Sudden Involuntary Braking

[911Cab2013]

Hello,

Over the past few weeks my 996 c4 cab (1999) has been doing great, but in the last day a rather huge problem has developed.

Out of nowhere, there will be a sudden braking motion followed by the yellow triangle (PSM info light) coming on until it disengages. This happens regardless of whether I have the PSM button on or off. No error codes being thrown, and I can't figure out a definitive way to trigger it, although it happens on almost all right turns, in addition to driving straight. I haven't taken it past thirty since, only driving to get it home. I feel as if it's a sensor problem, being that it is not always definitively triggered, although it does not happen in neutral, so I assume it is linked with the engine somehow.

If anyone has experienced this or has a suggestion, I do have a durametric (and I'll check for error codes again tomorrow and capture them. The light goes away after it decides to finish braking.)

Thanks!

[wvicary]

Are the brakes really being applied or the engine power being reduced? You need to check for fault codes or monitor live data ( get someone to help ) whilst the vehicle is faulting.

[911Cab2013]

Are the brakes really being applied or the engine power being reduced? You need to check for fault codes or monitor live data ( get someone to help ) whilst the vehicle is faulting.

The PSM intervention light is coming on, the brake pedal locks, and I hear it braking. It'll brake to a stop almost immediately on its own, rather than rolling to a stop if engine power was being reduced.

[Peterfrans]

A friend had similar problems with his c4s. It was ultimately diagnosed as a faulty master brake cylinder.

[Dennis C]

Have you installed tires that are different sizes that the Porsche recommended sizes? In an AWD car, this can make a big difference. If the variance in size is too great, it tricks the sensors into thinking that you have wheel slippage and it tries to correct the problem. Even when it's turned off, PSM is never totally off the way I understand it.