MisterMisfire

Got after-market 21" wheels. 285 40 R21 tyres on the front. 295 35 R21 on the back. No issues whatsoever with PSM.

Had the wheels aligned and within half a mile of leaving the place the PSM fired off and the car juddered. Took it back and they guy could see no reason why anything he has done would make this happen.

So basically, if I start the car and drive away the PSM kicks in when I go round a roundabout or swing round a corner. So I have a new routine. Start the car, switch off PSM, brake off, drive away, then about half a mile later switch PSM on again and all is dandy.

If I forget to switch PSM off then the flipping thing fires, I switch PSM off, drive for a bit, switch it back on again and it doesn't fire again.

I've got an iCarsoft reader and it doesn't have any stored faults for PSM.

Any ideas?

The only thing I've found is a suggestion that a tyres change or uneven wear left/right had mis-matched the diameters of the tyres and makes the car think it's wheel spinning.

stefan063

That is just NOT the good thing to do with your car! You should have all 4 tires of same size and width. Also you will destroy your transferbox.

garrett376

Most decent alignment places will give you an initial settings printout along with the new settings printout. What exactly was changed with your alignment? If you truly had zero issues before, and now have issues, it was something affected by the alignment and studying the before and after settings should help you figure out what's wrong.

Wisconsin Joe

A quick check shows a 0.9" difference in tire height. That's not good to run.

I don't understand how it worked before, but you are basically 'confusing' the PSM by spinning the wheels at different speeds.

And, as noted, it's not good for the clutches in the transfer case.

Petza914

Did they change the tire pressures when they rotated the tires. You should have about a 5 lb differential front to rear, like 38 F / 43 R. If they left the pressures where they were the pressures could also be effecting the rolling circumference of the wheels and confusing the PSM.

The Cayenne does not run a staggered setup. The proper tire sizes for a 21"x 10" wide, et 50 wheel is a 295/35-21 and those should be on all 4 corners.

hatchetf15

Petza914

I've heard there was one year or one trim where Porsche used a staggered wheel setup on the Cayenne, and maybe it's yours, but I believe for the rest of them, they all use a square setup - for both wheels and tire, at all 4 corners, like my 957 Turbo S was with the OEM setup and I continued this with the aftermarket setup. Not only does it make tire rotation much easier since you don't have to dismount and remount to move the tires around, but the same tire on a wider vs narrower wheel, creates a different profile and different rolling circumference (wider wheel flattens out the tread area and makes the rolling circumference smaller) and might be enough to throw of the PSM, especially if it didn't come that way.

Stringbag

It is possible that the alignment disturbed one of the PSM sensors. The yaw sensor and all 4abs sensors are inputs into it. I’d get underneath with a torch and see if there are any recent marks on the sensors/cables that might have been caused by a wrench. Undoing some of those bolts whilst aligning isn’t a gentle process...

nodoors

It is most likely the almost 1 inch difference in diameter between the front and rear tires. You may eventually burn up your transfer case running the car like this too.

Petza914

It's his wheel widths that are 1" different, not the wheel diameters. The width difference will result in a total diameter difference as well, but it won't be anywhere near a full inch.

nodoors

I tried two different calculators and it is of course going to depend on the actual model of tire he is using, but both came out to nearly an inch difference in diameter:

This one was the lower of the two at .85 inches:
https://www.tacomaworld.com/tirecalc...5r21-285-40r21

Your assumption might be correct if he has different rim widths, but I did not see that indicated in the post.

Petza914

Sorry, you're totally correct with respect to the OPs post. I thought you were responding to hatchetf15s more recent comment about the same tire size on 2 different width wheels
"My 955 TTS has 9” fr/10” rr shod with 275/40’s all corners, for example."

MisterMisfire

I replaced the rear tyres to match the front ones and the mysterious hypersensitive PSM has gone.

DOH!