MechanicalEng

can you have 235 40 18 and 295 30 18 in a 4wd 996TT?
the circumference of a 235 40 is 25.5" and 25.1" for a 295 30

wross996tt

Theoretically you have a 1.7% difference (and smaller in the rear) which is within the spec from Porsche. But remember these are theoretical calculations. You really should go out and take actual measurements as circumference from different manufacturers varies...

rocketrider2

Agreed within spec but that size front is not the match for that rear and I would bet that if changed the issue will go away.

Lane

Quinlan

Oddly enough, I just had my rear tires changed and had the PSM come on several times at a DE at Mont-Tremblant earlier this week. I had to get on the gas fairly aggressively to avoid it - I think the computer figures that if you are on the gas in a situation where the tires are slipping, you are doing what you intended to do. If braking or coasting when the tires start slipping, the computer thinks you have gotten yourself into trouble, so it intervenes.
This was my first experience at Mont-Tremblant, vs. earlier DE's at Mosport and Watkins Glen, where most of the corners have concrete strips on the driving line. Mont-Tremblant is pure asphalt, so I am not sure how much that contributed to it.
I will be going to Mosport in a couple of weeks for the Rennlist lapping day, so I will see if it continues to come on. If so, perhaps it is the new tires that are the culprits - slightly different diameter or compound?

tbarcelo

Digging up an old thread. Does anyone have resolution on this topic? I'm experiencing similar problems, and like many others, it occurred after replacing the rear tires and having a 4-wheel alignment done. I have checked the steering angle sensor, and it is fine.

2003 Turbo. 997 Rims (Fronts 8Jx19H2 ET57, Rears 11Jx19H2 ET51)
Older 235-35-19 Michelin PS2 front
New 315-25-19 Nitto Invo rear (had been running the "wrong" 305-30-19 Hankook Ventus in rear before issue with no problems)

Thanks.

dustinr

I only had the issue on one road in particular and it was only going down hill. But now that I've gotten used to driving a Porsche and it's rear weight biased tendency's the PSM is hindering my smoothness so I keep it turned off now to let the car slip a little more than the 7 degrees it allows, which of course alleviated the problem. The PSM works very well if your goal is to keep the car straight, but it's kind of ham fisted in its engagement, at least with the AWD cars. The AWD all by itself will have a tendency to self correct for any drift as long as you keep in it, but with the PSM also engaged it jerks you around a bit cause it's applying brakes too. The AWD all by it self has a tendency to make the car want to go in the direction that your steering, so counter lock is a brief action anyway and when the PSM starts doing it's thing and you're trying to do your thing the two don't mesh so well...so I keep it off unless its wet.

tbarcelo

Thanks Dustin. I need to experiment a bit more to figure out exactly when this happens and when it doesn't. Another forum recommended that I go ahead and replace the front tires to match the rears.

996tnz

Long post but here goes, maybe it will help someone with similar issues feel less alone. Have had my '02 996T for only 2 months and these PSM problems are driving me nuts too. Pushing it into a sharp right-hander with a strong dip does it every time - guaranteed to result in a "PSM Failure. Brake Fluid Low. Drive to Workshop" failure message. If left-foot braking into a intersection and giving the throttle an instantaneous blip to try to force the tiptronic into sport mode for a potential quick exit, it throws the same error. Why can't there just be a sport button for that like on my last few BMWs?

It also does it every lap or two on track, and in all cases it happens about 2 seconds after the offending corner, when the car is straightened up and steady again. Really annoying because it the car needs to be stopped and restarted each time - not good on the home straight. Same deal with the emergency lane change exercise - had to restart her everytime to get PSM back.

A guy at my recent Driver's Ed said all 996 Turbo's throw these errors under similar conditions if pushed and I'm inclined to believe that. Apparently those who have raced them with PSM usually make a momentary reset switch that cycles power to the PSM module to clear the fault without stopping the engine. I still can't believe I was having to convince several internationally known race drivers who were instructing us that despite the dash warning, we really weren't running out of brake fluid - including one as we piled down the straight at about 220 kph (140mph) towards the corner...

Other hearsay, which I'm also inclined to believe, is that the PSM turns itself off as a safety precaution after its inbuilt integrity checks find that the signals being sent back to it (from the wheels, accelerometer, sterring angle sensor etc) fall outside the allowable boundaries set at the factory. This is to return the car to predictable handling if the PSM itself were to go wrong, rather than have the PSM think the car is turning left when it is going right and intervene to make things worse.

Trawled the net and having also read some posts from a Bosch technician, I can well believe that this is what may happening as PSM/DSC etc relies on massively complex mapping of what the sensors say about the car behaviour to the computer modelling that determines the reaction of the PSM, and monitoring of the resulting feedback loops. Whenever the car's behaviour falls out of the mapped performance envelope, PSM seems to be giving up and turning off as soon as it thinks it is a safe time to do so (ie back on a straight). During motorway cruising and basic commuting it doesn't happen.

All very well, but if so, I don't want to keep stopping and restarting my car to work around the limitations of a computer program!

This same behaviour shows at 29 psi front, 31 rear and also at factory 36 front, 44 rear so I don't think it is tyre pressure though I suppose that their effective circumference and front-rear,left-right ratios would be affected by pressure, braking, cornering, and acceleration so there may be something in that too.

Has dustinr (the original poster) or anyone overcome similar symptoms (and not just by buying some other brand of car)?

For the record, mine's an '02 Tiptronic, ROW suspension, OEM approved factory sized N-rated tyres. More worn on the back but well within Porsche spec for circumference difference (at 2% measured). No mods I believe. Love it in every other way bar PSM suiciding and that it eats chin lips, and not just at low speed.

Never any PSM problems with my previous '79 928 of course, and no handling problems that wider tyres didn't fix ;-)

Has anyone beaten this?

Oak

I think this problem is common with the tip. from my exp PSM rarely kick in. need to be out of sorts so to say for it to kick on.

omahoney

DustinR, did you ever find out what was causing this. My car is doing exactly what you described.

dustinr

With my car I attribute it to the 19" wheels I was running. Even though they were lighter in weight than the factory 18"s , the car handled bizarre with them and the 997TT size tires. I spoke with a Porsche engineer in the Porsche garage at Laguna Seca last year and he said that when Porsche went from 18" to 19" wheels on the newer cars that they had to redesign and tweak the suspension to make it work with the larger wheels and shorter sidewall tires. He advised I go back to the 18"s because the 996TT wasn't designed with the 19" wheel in mind and said that they usually run 18" wheels on their race cars unless they need the extra room for larger brakes. They only went with 19" wheels on the new cars because that look was becoming more popular. So I switched back to 18"s and haven't had a problem since and the car actually handles a lot more confidently. That's not to say that you couldn't make a 19" wheel work, but it would take some tweaking to get it right I think.

If you're not running 19"s and still come across the problem, try turning the PSM off and see if that remedies it. I think the right combination of speed, cornering G's and bumps or vibrations may be enough to fool the sensors that a wheel is slipping more than it really is. It's definitely unsettling when it happens mid corner and gives you pause to continue on at the same clip...

Just my guess...

tbarcelo

I have come to the conclusion that either the PSM has some form of adaptive learning or something about a fresh (mismatched) pair of tires causes this problem.

When I bought my car it had 997 wheels with 997 tires on it. No PSM problems. I read all about chewing up the front diff and I needed new rears, so I bought some 315-25-19 Nitto Invos as mentioned above. Immediate PSM problems. Now that I have some miles on that setup, the problem seems to have gone away. I autocrossed the car for the first time a few weeks ago, and I was astonished at how little PSM interfered (when I forgot to turn it off

All that being said, I'm still on the hunt for a set of 18" wheels!

dustinr

You may be onto something here, I noticed more when my tires were new, then it started going away as the tires wore down. As I had just added two variables to my equation (wheels and tires) I figured to get rid of the problem meant getting rid of both changed variables which seemed to work. But tires might be the key...

Guybrush Wilkinson

I bought my manual transmission TT a couple of weeks ago with worn out rear tires. I had to order a pair from Germany to get them for a reasonable price. Just having spent a huge amount of money on a car I decided to go cheap on tires. New Ecstas were installed today. Fronts are Pilot Sports with abot 40% of tread left.

I did not push my car very hard with worn out rears. Today - with new tires - tried first time to take corners fast and got spooked by this PSM attack. In my opinion PSM kicks in way too early and with too much force. My friend was onboard the first time and he asked me "why did you make that idiotic braking?". I said I didnt - the car did!

Maybe miss match tire brand contribute to the problem.

I am driving the PSM off from now on.

(my car is 100% stock - excluding the rear tire brand).

DaveK

I do see the PSM light flicker - but it's pretty rare and certainly not enough to spoil my driving.

However, I did make the mistake of using Ecsta's on my 964 once and never would again - I do wonder if the problem is your tyres rather than the PSM itself. I hope not - because if it is, driving with the PSM off might not give you the result you're looking for!

I work with a number of guys in Finland - I'm assuming this is a summer car?