i have a 1999 911 966 c4 3.4 .-i have had a problem with the ABS appearing to pulse the o/s front brake as if its skidding at slow speed when its not ...This is from when i purchased it .i have had it for 5 years now.. And have since buying it changed at different interval's to following: brake fluid .front abs sensors new set of pads and sensors and full set of disc's.i have no sign's on dash at any time as in ABS fault or such..if i remove ABS fuse it cure's it ...now here's the curious bit if i put on rolling road so front wheels are on rolling road for brake test it doesn't do it !!! or even when rears are on rolling road... it only seems to do it when all 4 wheels are turning ..and only on last couple of yards of braking....at speed this doesn't happen..it can't be caliper as it woks fine when on rolling road ...or fuse pulled out .i feel the only thing left is ABS pump..and does the replacement ABS pump have to be programmed to my car ....can any one advise me please as this is spoiling the sheer pleasure this car brings me ...and driving me crazy !!????

I don't think its the ABS pump. I think the ABS pump is getting a false signal, telling it to start pulsing. Are you tire diameters the same front and rear? Same brand? Any codes or info with a scan tool?

What tire sizes are you running? Are the stock size?

Its probably not the ABS but the psm. You have the same make tires on all four corners with the correct sizes?
Tires that are ofspec from one another, makes the ecu think one tire is running quicker than the others and trys to slow it down to keep you from going into a skid, this condition usually only happens when turning around corners and twisties! The psm uses the brakes to perform this function.

Be aware with the C4 and its AWD system if the car needs to be put on a "rolling road" for some reason the rear rollers *must* drive the front rollers.

If this is not the case the viscous coupling that connects the rear diff to the front diff can be damaged.

In some cases when a "brake test" is done on a "rolling road" of the wrong style/type the car can react quite violently.

Do the front brakes actually cause the front tires to skid? If so this certainly reads like a bad ABS.

Be aware under low speed sharp turns the inside front tire will scrub and if one is applying the brakes this can make it appear the ABS is activating.

Also, occasionally I will make a sharp turn at a speed a bit higher than a walking pace and the inside front tire scrubs but it does so at a much faster rate. Again, this is not the ABS or the brakes but (at slow speed or above) it is a characteristic of the car and its steering geometry and alignment. It is normal. (Both my Boxster and my Turbo (with an AWD system similar to your C4) do this.)

5 years!!!!! If the shop you are using hasn't figured it out by now then immediately change the mechanic who is working on the car...even if it is you. This is your braking system for god's sake, and if you don't know what is wrong, you are putting yours and other lives at risk. Have you taken it to a Porsche dealer and let them have a shot at fixing this?

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