ABS light

Johnny C · Apr 5, 2006

The ABS warning light on my '96 993C2 started coming on at the start of last winter. I got Ninemeister to check it at it's 10K service, and though the warning light is on, the ABS is working fine. They cleaned all the contacts etc in case water was shorting something and it still came on but the ABS were working fine. It had new disks/pads at that service.

I've now realised that it seems to be the external temperature; it's been on more or less constantly since December when the cold weather set in, but now it's warming up, on days when it's over about 8C it goes off and only comes back on in the morning when it's cold, then goes off as the day warms up.

Anyone any ideas? If it is the temperature it won't bother me much over the summer, but I'd rather know that I can rely on the warning light

zwhitea · Apr 5, 2006

Can't really help here, but it could even be down to a faulty regulator on the alernator, pushing out too many volts.. I have had this before with AirBag warning lights..
You'll probably need to get it fixed before your MOT, as even if the ABS is working the warning light being on should be an automatic failure..

968er · Apr 5, 2006

I had a very similar problem. Don't think it has anything to do with the temp. Initially light would come on intermitantly, then would stay on for a while, the ABS still worked though. Eventually the light stayed on and ABS stopped working.

Bottom line was that one of the sensors was failing and eventually had to be changed. Once that was done the problem was solved.

sawood12 · Apr 5, 2006

There are a few threads on this issue over on the 944 forum. Is it intermittent? Mine suffered this when I first bought the car but the previous owner only used the car for a few thousand miles a year so as soon as I started using it pretty much daily the problem has gone away. The experience of other 944 drivers on this is that it seems to be the ABS sensor - the little plasic sensor fitted to each hub. The trick is to track down the one that is faulty and replace it. Some have taken their cars to a Bosch Brake service centre to get a full diagnostic check done but in almost all cases it's turned out to be the sensor i've mentioned.

Also beware - if the ABS warning light is on or comes on during your MOT test then it's a fail, however if it's off then it's a pass irrespective if your ABS system is actually functioning or not. The MOT doesn't actually test ABS function but if there is a warning light on then they will mark it down. Some of the more keen drivers who want to disable the ABS function have removed the ABS relay which illuminates the ABS warning light so to extinguish it they have removed the ABS warning light bulb and the '!' light bulb and have passed subsequent MOT's. I personally prefer having the ABS functioning!

Hope this helps.

sawood12 · Apr 5, 2006

Sorry, just re-read your problem and my suggestion doesn't seem to apply if your ABS is still functioning. Oh well, it's one to look out for anyway.

Johnny C · Apr 5, 2006

Thanks, I'll whizz it into a Bosch brake place. If it is the temperature then it would pass as it's not due until June, but better to be safe than sorry.

Johnny C · Apr 26, 2006

I took it into a Bosch place and they checked it out - no warnings or errors were found in the giagnostics. They went out and checked it out and the ABS works fine, and if there is a sensor light it should be disabled.

The chap said the systems fitted were similar to those on BMWs and they have a tendency for ABS warning lights to come on if you have a rear brake light failure. However I replaced both the bulbs last month so it's not that.

The chap said there's a possibility water has leaked into something and shorted a circuit that tells the car there's something wrong, but he thought in those circumstances the ABS wouldn't work.

HE also said on some cars the ABS works off the drive shaft and the shaft get grooved where the sensor is which fools the sensor, but from the 993 book it seems there's a sensor on each wheel rather than the drive shaft,

Stratstone were unhelpful, said I'd need to take it in and let them take it to bits but sod that at £75/hour, it'll be cheaper to get the bulb disabled for the MOT until someone can track it down

Any more ideas? The light is flickering a bit more which reinforces the suggestion it's temperature linked

Cheers

968er · Apr 26, 2006

Change the sensors. Not that expensive or get a good indie to track down the problem.

Still think that it is a failing ABS sensor.

Johnny C · Apr 27, 2006

Wouldn't that show on a bosch plug-in check?

Not being belligerent, I have no idea what the readings show on a check, I just assumed things like that would show up

it's due in for a service in 3 weeks, so I'll get them changed. New aircon radiator, new steering rack, oil leak etc... it's starting to add up [&o]

Johnny C · Apr 28, 2006

The Bosch people are going to run an oscilloscope on each wheel to try and track it down, but I'll get 9M to put new sensors on as I seem to remember Robin saying they need to drill them out, and I don't want someone who's never seen one before going near the brakes with a drill! When I took it in for the first check one of the junior mechanics nearly reversed a Rangerover into it, fortunately someone stopped him before he actually hit it

How much do the sensors cost?

968er · Apr 28, 2006

Johnny,

Sorry, made a mistake when I said they were cheap. Had been looking at the brake sensor price which was just above the ABS sensor price which was £64 excl VAT each

laura · May 1, 2006

We shall be interested to know what your Bosch people say, Johnny. The ABS light on Blue keeps coming on intermittently. Recently it's been after about 25 miles or so. I haven't noticed whether it has anything to do with temperature.

Laura

Siggy · May 1, 2006

ORIGINAL: Johnny C
HE also said on some cars the ABS works off the drive shaft and the shaft get grooved where the sensor is which fools the sensor, but from the 993 book it seems there's a sensor on each wheel rather than the drive shaft,
Cheers

On ABS systems there is a sensor on each wheel, normally on the hubs and pointing at the driveshaft, often you get dirt on the sensor 'ring' this is a toothed wheel. the fix is to carefully remove them and use an airline to blow any dirt out.
The sensor and ring work like a very low voltage generator, as the wheel turns it produces a very low current.
Other problems can becaused by

Worn wheel bearings, movment of the bearing causes the air gap between the sensor and ring to increase and the current lost.

Odd size tyres or 1 tyre that is badly worn, ie 1 brand new and 1 'bald' tyre, this causes the wheels to rotate at different speeds and thhe sensors produce unequeal current.

To much brake fluid in the master cylinder.

My experience and training has been on Fords

Good luck Dave

laura · May 2, 2006

Just been out tonight and the ABS light came on again after about 25 miles. Did a brake test and now have four flat spots on the tyres! So the ABS is not working when the light is on. Didn't do a brake test before the light came on so I shall try that next time I take the car out.

Laura

Johnny C · May 17, 2006

Not heard back from the Bosch people, so no joy there. I decided not to get each wheel checked as the cost of diagnostics would be fairly high, it won't cost much more to get the sensors replaced so I may as well get them all sorted now. If one or more has gone the rest are probably not that far behind.

I did wonder about the possibility of the fault being an electrical one behind the dashboard. I thought my aircon was gone, I haven't used it in the winter, and when I turned it on a few weeks ago it only blew hot air. Then I realised as the outside temp warmed up that there's no cold air entering the system, the aircon is cooling the warm air slightly, but with no aircon the temperature control does nothing and the system blasts out hot air all the time. So either the cold air blower or - more likely - the cups in the engine have rotted. You can usually tell when the cold air blower goes as you get a puff of smoke and a nasty smell out of the vents. I live in the North west where salt corrodes the heat intake cups regularly, on my old 3.2 they needed replacing every other year as they'd rot into a fixed position. I know on the 993 they're at the top of the engine but all my miles are on the motorway and I'm sure the whole engine gets a nice coating of salty spray. I need to check the fuse first though!

This may sound silly, but I'm a problem solver by profession and I've learnt that symptoms which couldn't ever possibly be linked often are.

However the lights come on as soon as I turn the engine on, and if they do go off it's usually after 30 miles or so in which case the air will be hotter than ever. So that's that theory kyboshed.

Johnny C · Jun 5, 2006

Okay, had the sensors replaced. Light still comes on, so 9M say the ABS unit itself might be knackered, a valve not working properly inside or something. That's going to cost a lot to replace.

zwhitea · Jun 6, 2006

Hi Johnny C
Have the same problem currently and we're also thinking it might be the unit rather than the sensor. Please let us know on here what the damage is?

Cheers
Ruth

38F3E1 · Jun 7, 2006

I have heard about a few problems coming to light on 964s with failing ABS units. Wonder if some 993s are starting to suffer the same problem.

Ian.

PS Ruth nice to meet you and other half on Monday night.

Johnny C · Jun 7, 2006

Hi Ruth
A brand new unit will be about a grand. You can pick up a recon unit for about £500, but there's not many about.

After I had the sensors replaced, the ABS was coming on when it shouldn't, i.e. when I pressed the brake pedal very lightly and kept the pressure constant. A couple of emergency stops from 60 on an empty dual carriage way seems to have cured it mostly, it still comes on as I coast to a stop if I keep the brakes pressed with even pressure, but a good stomp clears it. So maybe there's a dodgy sensor in the new ones, this is going to be checked out.