. The brake wear sensor system in 84-89 Carreras is NOT a single wire grounding system. It is a 2 wire open circuit detection system that will also work if the sensor wire is grounded.

2. The e-brake/low fluid level warning circuit is separate from the pad wear sensor circuit. I've seen some confusion between the two, thinking that both systems operate the same light. They don't. Having the e-brake on or the fluid level lights up the light the says "BRAKE". The pad wear sensor lights up the circle with 6 dotted lines around it. Low oil pressure lights them both up via a diode based "OR" circuit.

What do I have to back this up? Factory wiring diagrams, the internal schematic of the brake warning system, experience with my own 85 Carrera and a rusty but still valid EE degree.

From the factory diagram you can see that the system consists of a single wire loop with all sensors in series:



I was familiar with this circuit because I just this weekend removed my wear sensor circuit, as my car is a mostly track car and the pads I run don't have sensor cut outs (and would probably melt them if they did). If this was a single wire grounding system, I doubt Porsche would have run the series loop like they did. But, Porsche engineers have done weird things before, to lets look inside the circuit.

So how does it work? The basic premise is that when the pads get low, they'll wear through the sensor wire and sever it, creating an open circuit between pins 1 and 2 of the 3 light module. Inside, it's a very simple circuit that biases on a NPN transistor to turn the light off when the system is intact:



Pins 1-2 close the circuit between the 750 and the 1.6k resistors. So, when the loop is intact, there is ~ 9V at the base of the transistor, turning it on. When the transistor is on, it grounds the gate of an SCR, keeping it off. The SCR is what actually controls the light. When it's on, the light is on.

If the circuit between pins 1-2 is open - or grounded - the base voltage will drop to zero, turning the transistor off. This turns on the SCR which turns the light on. The oil pressure switch is simply diode-ORed to both brake warning lights at the end of the circuit, but has nothing to do with the open circuit detection part of the circuit at all. So yes, it does work if you ground the sensor wire, but this is not it's primary mode of operation. Fundamentally it's designed to detect an open circuit. Why do I say that? Well...

Originally posted by dentist90
Follow-up question: Just to clarify... if I want to check that my brake pad wear indicator is working, do I just attach an alligator clip to one of the leads to the pad sensors and ground it out? I guess the rotor itself is the normal ground source?

No. I have one of the sensors from my 85 on my desk and tested it. The main black part is plastic - and is not conductive at all. Furthermore, the wire looped through the plastic is insulated. So yes, if you ground the sensor wires you should see the light come on. But, there is nothing electrically conductive in the sensor itself except the wire loop. Which is why I say it's primary mode of operation is to detect an open circuit. Sure, it'll light up as the rotor wears through the insulation and grounds the circuit, but really - you'd have maybe 3-4 applications of the brakes before the rotor wore through the line altogether and opened the circuit.

For this system to be a single wire ground design, a significant portion of the wear sensor itself would need to be conductive to make reliable rotor contact as the pad wore. That's not the case with the sensors on these cars.

Actually, the bulb is molded into the lens and isn't replaceable.

You're right, it is molded in, but it can be replaced, just not super easy like. If you pull the unit out, the assembly unsnaps pretty easily and you can see the 3 bulbs soldered to the PCB. Unsolder the old, solder in the new to replace. Or, just take out the old.

[aj88cab]

I love this forum, the fact that after 4 years of owning this car there are still things I continue to learn. Albeit, sometimes I have this obsession to go beyond a point that could be considered reasonable and prudent by a sane individual The level of detail that folks bring to the table is amazing.

Jazzbass - that is a great detailed explination & diagram of the internals of the circuit, but I think I have to respectfully disagree with your conclusion:

Originally posted by jazzbass
So yes, it does work if you ground the sensor wire, but this is not it's primary mode of operation. Fundamentally it's designed to detect an open circuit.

I don't think "open circuit detection" is the primary mode of operation. My reason is this - the rotor is conductive, without the grounding feature it is possible that even after the wire loop in the sensor is broken the two ends could contact the rotor and complete (close) the circuit. In real world operation it is probably designed as a dual mode system (open mode & ground sensing) since either mode if used alone could give a false or inconsistent reading.

[jazzbass]

Originally posted by aj88cab

Jazzbass - that is a great detailed explination & diagram of the internals of the circuit, but I think I have to respectfully disagree with your conclusion:

I don't think "open circuit detection" is the primary mode of operation. My reason is this - the rotor is conductive, without the grounding feature it is possible that even after the wire loop in the sensor is broken the two ends could contact the rotor and complete (close) the circuit. In real world operation it is probably designed as a dual mode system (open mode & ground sensing) since either mode if used alone could give a false or inconsistent reading.

Well, from an engineering perspective, let me clarify what I mean by "primary mode of operation". Specifically, it's "how is this thing going to work 80-90% of the time". Take the 915 - would you say driving a car forward isn't it's primary mode of operation because it has a reverse gear in it? Same thing here. Yes, the sensor will work if grounded to the rotor. Yes, they most definitely thought about this during the design. But, if you look at the design of the sensor, there's no question that 90% case is open circuit mode.

The sensor is a small piece of non-conductive plastic with an insulated loop of wire in it. The wire is not secured in the loop. It won't take long for the rotor to wear through the wire. Yeah, while it's wearing through the wire it's probably grounding on the rotor. But, the wire is a small 18ga wire and that would probably take 3-4 brake applications before it wears through - maybe less. Now, at this point the circuit is open, maybe occasionally grounding through the rotor is one of the loose ends of wire in the sensor just happens to randomly contact the rotor. But, no engineer in the world will design a sensor based on random contact as it's primary trigger method.

That's why I say open circuit mode is the main design - grounding operation occurs 4 out of potentially hundreds of inputs and then randomly after that is maybe 10% of time it detects th pads are low. The other 90% of the time is all open circuit.

There are one-wire grounding systems that detect brake wear primarily via grounding to the rotor. This one isn't one of them. In these grounding systems, the entire sensor has a large conductive area to ensure reliable and repeatable electrical contact with the rotor. That is simply not the case with the Carrera sensors.

[aj88cab]

jazzbass,

No real argument with your conclusions. The good news is we now know that there are two modes that can trigger the warning lamp.

Yet another "Porsche Engineered" system demystified

[rick-l]

Is this the actual circuit that is in the car? Just want to add to my collection. Off to the right is just an SCR and a bulb?

Makes sense.

Originally posted by jazzbass
The basic premise is that when the pads get low, they'll wear through the sensor wire and sever it, creating an open circuit between pins 1 and 2 of the 3 light module. Inside, it's a very simple circuit that biases on a NPN transistor to turn the light off when the system is in tact:

[rick-l]

Originally posted by dentist90
Follow-up question: Just to clarify... if I want to check that my brake pad wear indicator is working, do I just attach an alligator clip to one of the leads to the pad sensors and ground it out? I guess the rotor itself is the normal ground source?

Why not just unplug one and turn the ignition on?

[jazzbass]

Originally posted by rick-l
Is this the actual circuit that is in the car? Just want to add to my collection. Off to the right is just an SCR and a bulb?

Makes sense.

The whole circuit. Reverse engineered from the module in my 85:

[aj88cab]

I might be splitting hairs here, but some folks do make a distiction between "igintion on" and "engine running". In this case the engine must be running, disconnecting the connector should turn the warning light on. Reconnect and the light will stay on until the engine is turned off.

[jazzbass]

Originally posted by aj88cab
I might be splitting hairs here, but some folks do make a distiction between "igintion on" and "engine running". In this case the engine must be running, disconnecting the connector should turn the warning light on. Reconnect and the light will stay on until the engine is turned off.

Not splitting hairs IMO, that's a good point. Because w/o the engine running, the oil pressure switch will ground out the circuit via pin 7 and the lights will be on all the time.

[2.7RACER]

This is the discussion I've been looking for re:the brake pad sensor circuit.
My question at this point is the proper removal of the module from the dash of my '85.
I have accessed the rear of the module by removing the clock and speedo.
At this point I have disconnected the connector at the rear and pushed the module from the rear by hand.
Is there some kind of retainer to be squeezed before removing the module?
Thanks jazzbass for your clear documented explanation of this circuit. Better than that found in my Bentley.

[Winterfarm]

Originally posted by jazzbass


That's why I say open circuit mode is the main design - grounding operation occurs 4 out of potentially hundreds of inputs and then randomly after that is maybe 10% of time it detects th pads are low. The other 90% of the time is all open circuit.

There are one-wire grounding systems that detect brake wear primarily via grounding to the rotor. This one isn't one of them. In these grounding systems, the entire sensor has a large conductive area to ensure reliable and repeatable electrical contact with the rotor. That is simply not the case with the Carrera sensors.

After reading Jazzbass' explanation it seems his observation is inescapable; if you're going to bother to design and build an open circuit system then that would be the primary mode of operation, particularly insofar as the open circuit system is more useful as once opened it will continue to give warning when brakes are not being applied, which certainly is better than warning only when brakes are applied.

[dentist90]

Exactly what I was thinking. Having the circuit broken forces you to tend to the pads or the light will not extinguish. A ground- indicating system would cause the light to flicker but be mostly off.

Another question: if one were to replace the pads before the light comes on, do you re-use the old sensors or get new ones anyway? Similarly, if your light comes on, but it turns out only one sensor has worn thru, must you replace all of them?

[jazzbass]

Originally posted by dentist90
Another question: if one were to replace the pads before the light comes on, do you re-use the old sensors or get new ones anyway? Similarly, if your light comes on, but it turns out only one sensor has worn thru, must you replace all of them?

Reuse if they're not worn. If one has worn, just replace that one. The goal is to keep continuity, which can easily be tested with a DVM. If the sensor has continuity, I'd reuse.

[Jean Berk]

Dear JazzBass, thanks for suberb enlightenment on this - I've been trying to solve the mystery for a while.
Thanks to you, I've carefully removed the light unit from the dash, bridged pins #1 &#2 and my light has gone out.
Thanks!

[jazzbass]

Glad it helped

[Mitch Leland]

Who's on first... I removed the pig tail that connects to the brake pad, installed a plastic plug into the female plug end that mounts to the strut and trailing arm. Dash light stayed on. Then I read you need to complete the circuit so I cut the old pig tail off leaving enough to twist the TWO wires together, found a perfect cap to fit over the bare wires and plugged the plug back into the female socket. Dash light stays on...

What am I doing wrong????

Thanks,

[aj88cab]

Sound like you did everything correct. Make sure you turn the ignition off then on agan. The only thing I can think of is that one of the twisted wires came loose when you attached the cap or one of the connectors is bad or dirty. A short somewhere else in the circuit is always possible. Low fluid could do it too but that would be too easy....even easier would be if it was the parking brake light you were seeing....Don't mean to insult your intellegence with those last two, just looking for options

Was the light off before you removed the wear sensors?