ABS Failure on the track - root cause
05-06-2009, 02:57 PM
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ABS Failure on the track - root cause
I recently experienced ABS failure at the track. Under hard braking coming into a 90 degree turn. Felt like there was minimal response, followed by no response. That led me to get on the brakes even harder, and then harder, and then the next thing I know, the wheels are locked up and I'm off. Luckily, no harm done other than flat-spotting a tire.
Issues with the wheel sensors seem to be the universal explanation for others who have lost ABS, but this turned out to be different. Turns out the left rear brake light socket had broken - could have been weeks, days, or seconds before the incident. In the process, a few stray broken pieces had found their way towards the electrical connectors, and at the moment of braking, shorted the circuit. (Must have been the negative g force under braking that jostled the broken pieces into just the wrong position).
Although ABS monitors brake pressure, I learned that it also monitors the brake light circuit. My theory is that with the circuit blown, the split second of feeling no brake response was caused by the sequence of applying current through the brake light, blowing the circuit, ABS getting confused by brake pedal pressure with no brake light current, and then finally disengaging... leaving me to me lock up the tires on my own, under even more aggressive braking.
The root cause was determined when the fuse was replaced, brake applied, and the whole thing blew again. Had to replace the whole brake light circuit wiring harness.
The really strange thing about the experience is that I subsequently ran in 5 more sessions throughout the weekend - apparently without a working brake light!
And nobody noticed!
I guess everyone figured that I am so slow that I don't need to use my brakes.
Issues with the wheel sensors seem to be the universal explanation for others who have lost ABS, but this turned out to be different. Turns out the left rear brake light socket had broken - could have been weeks, days, or seconds before the incident. In the process, a few stray broken pieces had found their way towards the electrical connectors, and at the moment of braking, shorted the circuit. (Must have been the negative g force under braking that jostled the broken pieces into just the wrong position).
Although ABS monitors brake pressure, I learned that it also monitors the brake light circuit. My theory is that with the circuit blown, the split second of feeling no brake response was caused by the sequence of applying current through the brake light, blowing the circuit, ABS getting confused by brake pedal pressure with no brake light current, and then finally disengaging... leaving me to me lock up the tires on my own, under even more aggressive braking.
The root cause was determined when the fuse was replaced, brake applied, and the whole thing blew again. Had to replace the whole brake light circuit wiring harness.
The really strange thing about the experience is that I subsequently ran in 5 more sessions throughout the weekend - apparently without a working brake light!
And nobody noticed!
I guess everyone figured that I am so slow that I don't need to use my brakes.
05-06-2009, 03:48 PM
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Seems odd that Porsche would engineer a sophisticated, modern ABS system so easily disabled by a shorted out brake light.
Professional Racing and Driving Coach
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05-06-2009, 03:55 PM
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I'm curious, with the brake light out on the subsequent 5 runs, did the ABS work?
05-06-2009, 04:26 PM
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No, it was out all weekend. I could reset the ABS and PSM warnings by turning the car off and back on, but the moment I braked hard again, the warning lights would come back on, and ABS would not engage.
Just to clarify the role of the brake light... the fuse protects the entire brake light wiring harness, and technically, it's the fuse that gets blown. Agreed, it sounds weird to simplify it down to the "light".
I'd be interested in other theories for simultaneous ABS failure, a busted bulb socket, and two blown fuses. And if the cost of the replacement wiring harness is considered, that alone should qualify it as a part of a sophisticated, modern ABS system.
Just to clarify the role of the brake light... the fuse protects the entire brake light wiring harness, and technically, it's the fuse that gets blown. Agreed, it sounds weird to simplify it down to the "light".
I'd be interested in other theories for simultaneous ABS failure, a busted bulb socket, and two blown fuses. And if the cost of the replacement wiring harness is considered, that alone should qualify it as a part of a sophisticated, modern ABS system.
05-06-2009, 04:31 PM
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From what I now know of the Porsche ABS,and all ABS production car systems, I can see this happening.
The systems are designed for street application, the system does not always like all the changees to stickier tires, better pad material, better suspension, etc etc.
The race cars come with a different ABS system.
The systems are designed for street application, the system does not always like all the changees to stickier tires, better pad material, better suspension, etc etc.
The race cars come with a different ABS system.
05-06-2009, 04:35 PM
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In the meantime, I've got a new wiring harness 
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05-06-2009, 04:36 PM
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From what I now know of the Porsche ABS,and all ABS production car systems, I can see this happening.
The systems are designed for street application, the system does not always like all the changees to stickier tires, better pad material, better suspension, etc etc.
The race cars come with a different ABS system.
The systems are designed for street application, the system does not always like all the changees to stickier tires, better pad material, better suspension, etc etc.
The race cars come with a different ABS system.
Well, you sure know a hell of a lot more about it than I do.
That said, the only time I have ever experienced ABS failure was also in a street-based car with street-based ABS....but it was after geting t-boned on track in a race, which broke 2 wheels & destroyed at least 1 ABS sensor.
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05-06-2009, 04:39 PM
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Well, you sure know a hell of a lot more about it than I do.
That said, the only time I have ever experienced ABS failure was also in a street-based car with street-based ABS....but it was after geting t-boned on track in a race, which broke 2 wheels & destroyed at least 1 ABS sensor.
Professional Racing and Driving Coach
That said, the only time I have ever experienced ABS failure was also in a street-based car with street-based ABS....but it was after geting t-boned on track in a race, which broke 2 wheels & destroyed at least 1 ABS sensor.
Professional Racing and Driving Coach
05-06-2009, 04:42 PM
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Well, you sure know a hell of a lot more about it than I do.
That said, the only time I have ever experienced ABS failure was also in a street-based car with street-based ABS....but it was after geting t-boned on track in a race, which broke 2 wheels & destroyed at least 1 ABS sensor.
Professional Racing and Driving Coach
That said, the only time I have ever experienced ABS failure was also in a street-based car with street-based ABS....but it was after geting t-boned on track in a race, which broke 2 wheels & destroyed at least 1 ABS sensor.
Professional Racing and Driving Coach
I have been surprised by how much I didn't know about the ABS myself.
As for your ABS failure, it must have had too much braking torque ! Or is that HP ?
05-06-2009, 04:43 PM
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05-06-2009, 04:44 PM
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05-06-2009, 11:03 PM
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