944 - antilock

[Globe_911]

Hi, haven't been on here for a while. basically i bought a 944 S2 late last year and stripped it out, put a half cage in, and bucket seats and harnesses to use as a track toy, only problem is after Croft last sunday the antilock light was coming on and th back wheels were occasionally lockign up. brake fluid level is good so any ideas? (I'm going to be away for the next week so if anyone replies I'm not ignoring you!)
Thanks

 

[Diver944]

The system is pretty straightforward and consists of the ECU, the ABS pump and a sensor on each of the wheels. At startup the ECU checks the pump and all 4 sensors and if it doesn't receive the correct reply you get the warning light.

The most common failure is one of the sensors (or the wiring to it). The fact that your rears keep locking up would make me think one of the rear sensors has failed, they are diamond shaped and can be seen once the brake disk is taken off.

 

[sawood12]

As Paul said but if the system detects a fault with any of the four sensors then the system will disable itself and the light in the dash will come on so it may not be the case that the problem lies with the rear sensors if it is sensor related. The rears are probably locking up due to that axel being unloaded under heavy braking.

Do you hear the accumilator buzzing when you set off and reach a few mph?

We've had this discussion before some time ago and I got some fervant disagrement but I believe this is the case - have you got non-standard sized wheels fitted and if they are non-standard how far away from standard total diameter are they? The handbook states that the ABS system will disable itself or 'not function properly' if the wheel diameter is outside a certain tolerance from standard (+ or - 17mm I think). This is because the ABS system tries to anticipate when the wheels are going to lock up and will engage the ABS before you actually loose traction (i.e. it is an Ani-lock Braking System). If the wheel diameter is too far away from stock then there will be a discrepancy between wheel RPM measured by the abs sensor and the indicated speed on the speedo and the ABS system will cut out. A long shot but worth checking just in case.

 

[will lyons]

I have a similar problem with my ABS history shows Discount9 have checked all sensors and replaced the rear final note suspects the pump but braking is normal with no adverse effects as in normal non ABS but at a few mph accumilator buzzes and light comes on.

This is the next on my things to do list so any help appreciated. I have arranged to visit a friendly porsche breaker who as offered to try changing the ECU to eliminate a fault here this would only leave your theory or to replace the pump. Any more information on the wheel speed etc would be appreciated right now the car is on standard Turbo teledials so speedo to speedo SHOULD be correct

 

[944Turbo]

Dont see how wheel size can affect it provided all 4 wheels are the same size- there is a speed sensor on each wheel which detects speed and on the gearbox (or front wheel) there is a sensor which sends a signal to the speedo. If you change the wheel size you change the speed seen by all the sensors - the speedo will read wrong if the size is too far out but all the speed sensors will also read wrong by the same amount.

Wheelspin doesn't create an ABS fault. A single or pair of different size wheels under braking may look like it was travelling slower (and closer to locking) but there must be some tolerance for turning, the outside wheels travel faster than the inside ones hence the need for a differential.

I think most likely is a broken wire (30 years of flexing) this would then appear as a locked wheel to they system which would then try and realease pressure on that wheel which of course if you are not breaking it cant generating an error. It would also mean it wouldnt 'see' a genuine lock up.

Pump failures also occur but these usually result in permanent failure a broken wire may be ok at the start up only triggering once flexed. ECU and sensor failure are rarer but do occur. Sensors do get dirty though which limits there effectiveness the rust from brake disc etc. sticks to them.

I think the buzzing at 3?mph is the pump so I think if you get it the pump is ok. If you dont then the pump might be ok if the ECU isnt telling it to run it wont run. Dont know if a failed sensor will stop the ecu from making the pump run.

ECU is in drivers footwell behind a carpet panel in the vicinity of the boot release.

Tony

 

[appletonn]

Speaking as someone who has 18" rims fitted to his 944, I did wonder about the effect of these on the ABS system, but mine does most definitely work, albeit in the somewhat clunky, mechanical fashion of last century ABS systems. I also have no warning lights to show anything is amiss due to the size of wheels fitted.

I must admit, I did think that any 'disagreement' between the ABS system and the wheels related more to variances in overall diametres between wheels, rather than in a change in total diametre over the stock setup?

As an aside, my speedo reading apears to be more accurate on 18" rims than that fitted to my 2 yr old work car, if the GPS speed reading from the old Tom Tom is to be believed. Barely 2mph over reading at an indicated 85mph!

Hope you get it sorted quickly.

 

[sawood12]

ORIGINAL: 944Turbo

Dont see how wheel size can affect it provided all 4 wheels are the same size- there is a speed sensor on each wheel which detects speed and on the gearbox (or front wheel) there is a sensor which sends a signal to the speedo. If you change the wheel size you change the speed seen by all the sensors - the speedo will read wrong if the size is too far out but all the speed sensors will also read wrong by the same amount.


Tony

Well i'm not quite sure exactly what it does but nevertheless it does state in the manual that your wheel diameter should be within a certain tolerance for the ABS to function correctly so there must be some calculations going on in there where a key assumption is the rolling circumference of the wheels. The 944 has a mechanically driven gear off the final drive rather than speed sensors but I agree that the relationship should remain the same ie. both the ABS sensors and the speedo will be tricked the same. Maybe there is some Porsche Witchcraft going on.

 

[Fen]

As Tony has said there is no way the wheel diameter can have any effect on the ABS so long as all 4 wheels are within tolerance relative to each other. As has also been noted it's an old ABS system and while I find it works better than many new ones I've used it is hardly "fancy" in terms of what it does compared with DSC / ESP etc. that is now commonplace.

The pump groans at just over 6mph I believe (10kmh in fact) but that doesn't indicate in itself that everything is in working order. Sensors are the usual culprit, but I have heard of failed pumps and a failed ECU that didn't display a light nor cause the pump to groan.

In the case of the OP's situation it may not even be a fault - the car has been lightened and is being driven exceptionally hard. Is the suspension also past its best (i.e. is it not new)? Does the car have grippier/bigger tyres than standard?

Combining the weight removal changing the front/rear distribution of the chassis, greater braking force from the front (through a grippy track surface and modern tyre technology plus probably bigger/wider tyres and maybe bigger brakes/better linings) and possbily more dive than when new the brake proportioning valve is very likely giving too much braking to the rear. It might then be possible that the rears are so unloaded that they lock very easily and don't unlock as quickly as the ABS is programmed to expect, hence it triggers a fault indicator. Does it happen regularly at the same point(s) on the track but never on the road or on less heavy braking? My car didn't do this and I haven't heard of anyone else's doing it either, but then my car put down 359bhp in the wet without so much as a squirm and some people's back ends are all over the place with under 300 so suspension and set up have lots to do with traction.

Of course it could be a fault (and a fractured wire is a good suggestion as the suspension is also probably at greatest extension when it happens).