Odd ABS symptom

1999 Carrera 4 I've noticed for a while that the ABS seems to cut in with very little provocation. Since the car's MoT, however, when it cuts in it makes a weird 'twanging' sound, a bit like bed springs. Any ideas? Is it something I need to worry about? Thanks Ed

Sounds really odd. ABS cutting in usually gives a hammering sensation on the pedal. Do you get that? PSM also makes a helluva hammering sound from the ABS pump, but nothing like a 'twang'. Just a couple of thoughts about tyres, which might have an effect. Are they evenly worn (similar tread depths all round)? Tyre pressures correct? Tyre age? If you haven't got a constant ABS or PSM light, I doubt there is a fault, but a diagnostic check might be worth doing. Of course, the wet roads are a lot more slippery at the moment after a relatively long dry summer, so it might be as simple as that.

Thanks for your reply. Yes - I do get the hammering sensation, and it's happening a lot more in the last year or so than I would expect - ie nearly every time I brake, and this is just with normal braking, driving around town. However, there is the sort of 'boing' sound with each hammer. It's not the end of the world, but I do need to be sure than there isn't a braking fault, as I'm planning a track day soon.

Could the twanging noise point to an issue with the balljoints on the coffin arms?

Given the progressiveness of Porsche brakes, unless you've put particularly grabby pads on or are literally jumping on the brakes every time, I reckon you've got a fault. A thought: have you checked your handbrake shoes recently? They act on the inside of the rear brake disk bells.

What I don't understand is that the car has only relatively recently passed its MoT, and this fault, if that's what it is, was evident then. I guess I'm going to have to get it looked at...

Ed, Could the twanging sound perhaps be caused by one of the brake disc shields being excited by the ABS? Jeff

I've had a garage look at this now and it turns out that the N/S ABS ring is actually broken - a missing tooth, which causes the sensor to fire all the time. Apparently it's part of the front outer CV joint. Does anyone know where I can get this part at a sensible price, please? Porsche have quoted something like £350 for it! Haven't been able to find anything online, or on eBay. There are places that sell the rear inner joint, but I have to admit I can't understand why there have to be CV joints at the rear at all. I thought the CV joint was needed on front wheel drive cars to transmit drive through a flexible coupling? Ed

Any use? http://www.reluctorrings.com/cars/porsche

Good spot Chris! On the C4 the front ABS sensor ring is fitted to the hub of the outer CV joint, but Porsche only supply the whole drive shaft at about £350, so I expect that is what they quoted for, not the CV joint alone. If the part in Chris' link is correct for your car, it could be a result.

Perhaps a u/j would work and indeed despite the complexity of F1 suspension back in the late 60's and early seventues many a car had u/js on the rear axle. That said, cv joints are superior in a number of ways. The uj does not like being continuously deflected through too big and angle, and to keep the rotational speed contant, the stub axle for the wheel must remain parallel to the output shaft from the gear box. With fully independant multii link passive rear wheel steering this is not always the case in modern cars, in general and you can imagine how a wheel not rotating, accelerating, or decellerating evenly would upset the handling let alone the way the pasm works. I venture to suggest that the unsprung weight is greater with a uj than it is with a cv joint, and a rotating uj would then have greater momentum to be slowed/accelerated. Notwithstanding all that, u/js were not protected from the elements in the same way that cv joints are

Thanks for this John. I must say that your technical knowledge has rather taken me out of my depth! I'm hoping that the link has Chris has sent is going to get me out of trouble, so although this is interesting, it's rather of an academic nature. I'm still puzzled as to why *any* kind of u/j is needed at rear. I didn't think this car had any rear wheel steering at all! Now I'm thinking it through, I suppose that if the wheels are moving as the suspension flexes, there has to be some flexibility in the drive. Sorry for being so dumb! I guess I'm just used to driving front wheel drive cars with stub axles at rear. Ed

Thanks very much Chris. Looking into this, and hopeful it'll sort the problem. Many, many thanks! Ed

My pleasure Ed. If they do not suit, do a Google search for 'Porsche ABS Ring' it may throw up some others that may fit if the first link does not work out.

It is not generally understood or appreciated that provided the input and output shafts remain parallel the angular velocity of the driven shaft will be constant, given a constant input speed, wih a u/j. The problems arise when you need the two shafts to go out of parallel, which would give changing speeds through one revolution; though I am not sure how Messers Ciroen overcame this with the "Maigret- traction avant". As its name implies a c/v joint gives a contant velocity out, given constant velocity in, no matter how out of parallel the driving and driven shfts are. If I was to add spanners to my signature to indicate competency I would be confident to add five.