dustinr

Went for a spirited drive today up one of our local mountain roads, car drove great up and most the way back. At one point I slowed to a "mellow'er" pace because I had earlier seen a cop sitting at the crest of this hill. So I'm in a left hand sweeper and right about the time I hit the apex, the car does something scary. There's a quiet sound like I just ran over a mini cattle-guard and the car starts to slow and pull to the outside of the corner, instant understeer. So I ease off the throttle and it regains it's composure. Now I'm paranoid because I don't want it doing that when I'm actually driving it hard, because it would easily cause a shunt.

So I drive it mellow for a while, no fault codes showing, it's driving normal again. So I up the pace a bit but I'm still nervous about it's previous behaivor so now I'm switched on to any discrepancy other than my driver input. So I go into another corner and it happens again; this time I noticed that it happens at right about the same time that I run across some very small wash-board type bumps, bumps you would normally never even notice.

So I turn off PSM and drive on at the same pace, no problems. I'm now aiming for any small bumps that I can see ahead in hopes of duplicating the issue but with PSM off it did nothing.

So I turn PSM back on and continue my driving, aiming for bumps the whole time, and it's fine. Then just toward the end of the curvy sections I take one last fairly descent sweeper with some small bumps and it happened again. Car slows like a brake is being applied, it lurches to one side, and there is a faint sound that sounds like rapid brake pulsing, like you just drove over a micro cattle guard.

Anyone have a clue?

There are no fault codes on the dash "Check" screen.

wross996tt

Could you please provide information on your car? What is it MY type etc.? any aftermarket equipment? How about tires what are they? Inflation pressure? Stock suspension? Did you get codes read with a code reader?

dustinr

2003 Turbo X50; stock. Tires are Michelin Pilot Sport Rib, OEM sizes. Front pressure +2 over stock and rear +2 over stock (38, 46)

Did not have it read with the code reader, just no faults displaying on the car's in dash "Check" function.

wross996tt

First I would lower the tire pressure...whether there are codes has nothing to do with the idiot lights. How do the tires look? My guess is you are fine, just too much tire pressure.

dustinr

Tire preasure shouldn't effect PSM unless they're way out of wack. I don't go off recomended tire pressures; I start there and then use a tire pyrometer to adjust them to what works best. The +2 feels better in the tighter corners, less sidewall flex, quicker turn-in. Are the "N" tires approved to top speed at recomended tire pressures or do you need to add more psi? Anyway, back on topic... say there is a fault in the PSM system, it won't register on the cars "check" function? It will only show up using the PWIS or whatever it's called?

deandob

I have a similar problem with a sweeping right hand bend near my house that I usually take with "some speed" and the pavement is wavvy near the apex, which causes the PSM to kick in for a split second on the right hand tire, braking it an causing the car to swerve a bit, but still under control. The sound that is like a cattle grid I think is the brakes being pulsed.

I too would like to know why this is happening, I assumed it was because my front wheel was losing traction hence the PSM kicks in, although it is too aggressive braking the wheel and gives you a bit of a fright. My BMW M5 had a pretty intrusive traction control, but not this bad and never made the car swerve.

I assumed this is normal behaviour for understeer correction (I have only had the car 6 months so I don't really know any better).

seattle_sun

Not that I'm a track guru, but all 911's are going to have a tendency to understeer. If you come into a corner too hot, the PSM will kick in. A lot of guys at the track will either have the car very balanced coming into a corner or will trail brake into the corner. Slow in, fast out.

At the track and taking corners much faster than the norm, I don't have PSM kick in.

dustinr

The problem actually caused oversteer... the car was tracking fine till the PSM or whatever problem it is caused the car to suddenly head for the outside of the turn.

It is frightening when you're going through a corner at speed with the car balanced on maintenance throttle and then it all of a sudden decides to apply a brake. Balance gone! someone who didn't know any better would probably come completely off the throttle and bin it into a guard rail. Not good...

This would be the only reason that I'm not a fan of PSM; if the system does wierd things like this it can cause more harm than good. If not for this, I'd say it's great. It can do what is impossible for us to do without 4 brake pedals. I'd prefer to use it, but if it keeps up these shinanigans it's off for sure.

slant911

I agree with Statman on the tire pressure. Not sure where you are at but with the summer heat here I have to go down to about 32F/36R cold on my car to hit the recommended PSI while driving.

The rest of it though sounds pretty normal for PSM. It's hard to say for sure as your idea of aggresive driving may not be mine. However, I have felt this same sensation when going a bit too hard through corners. So I think it's normal.

1AS

I believe "when it heads for the outside of the corner" that is UNDERSTEER.
That can happen with skid and traction controls when you hit gravel, or very fine ripples in the road. The only way to test is on a track or huge lot with TC controls off. This was a huge problem in my (long ago) 93 Corvette, though the more-sophisticated Porsche system was never as badly afflicted. Tire pressures that are way off (like 20 lbs. off) can do that, but I'd be surprised if it was a minor pressure misadjustment. The car is sensing something sliding, and applying brake. Could potentially be a bad sensor, but then should get a warning light. My advice- try it at the track. AS

dustinr

Doh!.. that's what I meant...understeer, not oversteer, I typed the wrong thing.

dustinr

You guys are right about the reccomended tire pressures being too high.
I ran 34 Front and 43 Rear today and that was too high. Car feels fine, but tire pyrometer tells the truth.

So today... take her out for a hooligan run on one of our local roads (Virginia City Truck Route; a road that is frequented more by sports cars and sports bikes than by actual trucks. Funny thing is that there is a Police sub station right where it starts and they could seem to care less. In all of the years I've ridden or driven on that road I've never seen a cop pull someone over on it... yeah for us! It's also the road where the California Ferrari Club and the Audi Club hold a time trial twice a year... way cool.) She drives great the whole time I'm hammering on her... then on my cool down trip back home which is basically going down a very windy mountain road, it does the stutter, and jerk thing again. Intermittent, usually when I'm only going about 48mph, coasting through coners, not pushing at all. This time I noticed that the little yellow triangle light with the "!" comes on when it happens, then goes off as soon as it stops doing the lurch. It sounded pretty crabby for a second or two, almost like something came loose or a tire went flat, then it was fine again. No clue...

Here's a video from the Ferrari Club Event.. .pretty cool.

http://www.ferraripacific.org/pastev...limb-Video.htm

rwilliamtaylor

Same thing happens to me... I believe it is "normal" operation of PSM. The PSM detects oversteer due to your coasting around the corner (trailing throttle oversteer) rather than typical slow in fast out driving and corrects. It is usually triggered by a bump or wave in the road. I've had my car thouroughly checked out including recalibrating the steering sensor... no change.

If I'm going to push it a bit, I'll turn PSM off and use the old fashion butt sensor for over and understeer.

BlackBear

OK guys, count me as member of the random PSM club.

I have owned three 996 Turbos and one regular 996.

Two of the Turbos have developped this issue. It first happened when I changed from Conti to Michelin tires. I thought maybe the diameters might be off and went back to the old tires. But no luck.

My newest Turbo only has 9,000 miles on it and it first happened the day I bought it at about 8,200 miles on the odo in a 110 mls/h turn. Scared me straight. This was with new dealer installed OEM spec tires.

Today I just brought this problem up with Tom at Lucent. He suggested to clean the debries off the ABS sensors in all four wheel hubs. The sensors are magnettic and may give a bad reading with metallic debries clustered around them.

rwilliamtaylor

Great idea, I'll try anything. Where are the ABS sensors... Is there a DIY explanation on how?