My struggle with the dreaded ABS/PSM warning and 4460 code

My 996TT recently had a brake line and the master brake cylinder replaced. After that, I was getting the ABS/PSM warning. My indy mechanic can rebuild a Mezger engine blindfolded, but diagnosis of electronic issues are probably not his strong point. He pulled the codes, replaced the brake pedal switch, fiddled endlessly with the adjustment of that switch, but the issue remained. When we got the car back, I read error codes 4460 (pressure sensor) and 4340 (stop light switch) using the Durametric software. The ABS/PSM lights would come on after driving a couple of km / 10 minutes. I cleared both errors. Only the 4340 error ever came back. I took the switch out and it seemed a bit dodgy, the adjustment ratchet felt very loose. Got a new OEM switch from Porsche for �20 and installed it. Adjusted it so the brake lights would come on after a small (1cm) amount of travel on the pedal, before it hit resistance from the cylinder, as per the instructions. When driving, the ABS/PSM lights came on after 100m of driving or so, when hitting 50km/h. Only got error code 4460 Cleaned the MAF sensor as apparently it can cause this error to appear. No change, and readings from the sensor appear to be within valid range. So a few suspicious things: The error appeared after the brake system had been worked on. Replacing the switch did change the behaviour of the ABS system I ran into this interesting post here on the forum. Apparently: The car performs a little self test on the brakes when you first hit 30MPH / 50km/h, and failing that test brings up the 4460 error. My car's ABS/PSM lights ome on at that exact moment, so apparently it fails that test People who have replaced their 996 master brake cylinder with one from a 997 (which has a different brake circuit) have experienced the same issue. So prompted by that information, I hooked up a voltmeter to the pressure sensor and fired up Durametric. I got: 0.6v at idle, no brakes 1.8v when pressing the brakes. A little below the 2.2v reported in that linked post, but this seems within reason. Durametric reports the brake pedal switches activated correcty, before the brake pressure registers on the voltmeter (otherwise a 4460 can result) When activating the boost pump with Durametric, no pressure increase registers on the voltmeter. When turning off the pump after a few seconds, the pressure spikes briefly (probably through a return line). This is essentially what that self-test does, and a 4460 error occurs if no pressure increase is measured when activating the pump without the brakes being applied. I also performed the brake pedal test, repeatedly pushing the brake pedal while running stationary. An incorrectly adjusted switch will apparently set off a 4460 error when you do this. I got no errors however. Could my mechanic have installed a 997 brake cylinder, and how can I tell? The brake cylinder sadly has no markings on it whatsoever, and both models kind of look the same at a glance. I've asked my mechanic what he installed, he hasn't replied just yet, but it's been a while and perhaps he no longe rhas a record of the part he used. I also wonder why the errors progressed the way they did. After we got the car back, the system passed the brake self test and the ABS/PSM lights came on way later. Perhaps the original as well as the replacement brake light switch was buggered, and having a broken switch would cause the brake self test not to run? By all measurements, the new switch functions well and is adjusted correctly, and I'm only now getting errors on the brake self test. At least I have eliminated some possibilities and zeroed in on a few suspects thanks to the great info on this forum. So at this stage I suspect the brake cylinder, and I'll wait for info from my mechanic. In the mean time, if anyone knows how I can tell if mine is a 996 brake cylinder, I'd be grateful. Also, I was hoping for any further information on that brake self test anyone might have.

It really sounds like it could be the mc. i followed that upgrade thread and its real technical. If i remember correctly the fault didnt always come up immediately after the swap for some. The first place i would start would be replacing the mc with one that has a part number that can be verified correct.

I had the same error after replacing the brake booster, master cylinder, and brake pedal switch. The problem was the master cylinder linkage was incorrectly adjusted. The spec says the master cylinder linkage must be 45mm higher than the throttle pedal. That's according to the service writeup. Haven't had a problem since this adjustment.

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Thanks, that sounds like another thing to check. How is that linkage checked and adjusted? I couldn't really find anything about it.

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I'm sorry but the guys at the indy shop did the work and let me know. I'm sure someone else with more hands on can help here.

No joy so far. The mechanic gave me the number of the part he used, it's not an OEM part but definitely suitable for the 996. To double-check, I hooked up a recording voltmeter to the pressure sensor and started driving. Sure enough, when the ABS/PSM lights come on, the meter records a small spike in the brake pressure, so the pump is able to pressurize the system during the self test. The spike is not too great though, it goes from 0.6v to 0.75v. Maybe it's not enough? Tried turning on the brake boost pump again using Durametric, but nothing. Supposedly I should be able to hear it, but there's no sound nor any change on the voltage on the vacuum pump's terminals. I'll contact support, but apparently other people have had the same issue with this software. The self test seems to indicate that the pump is at least doing something. The voltage on the pump terminals is weird though; 12v when open (pump unplugged), 4.5v with the pump hooked up. Is there a way to test this pump manually? I also ran through the tests in the diagnostic manual. The one deviation is measuring the voltage on the sensor lead with the sensor unplugged. Reads 5v, should read 0v according to the manual. There's no short though, with the sensor hooked up I am getting good values from that lead. Other people reported seeing 5v there as well. I'm out of ideas at this stage, short of replacing stuff by trial and error (and 1000s of €). And I am kind of scared that the dealer will follow the same pricey pattern if I have him take a look at it.

I had a somewhat similar scenario with scary codes that was eventually solved (after several expensive yet non-successful guesses) by me installing a $16 brake switch. Chances are your scenario is not the same but goofy little things can sometimes trip up our cars.

Back to suspecting the brake cylinder. The replacement part is listed as suitable for the 996TT... and for all other 996 models. That can't be right. I also noted that sites like PelicanParts and Design911 carry several replacement brake cylinders for various 996 models, but only offer the OEM part for the TT. Should I spring for the OEM part (given that it's not sure that this will even fix the ABS light issue), or does anyone know of a working aftermarket part?

C4S uses the same master cyl as a Turbo. Part# 996 355 910 50. Gaudin Porsche Parts lists it as $455 (as opposed to $565 at Pelican Parts now owned by ECS Tuning), so I can see why some might want a "universal" part that's probably cheaper. What I can't understand is why it was changed. I have never heard of a Turbo m/c going bad. Could you put the old one back for a comparison? Or I'd get a used one rather than throw another half-thousand at *possibly* fixing a problem. I'll also note that lots of ABS/PSM issues with the Turbo are cleared up be replacing the MAF, cleaning it doesn't really seem to work.

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The Porsche dealer quoted just over €500, not too bad considering there's 21% VAT in that price. I'll just go and get the genuine article from them, so far everything seems to point to that cylinder (it was replaced after all, and the warning lights come on during the brake self test). We had it replaced when the shop had to replace a rusty brake line, as the brake action felt a bit spongy. This car has almost 400.000 km on it so maybe the thing wore out just a bit.

Finally got around to addressing this. I got an OEM brake cylinder from the dealer and had our shop put it in. No more errors!

Same here re: Master Cylinder. Interestingly, I'm still having a problem with the PSM - if I disable it the car drives ok. If not, it goes into limp mode over 30 (ie; it won't let me drive over 30). When disabled there's no limp mode, but I've found that if I make a left turn with the brakes applied the ABS kicks in. Wheel speed sensor? There are no codes...

So, I drove the car up to Woodstock over the weekend (about 600 mi. including driving around for 4 days up there). It was fine with the PSM turned off, but the ABS was still touchy at certain times (as described in the post above). I ordered a new wheel speed sensor while I was away and it just arrived when I got home. I did check the ones that are installed now and I don't see any obvious wear or breaks in the wiring. I'll install tomorrow and see what happens.