How does it feel when PSM engages?

[PhoenixBlue]

Curious about how it feels when PSM engages - I read that it brakes the car, but is it an ABS-style type of juddering braking? Also, if it engages, will a light somewhere blink (as in some of the newer cars)?

Asking cos I’m having weird ABS-type braking feel if I power out of a corner, but with what I feel is not enough to be considered a “slide”. Is PSM or ABS somehow engaging prematurely?

BTW the car behaves the same way if I have the PSM off button switched on or off. Any ideas?

[ike84]

If your LCAs in the rear are shot, you will get wheel hop coming around hard turns. Do you ever hear a clunk coming from the rear if you hit a box around 15-20mph? If so, it's the LCAs.

When traction control activates, all it does is kill power momentarily. No braking to my knowledge.

If it were your traction control, the juddering should stop when you disable it.

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[Gilles]

If I remember correctly, on the 987 you can feel the engine reducing the power and also the inside (side of the turn) rear brakes activate as well.
And on the 981, after enjoying the car on a tight twisty road you can smell the rear brakes even if I was not braking aggressively, perhaps the PSM is activating the rear parking brake to help keeping the rear end under control..?

[JFP in PA]

Originally Posted by Gilles

If I remember correctly, on the 987 you can feel the engine reducing the power and also the inside (side of the turn) rear brakes activate as well.
And on the 981, after enjoying the car on a tight twisty road you can smell the rear brakes even if I was not braking aggressively, perhaps the PSM is activating the rear parking brake to help keeping the rear end under control..?

PSM has no control over parking brakes, which are mechanically activated. PSM uses the ABS hydraulic system to activate your main brakes according to preprogrammed yaw parameters.

[986 Boxster]

In my experiences the PSM on, it did reduce power to the car and it also braked the car when it engaged, and the psm light will come on momentarily on the cluster. I've had it come on several times on turns that the back end started to slide and it did brake and the car slowed down also with power being reduced, It didn't feel the same as the abs coming on. It will come on also when you try to spin the rear wheels. When I turn it off, it doesn't do anything to the car when I try to spin the wheels and I can slide the back end of the car if I want.

[Stl-986]

From what I read even if you disable PSM, it is actually still on, it's just reduced. So there is really no way to completely disable it.

[Gilles]

Originally Posted by JFP in PA

PSM has no control over parking brakes, which are mechanically activated. PSM uses the ABS hydraulic system to activate your main brakes according to preprogrammed yaw parameters.

Thanks JFP,
So if the smell comes from the rear brakes (not parking brake), then there is no need to remove the caliper to inspect the parking brake as long as the rear pads are ok is this right?

[JFP in PA]

Originally Posted by Gilles

JFP, So if the smell comes from the rear brakes (not parking brake), then there is no need to remove the caliper to inspect the parking brake as long as the rear pads are ok is this right?

Correct; the rear parking brakes are drum style assemblies inside the center of the "hat" on your rear discs.

[986 Boxster]

Originally Posted by Stl-986

From what I read even if you disable PSM, it is actually still on, it's just reduced. So there is really no way to completely disable it.

From my understanding it's off until you turn it back on, or if turn off the car and start it back up it's back on again. And only the front outside wheel brake is activated when it comes on, on a turn. You have to try it out with it and on and you will see that you can't spin the wheels without losing power and a bit of shaking when the abs kicks in. When it's off you can spin the wheels without it coming on, at least that's my experience.

I forgot to mention, I read somewhere that if you brake hard enough that the car needs the assistance of the abs system it will come back on, I've never tried it, so I don't know if that is correct.

[PhoenixBlue]

Sorry to add, mine’s a 986, so not sure if it is different from the newer cars?

Hmm… mine doesn’t change whether the button is on or off… Also, upon “activation” mine seem to come from the inside wheel (instead of the outside wheel). I just checked with a mechanic and he is suggesting my speed sensors and wiring loom at each wheel needs changing.

Is something broken?

[PhoenixBlue]

Was thinking it might be the arms, but it’s not a clunk sound, more like a series of hard rattles. Will a worn LCA do this?

Originally Posted by ike84

If your LCAs in the rear are shot, you will get wheel hop coming around hard turns. Do you ever hear a clunk coming from the rear if you hit a box around 15-20mph? If so, it's the LCAs.

When traction control activates, all it does is kill power momentarily. No braking to my knowledge.

If it were your traction control, the juddering should stop when you disable it.

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[edc]

If you press the PSM button it turns off but is really still on. If you hold the button down you should hear a beep and a message on the display. It's been some time since I did this on a skid pan but I think this is full off.

[986 Boxster]

Originally Posted by PhoenixBlue

Sorry to add, mine’s a 986, so not sure if it is different from the newer cars?

Hmm… mine doesn’t change whether the button is on or off… Also, upon “activation” mine seem to come from the inside wheel (instead of the outside wheel). I just checked with a mechanic and he is suggesting my speed sensors and wiring loom at each wheel needs changing.

Is something broken?

It could be a wheel speed sensor. Did your mechanic check for codes with Porsche OBD scanner? Check for codes first, it should tell you where the problem is. If it`s a wheel speed sensor the ABS light usually comes on, and some times even the engine light.

[PhoenixBlue]

Thanks! I’ll try this!

Originally Posted by edc

If you press the PSM button it turns off but is really still on. If you hold the button down you should hear a beep and a message on the display. It's been some time since I did this on a skid pan but I think this is full off.

[PhoenixBlue]

No codes as far as I know.. he just sent me photos of the wiring that look like they were taped up by the previous owner (see pics)!

The ABS and PSM lights (but not engine) did come up recently, but an alternator change sorted that out - or at least both lights remain off.

Originally Posted by 986 Boxster

It could be a wheel speed sensor. Did your mechanic check for codes with Porsche OBD scanner? Check for codes first, it should tell you where the problem is. If it`s a wheel speed sensor the ABS light usually comes on, and some times even the engine light.

[986 Boxster]

That looks like it could the problem. But like I said before, you should have some codes coming up if it`s a wheel speed sensor or something else related to the PSM and ABS systems.
A regular OBD scanner can't read Porsche faults codes in the PSM and ABS modules. You need a Porsche code reader, like Durametric, etc.

[ike84]

Originally Posted by PhoenixBlue

Was thinking it might be the arms, but it’s not a clunk sound, more like a series of hard rattles. Will a worn LCA do this?

When accelerating through or out of a hard turn a bad LCA will cause wheel hop. It can be a rather violent (depending on the amount of acceleration) juddering in the rear as the wheel literally bounces in the vertical axis, losing/regaining/losing traction as it does so. Google wheel hop or wheel skip and I'm sure there will be a good video on it.

Truthfully, if you're on dry haven't and on a street, it's really quite difficult to break the rear end loose. I'd be surprised if that's what's going on, unless you are intentionally trying to do so.

The "clunk" sound does happen during turns, it is only when going over bumps during straight line driving at low speeds. It's just another tell tale sign of a worn LCA.

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[PhoenixBlue]

Is the LCA the “Coffin” control arm? I’ve replaced that recently, but not the longer (trailing?) arm as circled in the photo attached. Could it not be the sensors and this mechanical part instead?

Also, this sound comes from the front end, and not the rear…

Originally Posted by ike84

When accelerating through or out of a hard turn a bad LCA will cause wheel hop. It can be a rather violent (depending on the amount of acceleration) juddering in the rear as the wheel literally bounces in the vertical axis, losing/regaining/losing traction as it does so. Google wheel hop or wheel skip and I'm sure there will be a good video on it.

Truthfully, if you're on dry haven't and on a street, it's really quite difficult to break the rear end loose. I'd be surprised if that's what's going on, unless you are intentionally trying to do so.

The "clunk" sound does happen during turns, it is only when going over bumps during straight line driving at low speeds. It's just another tell tale sign of a worn LCA.

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[ike84]

Originally Posted by PhoenixBlue

Is the LCA the “Coffin” control arm? I’ve replaced that recently, but not the longer (training?) arm as circled in the photo attached. Could it not be the sensors and this mechanical part instead?

Also, this sound comes from the front end, and not the rear…

No, it would be part 5, the part that runs front to back.

The rear LCA helps to keep the wheel planted, so when making a hard turn under acceleration the inside corner will hop. I think this is true especially since (in the rear) these are the drive wheels and are more prone to doing so.

I don't know if what I am saying is true for the front end though. A bad rear LCA will not affect the front wheels, but I would assume a bad front one might. Someone else may be able to chime in on that.



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[PhoenixBlue]

Thanks! Then it might be these arms in the front then. Maybe I’ll get them changed and see how it goes. I’m just entertaining the speed sensor/wiring hypothesis as the “juddering” feels rather regular, similar to an ABS feeling, but only on the inside wheel when turning.

Oh and to your previous question, yes I was intentionally taking the sharp corners at speed to try to get close to the grip limit of the car. Feels like I can get to within 80-90% of the limit before the juddering comes on (but only on power).

Originally Posted by ike84

No, it would be part 5, the part that runs front to back.

The rear LCA helps to keep the wheel planted, so when making a hard turn under acceleration the inside corner will hop. I think this is true especially since (in the rear) these are the drive wheels and are more prone to doing so.

I don't know if what I am saying is true for the front end though. A bad rear LCA will not affect the front wheels, but I would assume a bad front one might. Someone else may be able to chime in on that.



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