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So, did you ever wonder how many miles you have left on your front pads after you wear light pops? Short answer: many so don't sweat it.
My light popped on at just under 160K miles as I started on a journey south to Atlanta among other destinations... crap. I looked through the wheel spokes and they sure looked fat and healthy to me... wasn't sure if it was the fronts or the rears that popped as all looked about the same. Anywho... screw it, keep driving. At Thanxgiving, I changed my wheels over to snows and got really sick of seeing that light on my dash, so I cut the sensor cables, stripped the ends, twisted them together, wrapped a few inches of electrical tape on them and... thank goodness it was the fronts... and now no more dash warnings. I did put new sensors on this change.
So... here is the math:
Miles driven since light popped on >7K miles
Thickness of back plate old and new pads: 4.5mm
Total thickness of new pad: 16 mm
Pad thickness new pad: 11.5 mm
Pad thickness of thinnest part of thinnest old pad: 3.2 mm
Total pad burned up over 7K miles: 12.8 mm yielding about 550 miles per mm
Miles left on pad: 1,750 miles
So... max amount of miles you have from when the light pops: about 8,000 miles and you will be fine.
Given so many of you drive so few miles per year... heck, you may be able to go a year or two!!!!
Yep, that is a ton of miles left... so don't sweat rushing out to replace your pads.
Oh, my rear pads and rotors are original but they are ready to pop the light... so: rear rotors and pads last at least twice as long as the rears.
My rotors wore down below the dimples so pads and rotors are timed to fail at the same rate ... at least for a driver like me.
Oh.. no track days, just my everyday driver... fast when I can.
Some more information... some think the light pops on when the sensor is "worn through" or "cut". Not so. The light pops when the wire touches the rotor... it is grounded out. That explains why the light will be intermittent in the beginning... then constant as there is more wear. It doesn't always touch. None of my wear sensors were cut... two were worn to where the shine of the wire was exposed (the wire insulation wore away.. that was all).
Peace
Bruce in Philly
The worn sensor:
The thinnest pad.. shows some break down but the pad was really good. The wear on this one pad was tapered forward... I thought Porsche brake systems were designed to not do that?
Last edited by Bruce In Philly; 01-03-2021 at 11:45 AM.
Great info Bruce. Keep in mind that those who drive their 997s hard can actually see pretty even wear between the front and rear rotors and pads due to the weight distribution of the 911 with the engine over the rear wheels sharing a lot more of the braking than in a normal front engine car and with PSM on, rear wheel slip is mitigated by the brakes being applied to the rear wheel that's slipping.
Was there any squeal during those miles after the light showed?
I don't like seeing light in my dashboard but I really appreciate the thought and time you put into this!
No squeal... no reason to squeal.
Regarding the light... yep drove me nuts, that is why I went and cut the sensor cable and tied them together. IMO, you pretty much have to replace these sensors... some have been able to remove them and re-install them and that would be OK as the cables are not cut (assuming the pad material is not conductive and therefore ground out the circuit and pop the light.. but they get brittle from the heat cycling and you have to know exactly how they go in to remove them without breaking them. For me that is always impossible because I am a dope.
There is so much meat left on these pads, why throw money away before it is required?
As long as the plastic isn't worn through the sensor is still good.
I'm wondering ... The warning light turns on when the sensor supplies a connection to the frame via the rotor (after it wears through the plastic cover). Under normal operation the two sensors (inner and outer) don't connect to each other. Bypassing a sensor should only require that you break the connection to the rotor (remove the sensor or cut the wire). Did you really need to connect to inner and outer sensor wires to turn off the warning light?
As long as the plastic isn't worn through the sensor is still good.
I'm wondering ... The warning light turns on when the sensor supplies a connection to the frame via the rotor (after it wears through the plastic cover). Under normal operation the two sensors (inner and outer) don't connect to each other. Bypassing a sensor should only require that you break the connection to the rotor (remove the sensor or cut the wire). Did you really need to connect to inner and outer sensor wires to turn off the warning light?
I don't know.
I cut the wire coming out of the plug and not at the two sensors... I just bypassed both sensors by cutting upstream and twisting the leads together. I used this approach as I read track rats commonly do this to bypass the wear sensors.... so I knew it would work. I was not sure if it was the fronts or the rears as they all looked about the same wear thickness. I didn't think of just unplugging the sensor from the socket and seeing if the light stayed on... again, I just did what the track rats did.
I asked in another thread how this system worked, and apparently the system is just one big dumb wire loop going from wheel to wheel, sensor to sensor. The leads are not star-wired back to some junction. So the sensors, inner and outer, are connected to each other... they are just part of a loop. Said another way, if you had some sort of electronic sniffer to tap into the car's system, you could not tell which sensor popped, only that the system became grounded somewhere.
Peace
Bruce in Philly
Last edited by Bruce In Philly; 01-03-2021 at 04:25 PM.
As long as the plastic isn't worn through the sensor is still good.
I'm wondering ... The warning light turns on when the sensor supplies a connection to the frame via the rotor (after it wears through the plastic cover). Under normal operation the two sensors (inner and outer) don't connect to each other. Bypassing a sensor should only require that you break the connection to the rotor (remove the sensor or cut the wire). Did you really need to connect to inner and outer sensor wires to turn off the warning light?
Think it's the opposite. When wires touch, no warning light. Light comes on when circuit is broken .... nothing about a connection to the frame.
I snipped my wires, twisted them, soldered and added heat shrink tubing. No light.
Think it's the opposite. When wires touch, no warning light. Light comes on when circuit is broken .... nothing about a connection to the frame.
I snipped my wires, twisted them, soldered and added heat shrink tubing. No light.
That is what I thought, but none of my sensors are cut. None. That picture is the "worst" of them. I will go out to my bench and put a continuity meter on all four (actually two sensors).
Update: Yep, complete continuity. No wires are cut. I just put my fluke on the sensors from where I cut the lead cable. All sensors are intact except for the insulation.
Peace
Bruce in Philly
Last edited by Bruce In Philly; 01-03-2021 at 05:44 PM.
Think it's the opposite. When wires touch, no warning light. Light comes on when circuit is broken .... nothing about a connection to the frame.
I snipped my wires, twisted them, soldered and added heat shrink tubing. No light.
The sensor is OK when the plastic encapsulates it so it does not connect to the frame. The sensor indicates failure when its plastic wears through allowing the conductor within to touch the rotor.
Assuming the wires start at the same sensing point (directly at the warning light or indirectly at the computer) and these wires fan out to each sensor at each corner, connecting them asst the sensor end should not matter.
I'm not trying to be difficult here. I don't deny what I've read track rats do. I'm just trying to understand why it would be necessary to cut and connect wires rather than just unplug the sensor.
To be a bit clearer, from the plug at the rotor, going to the sensors, you have two wires. One runs independently to an encapsulated end on the inside of the caliper. The other runs independently to the outer side. I don't see any place that they are connected when the system is OK.
The sensor is OK when the plastic encapsulates it so it does not connect to the frame. The sensor indicates failure when its plastic wears through allowing the conductor within to touch the rotor.
Assuming the wires start at the same sensing point (directly at the warning light or indirectly at the computer) and these wires fan out to each sensor at each corner, connecting them asst the sensor end should not matter.
I'm not trying to be difficult here. I don't deny what I've read track rats do. I'm just trying to understand why it would be necessary to cut and connect wires rather than just unplug the sensor.
To be a bit clearer, from the plug at the rotor, going to the sensors, you have two wires. One runs independently to an encapsulated end on the inside of the caliper. The other runs independently to the outer side. I don't see any place that they are connected when the system is OK.
What I suspect is happening: The system measures three states
Full continuity tells you you have a healthy cabling system
Grounding is the switch to pop the wear light - your pad is worn
No continuity tells the system there is a system failure... what if road debris ripped out a cable?
Three possible states from a single wire. Not a bad system eh?
If you think it though, the grounding approach makes sense. Think about a cut wire... as the pad wears, the ends of the wire keep wearing down.... and the rotor is connecting the wires.. the rotor keeps continuity. Cutting won't work. Grounding.. brilliant.
I wish I checked it with just being unplugged. Maybe when I need to pull a wheel or put my summers on again. Maybe the track folks never tested it either.. just made an assumption.
Peace
Bruce in Philly
Last edited by Bruce In Philly; 01-03-2021 at 07:37 PM.
What I suspect is happening: The system measures three states
Full continuity tells you you have a healthy cabling system
Grounding is the switch to pop the wear light - your pad is worn
No continuity tells the system there is a system failure... what if road debris ripped out a cable?
Three possible states from a single wire. Not a bad system eh?
If you think it though, the grounding approach makes sense. Think about a cut wire... as the pad wears, the ends of the wire keep wearing down.... and the rotor is connecting the wires.. the rotor keeps continuity. Cutting won't work. Grounding.. brilliant.
I wish I checked it with just being unplugged. Maybe when I need to pull a wheel or put my summers on again. Maybe the track folks never tested it either.. just made an assumption.
Peace
Bruce in Philly
There's not a lot of circuit continuity in a single wire. From what I can see as long as that wire hangs out in the air (all OK as long as the plastic coating remains intact so that the wire doesn't connect to anything) all is fine.
It could be the "connect both wires" option is simply viewed as the easy way to insure the wires are terminated to avoid any possibility of them contacting the frame. But it would seem that simply removing the "connector plug to sensors" assembly should work.
Again, not trying to be difficult. Just curious. And as you say, it may require a test.
I've got Pam's car sheltered in the garage right now while we continue to shelter in place. Next time I get out for a drive I'll flip flop them so I can lift the 997 and see what happens with a simple disconnect. But that may be a while from now.
01-02-2021, 11:29 PM
Rotor wear sensor - how much mileage after light?
