wanna911

I have a 996 TT in which I have disabled the PSM, meaning disconnected the charge pump for PSM which completely disabled it. I ran it for a year and a half like that with no problems whatsoever. Then one day this summer, heading in the Turn 10 at Road Atlanta at 150 POOF, SCREECH, SMOKE. ABS failure (flatspotted a complete hole in the tire). I was running Yoko Slicks that I had run several times before. Turned the car off in the paddock, when I turned it back on to drive on the trailer, no lights nothing. Over the next few weeks before I tried repairs just driving around the ABS failure light would come on and off sporadically, never leaving a code (even for PIWIS at the dealer).

I replaced both front wheel sensors and harnesses (to abs unit).

Fast forward to September, ran Road Atlanta Sept 12 with zero issue, felt great (pagid blacks in front, Kumho V710's all around). Then went to Barber two weeks later and on my second session going into museum turn (7b), POOF, SCREECH, SMOKE. This time the ABS failure light was on, but not red like before, it was just a yellow light which went away with a power cycle of the car. I was driving fairly hard at the time so with some nudging I went back out and really took it easy on the brakes and running laps just not to waste the weekend. Sure enough, barely braking (50%) into (7a) this time, same thing again!!! I was hardly touching the brakes at all. This time drove it into the paddock and without turning the car off got some codes.

Left Front Wheel Sensor
ABS Booster Pump
CAN signal implausible


Mind you the wheel sensors are brand new and so are the harnesses, so what gives? Anyone had an issue like this in 996 car? I find it hard to believe that debris in the sensor could have caused this as I've never had this issue before. Any ideas?

jrgordonsenior

I had debris, specifically small pebbles, cause the exact same problem in my 99' 996 earlier this year. Cleaned out the sensors, cleared codes, adn all was well. I've also experienced the same intermittent issue when the headlight ground wire nut worked loose and it was shorting. That brown wire is also the ground for the ABS pump....

wanna911

Where does that brown wire ground to?
Thanks a lot, this is getting very frustrating. Did you record the codes? Were they the same?

ZSA Motorsport

yeah, check all the wires, ensure nothing is shorted, put the durametric on it, and wiggle it a bunch and see if errors pop up. sounds like you have a short, a broken wire, etc.

the minute that ABS senses a failure in the sensor or otherwise, it shuts down the system and if you're in th ebrake zone, you're probably utilizing th eabs and it goes instant lockup.

we had this a lot (ghost pedal, lock ups)wiht the grand am boxsters till we finally got grand am to permit bosch ABS units . but ultimately, you should strive to stay out of ABS as much as possible for this reason. ultimately you sohuldnt be in the ABS anyway. heats up easily and well, its unreliable in the road-based porsches.

Viperbob1

Have similar issue right now on a TT (left rear sensor). Everything in the shop looks good, go out on track and do some laps and ABS light comes on. Clear code and fine till next session. Just rewired that sensor from the pump all way back. Pulled axle (wheel bearing was already checked long time ago for this issue) and look for debris. Nothing really. Cleaned it all anyway. Heading to Daytona to race PCA this weekend. Fingers crossed.

This same car did have an issue like you are describing couple years ago on the left front. Seems that sensor wire rubs on the chassis and will short causing intermittant ABS lights. I found this out on Rennlist posts at the time. You replaced the harness so I would assume this would have been replaced. We used a GT2 ABS sensor repair kit from Porsche and that solved our issue..

Veloce Raptor

Running Yoko slicks, could it be possible that there is a huge amount of hot molten klag flying everywhere, including around the wheel backing plates, messing up ABS sensor clearances, etc.?

wanna911

Not possible, since that event in June, I replaced all the wiring from the sensor to the abs unit and had only run V710's since then and only two days.

I think it's a short in a wire somewhere because the second time this happened I was no where near braking hard enough to engage the ABS. Which would explain the booster pump fault issue, seems like the booster pump went haywire and increase brake pressure to some ungodly amount.

Car is at Goldcrest now and I'm sure they'll figure it out, but this is good info that I can pass along to them. Could still end up with a 996 Cup ABS. If I do, I'm really excited about the possibilities.

Veloce Raptor

Ah...damn, I hate electrical/electronic gremlins.

Hell,, man, just get a 996 Cup and we can have some real fun!

wanna911

Race car in the not so distant future. Want to max out this Turbo first, would like to see a 1:26.x at Road A.

Veloce Raptor

Ahem.

924RACR

I'm sorry, let me understand this correctly. You disabled PSM by disconnecting the pump motor? And now ABS is failing? And you're not sure why?

wanna911

I disconnected the PSM charge pump, not the booster pump, and have run for well over a year without an issue as have many other people done the same thing. I feel confident saying my issue is not related to the disconnection of the PSM in any way. But disconnecting the PSM from the ABS is the easiest way to turn if off (completely) without all kinds of weird hacks.


I will see how close I get in December, need two seconds. Going to be a big year next year though as I intend on going to NASA Nationals for Time Trials so will be looking for some help.

2BWise

I've got the same confusion Vaughan has. How can you disconnect the PSM from ABS? Aren't they the same unit. Common pump motor and common ECU. Did ABS ever function with PSM disconnected?

jrgordonsenior

Codes was only for the wheel sensor....

IIRC there are 2 brown wires that come together and ground by bolt to the frame rail adjacent to the driver's headlight inside the front trunk. One goes to the back of that headlight, the other to the ABS pump. I've also seen only 1 wire in the trunk with the wiring continuing on from the back of the headluight to the pump. I'd check for continuity...

wanna911

Yes, ABS functions fine. There is one plug in the bootlid that can be disconnected. Goes from the charge pump for PSM to the booster pump for ABS. You get a PSM off light on the dash and fault light on the display, but ABS works fine.

Pictured below.



Great, they will be thoroughly gone over! Thanks.

BTW, are those the two brown wires that you are referring to in the picture?