WHEN A CHANCE REALLY ISWORTH TAKING
As many of you will probably know by now, I am not at all keen on improvised fixes to any car, and least of all to a Porsche – especially when genuine brand-new spare parts are readily obtainable. Do it right, do it once, and all that. Sometimes, however, you simply have to make the best of the resources available to you at the time – particularly when it’s a billpaying customer calling the shots. And even more so when he and his car are booked on a ferry to France the following day, and the new components wouldn’t be available until some time the following week.
My friends at Auto Umbau, Robin Mckenzie and Terry Parker, told me recently about a rather unusual repair they’d had to carry out on a 996-model Carrera 4. The owner, perhaps a little optimistically, brought it in the day before his holiday complaining of a rhythmic grinding sound from the vicinity of one of the rear wheels. Try as they might, though, they couldn’t isolate it. Temporarily taking out the brake pads and spinning the wheel soon eliminated those or the disc as the culprits (quite often you can get a noise like that if a stone becomes trapped between the disc and the caliper or the backplate) and, although it then seemed to be coming from inside the hub, it didn’t sound at all like a bearing.
But then eagle-eyed Robin spotted it. Looking more closely into the inner part of the hub assembly, where the constant-velocity joint passes through it, he realised that the slotted ring upon which depends the ABS for its wheel-speed signals had been forced out of round by corrosion on the unprotected metal surface beneath it. The sound, unsurprisingly, was the single high point on the ring hitting the hapless sensor – and obviously wearing it away in the process. Oddly, though, the ABS light had not switched on – had it done so the diagnosis would surely have been a lot quicker.
But what to do about it? The sensor, assuming that the fixing screw came undone without breaking, would at £140 from Euro Car Parts be an easy replacement – if not exactly inexpensive. But plainly there was absolutely no point fitting a new one until the cause of the problem had been fixed; it would soon be destroyed by the distorted ring. Unfortunately, however, you cannot buy the ring separately, but only as an integral part of a complete brand-new drive shaft. To you, sir, that will be around £650 plus VAT.
After a bit of thought and discussion, our heroes hatched a cunning plan. By carefully bead-blasting the outside of the CV joint (with any sensitive areas masked off with tape), they dispersed enough of the rust to be able to slide off the ring. Further blast-cleaning of both items removed all remaining traces of corrosion, and by heating to cherry-red and then rapidly quenching the ring alone they were able to shrink it back down to more or less its correct internal diameter. It was still a little oversize, of course, and thus not quite a tight enough fit on the outside of the CV joint, but that was solved by securing it with an epoxy adhesive. A thin but still useful coat of paint, and it was (almost) as good as new.
But would it work? The short answer is yes. Perfectly. There is no guarantee for how long, and if it does later fail for any reason then the customer will probably have little choice but to buy a new shaft (or perhaps a good second-hand item, if one can be found). Either option brings with it the obvious financial risk of having to pay twice for the required labour, but if it saves the thick end of £800, even for a year or two, then I reckon that’s a chance well worth taking. PW
The car enthusiast’s constant enemy, corrosion, strikes at Porsches – which famously have rust-resistant, zinc-coated body shells, of course – in the oddest of ways. Here, rust on the body of the constant-velocity joint beneath the ABS ring on a 996 Carrera 4 had gradually forced the ring out of round. This had caused the high spot (arrowed) to hit and obviously wear away the sensor, mounted on the hub carrier (right). The ‘factory’ repair would have meant fitting a complete new drive-shaft – you cannot buy the ring separately – but at nearly £800 for the shaft, and a delivery date several days after the car was due on a ferry, that wasn’t an option. Time for some ‘desert engineering’, then