George_B

Hi,

Last weekend i drove a DE at Watkins Glen and i had an issue with my 2001 996 turbo with a Tiptronic transmission. The PSM was kicking on in places that it should not have been on at all. This is not my first event i have been doing DE's for three years now with this car and it has NEVER happened before. For the rest of the event i simply shut off the PSM and the car was fine. This car is well maintained.

One of my good friends asked me if i had done anything to the transmission. Last fall after my last DE i had the transmission flushed. The car has (52k on the odometer). He thinks that maybe they did not follow proper procedures when changing the fluid. My guess is that i'm going to have a difficult time proving to the shop that they caused the problem.

The only other thing i did to the car (other than oil changes and flushing the brake fluid) was installed new rear tires (proper sizes all Hankook V12Evo) the fronts still had more than 60 % tread life.

Any help would be greatly appreciated.

Regards,
George

Kevinmacd

Maybe a wheel sensor going bad, or a bad electrical connection or wire break to the sensor.

"02996ttx50

^^ there's your culprit.

rmc1148

I would guess rear tires also would put old back on for test or replace as a set.

George_B

Really ??? i know tire diameters are important on the AWD system, but that critical ??? Wow !!!

Thanks for the help.

George

jumper5836

Yes I agree. different level of grip from front to rear causing PSM to come on.

Macster

Never had any issues with new tires at the rear vs. old/worn but still legal -- not to the wear bars and otherwise in good shape and safe to run -- tires at the front.

However, I have never tracked the car. But it has seen some mountain road usage...

In any case, I do not think the different diameters are to blame as I think you would get a warning even on the street, but on the track grip differences -- as 02996ttx50 mentioned -- might explain the behavior.

"02996ttx50

agree it doesn't sound like rolling diameter issue ( though in fact, it is.. hence the cel! ) but rather the ecu detects the new vs old fairly easily. it might also be exacerbated by the very real fact that v12 tires are greasy. once you really scrub them, i would think the psm should settle down. but my dislike of hankook v12's has been widely debated in here. so i'll leave that part out. whoops. lol. i'm sure they'll be fine.

most of us probably go thru 2 sets of rears to one set of fronts. ( maybe even 2.5 sets of rears ) and the only time i feel i'm matching too much rubber ( new rears ) against too little ( older fronts ) i get abs/psm activations and cels. but i match my tread depths fairly closely for both aesthetic and practical handling reasons, so it's a rare event.

George_B

Point taken. On the road the car is fine. Didn't get a CEL only the PSM light. My durametric tool showed no faults. Damn i wish i would have stuck to my guns and bought a manual trans car but the price was right.

Once these Hankooks wear out i will be looking for something else. Not sure what to get yet, will do the homework before my next purchase.

Probably preaching to the choir here but what a car.......... drive it to the track and flog the heck out of it for three days then turn on the A/C and drive it home again.

Thanks again for the help.

George

Kevinmacd

You will get the PSM kicking in regardless of tire make if it detects one or more tires rotating at a different speed. If your fronts are worn and you put on new rears the PSM will kick in depending on the difference. V12 have a little bit more tread depth than some other tires when new. I would suspect that once you put a few miles on the rears and the tread depth comes down, the PSM will not be an issue.

"02996ttx50

^ that's absolutely true. as is the reality that as tires go, v12's are even more squeamish when new, than most other tires. which might even temporarily increase the cars propensity toward abs/spm cels, again, until scrubbed.. though you might differ with me on that point.


...george yes, amazing car, no doubt!

996tnz

Agreed, but it may take a long time to wear down enough. I believe Porsche approved tires for the 996 Turbo normally have slightly smaller rear rolling diameters than the fronts, so as to preload the viscous front diff by having the car think the rears are slipping a little already even when going straight with no actual wheel slip. Think this is the origin of the 5% minimum torque that always goes to the front (Porsche says it varies from 5-40% depending on conditions), and it keeps the system primed for quick action when needed. Half worn fronts and new rears reduce or reverse that intended preload, so I would expect the car to need more rear slippage and time before managing to transfer torque forwards.

Not sure what PSM makes of that situation but I know my PSM is very sensitive to mismatched wheels (ran some 10 inch rears for a few days while my 11s were being coloured up) and also to having any underinflated tires.

I normally just change all 4 tires at once with matched sets, but you could test the above theory perhaps by raising your fronts a few PSI and dropping the rears by 3 or 4 to better even up the rolling diameters. You may not want to keep running like that for long but for the short duration of the test you'd probably just see a bit more understeer than usual.

"02996ttx50

that works for me. well put. my solution to this occurence has been to disconnect the cardan shaft lol and drive psm OFF

mcbit

I've had no issues with worn tyres of the same brand but mixed brands front are rear will cause issues close to the limit due to inherent difference in grip.

Kevinmacd

996tnz - not sure your observations are correct. The Michelin PS2s N spec tires, 225-40ZR18 has a diameter of 25.1, and the rears 295-30ZR18 have a diameter of 25.0. Hardly enough to make a difference.