They are stock 997tt wheels with 235/35/19 size tires (25.5'' diameter) in front and 315/25/19 (25.2'' diameter) in rear. I did a lot of research into this issue and am aware of the max ride height, viscous coupling, ect, and this setup should be in spec and not be the problem, but I will keep it in mind as I narrow out the other potential causes. I'll also add that for now, I'm using different front tires than rear (fronts are PS2's and rears are Nitto Invos, PS2's are 80% new, Invos are brand new -- after PS2's wear, I'm going all Nittos to save the major $ in these sizes, but I can't believe this is the problem either). I'm going to try to install the new ignition switch tonight and see if that fixes it. If not, I'll go from there.

 

Good luck sorting it out. I'm no mechanic, just trying to pass along info in case it helps. The owner's manual does say to recalibrate the steering sensor whenever there is any suspension work done. Also, two different tire compounds on the same car could be setting off the PSM. That is a no-no in the owner's manual.

 

To update, I had Porsche reset the steering angle sensor to fix the PSM problem I described above. It didnt' help.

I've read all I can find on this PSM issue and think my next step is to take off my front 235/35/19 PS2's and put on 225/35/19 to exactly match the rear 315/25/19 diameter. That 3/12'' tire difference seems like its most likely culprit if its not the steering angle sensor.

After that, maybe wiring harness, or wires within it could be broken? I read comments about the MAF, but dont' understand how that is related.

I'll also add that the car does feel quite twitchy after putting the 19'' wheels on. I realize there is less sidewall, so this is expected, but I think my car may not be aligned properly as I've seen others have this problem with the alightment with 19'' wheels fix the twitchy problem. Really, I'm not ever comfortable driving the car on the highway with just one hand b/c its so twitchy it just darts from lane to lane if I don't have two hands on the wheel. I'm thinking this is definately not normal for just 19'' wheels.

Let me know if I"m missing anything. Again, my PSM is activating around corners and causing what feels like just one caliper to set off. I will have to test this again, but I think it may just be during right turns, with the front right wheel's caliper locking. I don't know if I recall any left turns setting off PSM. If this is true, then I'd think its more likely wiring harness than tire size. I saw one person's report of this activity due to a broken wire to the PSM unit from the right tire area.

Edit: My car has not been lowered or had any suspension changes. Tire pressure is 36 front and 44 rear.

 

Try swapping to a set of OEM wheels and tires to see if the symptoms go away. That may be a cost efficient way to narrow it down.

 

I took the car out today for some testing. Definately getting the PSM problem around right turns with front right caliper engaging and PSM/ABS lightly flickering around sharp/moderate speed rights. On left turns, I can't get this problem to reproduce, and everything is fine. So, this to me rules out tire size as the problem, and is more likely a broke wire in the right side wheel attached to the PSM computer, or something with the wiring harness (not that I really have any idea what I'm talking about here). Or, perhaps Porshce screwed up the steering angle sensor reset, or simply never did it, and its skewed right? Thoughts?

I have a durametric cable from my flash, but never used it for anything except the flash. How can I use this cable to see if the computer is giving any error messages, or reset the steering sensor myself before taking it to Porsche for $120 an hour labor?

 

You need to change your tires. Steering angle sensor makes no difference. I had this problem before and the solution was new front tires. Change your tires and let us know!

 

I would add that coasting at 100mph through a corner is probably not the best thing to do in a 911...

These cars like maintenance throttle... 10-15% will settle the rear end and lock the differential up... keeping the car steady through the corner.

 

Thanks .

So you guys still think its tires despite the fact that the PSM problem only seems to be around right turns, not left turns? This seems strange to me, but in the end, would be a reasonably easy fix.

I have an appointment with Porsche Tuesday for a tech to go out with me an see what its doing so we can try to figure it out. At a minimum, I'll see if they can loan me a pair of wheels/tires. Hopefully they'll cooperate considering I just forked over $2k for a belt/tensioner repair last month (well my warranty covered $650 of it).