ABS Problem, Searched And Found Nothing!

[944Fripp]

Hi all, I've had an ongoing ABS problem which leaves the ABS light warning light on. Not only is it annoying but very soon it'll be an MOT failure! It's made all the worse by the fact that there are a lot of posts about this but none that really describe my car.

It started at 10 mph (which suggested a sensor problem). Now however, the lights come on on tick over (after about 10 seconds) even before the car moves!

Last weekend I plugged up a multimeter to each sensor (+ve and -ve in each respective female socket on the output from the sensor) and tested the resistance as I turned the wheel. Each one showed a change in reading which I assume (I'm no electrician!!) means they are all working. The next thing I tried was to change the ecu with a spare I have; still the same. Next I changed the relay so I bought a second hand one which I am assured works... still the same.

Given that the light comes on before the car even moves and that each shows a resistance change I have ruled out the sensors. Right before the problem started I could hear the pump priming so for now have ruled this out too (this is also hopeful thinking!!). Given that the lights come on when the car is on tick over I guess it's an electronic problem and perhaps there is a short or a ground on a circuit somewhere. My next move is to whip the driver side wheel inner arch out and clean up the earth connector by the pump. If this doesn't work I might try another ecu just to completely rule this out as its the next cheapest thing!

Any other suggestions (not including black tape over the warning lights, removing the bulbs on the warning lights, removing the relay all together!!!! Sensible ideas please!! ) would be much appreciated. []

Are there any diagnostics available for S2 ABS systems?!

Thanks as ever!

 

[944 man]

Isnt the multimeter test fallible where they have corrosed internally? You might be as well obtaining a known-good sensor and swapping each position in turn.

 

[944Fripp]

Wow! Quick draw McGraw! That's the fastest reply ever!!

Good point but why would the lights come on when stationary? Does the ECU have memory?

 

[ALZ 1933]

If it comes on when the car is stationary & hasn't turned a wheel I'd expect the pump to be faulty as it primes itself on start up.

 

[tref]

OK, I posted this under "2.7", but only a couple of days ago... funny how things come around...

"OK... there is also something subtle to look for on ABS sensors... just got to remember it... Ah yes! OK... If you test the sensors "by the book" measuring the resistance, including measuring the resistance of the loom they can check out perfectly ok... however...

What can happen apparently is that they corrode and short out internally between eth casing and the windings inside... of course, measuring the resistance of the sensor itself doesn't show that out, as the resistance between the two pins remains exactly as it should... so, after doing the check according to the book, also check the resistance of each pin of the sensor to earth on the car. Now I have to make it up as I go along, but I think they should be open circuit (assuming you have the sensor disconnected) but will either show the same resistance as the sensor should, or show a short if they have corroded internally. Certainly this identified a problem on my S2 (on which I subsequently had to drill out the sensor - great fun, you wouldn't believe the dusty bits of copper it generates!)."

Regards,

Tref.

 

[944Fripp]

I'm 99% sure the pump prime is triggered ONLY by a positive reading (showing equal speed) from all sensors at 10 mph. You can hear the prime if you are very quiet in a fully working S2 (assume its the same for all late model 944's, Turbo and possibly 968's).

I think the system does a general system check on start up (relay, circuit, ECU), the sensors run a test at 10 mph and the pump then primes. If my pump is faulty I've got no way of knowing unless the fault comes on at 10 mph in which case it's the pump or the sensors. This is why I've changed the ecu, the relay and am going to check the pump ground.

It's also why I don't think it's a sensor because the light comes on after start-up and before the car actually moves even with 2 different ecu's!!

Unless anyone knows better?! I can't find anything online about the ABS start up sequence.

Big Phil never makes these things easy!! []

 

[944Fripp]

ORIGINAL: tref

OK, I posted this under "2.7", but only a couple of days ago... funny how things come around...

"OK... there is also something subtle to look for on ABS sensors... just got to remember it... Ah yes! OK... If you test the sensors "by the book" measuring the resistance, including measuring the resistance of the loom they can check out perfectly ok... however...

What can happen apparently is that they corrode and short out internally between eth casing and the windings inside... of course, measuring the resistance of the sensor itself doesn't show that out, as the resistance between the two pins remains exactly as it should... so, after doing the check according to the book, also check the resistance of each pin of the sensor to earth on the car. Now I have to make it up as I go along, but I think they should be open circuit (assuming you have the sensor disconnected) but will either show the same resistance as the sensor should, or show a short if they have corroded internally. Certainly this identified a problem on my S2 (on which I subsequently had to drill out the sensor - great fun, you wouldn't believe the dusty bits of copper it generates!)."

Regards,

Tref.

Tref,

Thanks for that!

That's interesting, I didn't think of that, certainly worth looking into. How do I check resistance to earth on the car with a multimeter? Where do the probes go and what's the procedure?!

Still, it doesn't explain why the fault light is on before the car even moves unless the ABS ECU has memory! [&:]

 

[chrisg]

The 944 abs system is very basic and only has the following components:

Abs ecu or brain
Abs pump
4 x wheels sensors
Wiring between the 3 other components

From the symptoms you've described it sounds as though there is a 'fault' even before the car has turned a wheel. So almost by default it can't be a wheel sensor as they only work at speed and detect a difference in front to back wheel speed.

If the pump is priming with the ignition on both it and the ecu are probably ok.

My thought would be its a wiring fault most likely a rusty earth strap from a barrel connector up around the rear axle/ petrol tank. These rust very easily and can readily snap off.

Yours

Chris

 

[eddieedmo]

I had problems with mine last year , intermittent light when at low mph , eventually pump stopped priming/self checking at low speed and light on all time.
When investigated went through whole system step by step from sensors , loom, ECU AND PUMP.
Found sensors all ok ,barrel loom sections from each sensor to main ABS loom all badly corroded no earthing,had to replace all 4 ( £90 each ) still fault showing , so changed ECU with a known working one , still faulty so last was pump out and completely swollen and siezed so replaced and all ok.
So initial intermittent fault was due to poor earthing and eventually due to lack of use at self checking.priming pump siezed and so light on all time.
I have good sensors and a Bosch centre tested ECU if you need to borrow to test yours.

Nick

 

[944Fripp]

ORIGINAL: chrisg

The 944 abs system is very basic and only has the following components:

Abs ecu or brain
Abs pump
4 x wheels sensors
Wiring between the 3 other components

From the symptoms you've described it sounds as though there is a 'fault' even before the car has turned a wheel. So almost by default it can't be a wheel sensor as they only work at speed and detect a difference in front to back wheel speed.

If the pump is priming with the ignition on both it and the ecu are probably ok.

My thought would be its a wiring fault most likely a rusty earth strap from a barrel connector up around the rear axle/ petrol tank. These rust very easily and can readily snap off.

Yours

Chris

Chris,

Thanks for this, I totally agree and it's a great suggestion.

Please see my auto signature....novice with a spanner but willing to jack it up on axle stands and give it a go! Any pics or pointers for this? Even a part number?! What is a barrel connector and is this under the rear axle or in the boot somewhere?

Thanks, hope is restored! []

 

[944Fripp]

ORIGINAL: eddieedmo

I had problems with mine last year , intermittent light when at low mph , eventually pump stopped priming/self checking at low speed and light on all time.
When investigated went through whole system step by step from sensors , loom, ECU AND PUMP.
Found sensors all ok ,barrel loom sections from each sensor to main ABS loom all badly corroded no earthing,had to replace all 4 ( £90 each ) still fault showing , so changed ECU with a known working one , still faulty so last was pump out and completely swollen and siezed so replaced and all ok.
So initial intermittent fault was due to poor earthing and eventually due to lack of use at self checking.priming pump siezed and so light on all time.
I have good sensors and a Bosch centre tested ECU if you need to borrow to test yours.

Nick

Thanks Nick, really good of you to offer. I'll see how I get on with checking the wiring first but keep it in mind. How did you get this tested by the way?

 

[Dockrell]

Tom,
Both times I had the ABS warning light illuminated the problem was the pump. I found that a good source of second hand pumps was via e-Bay from the USA. The pump from the USA had suffered from very little corrosion, was in better condition, and cheaper than one from the UK.

Mike
White 2.7 automatic

 

[944Turbo]

I investigated something very similar for someone a few years back, when jacked up and the suspension drooped the sensors worked, back on the ground the wiring flexed back the other way and they failed, simple to repair a b*tch to find, we got lucky wiggling the wires in the air.
Tony

 

[944Fripp]

It was the pump, all connectors around it were rotten (that's what was showing the fault on start up) and the pump had seized. Luckily I saw a good second hand one on eBay about 6 months ago for £75 and purchased it! Turned out to be a good call!! [] Phiew!!

Having a new clutch master cylinder fitted at the same time. The pedal falls when it's left over night and there's a little bit of slip. It's done it since I have owned the car (about 5 years now!) and all the other bits have been changed. Last in a long list of the bigger jobs that I have wanted to get done!!

Thanks for all your help gents.

 

[edh]

good to hear you've fixed it

Are you getting the clutch slave cylinder changed as well? - not an expensive part, and "while you're in there..."

 

[Keith D]

Hi all, re the ABS warning light on my 944s2 1991. The abs warning light came on so knee jerk reaction bought a new relay simple fix so I thought. No change except for £75 lighter in the wallet, following the advice on the forum I have spent the day under the car re doing and cleaning all the earth connections, but still abs warning come on just as you think you have fixed it at approx 8 mph.
I have to say that the wheel sensor on the front n/s looked a bit chatty other than that all seemed ok, new nuts and a little copper slip and a good sanding to all 8 earth connections and I am confident that the problem must be something else. Please any advise at to what to do next, might it be a wheel sensor ?
Kind regards
Keith

 

[944 man]

Sensors and they can be faulty even if they pass a multimeter test, unfortunately.

 

[Keith D]

Thanks for that, any one know we're i might get a replacement.

 

[sulzeruk]

Your pump will be knackered.
Alasdair

 

[scam75]

As an aside I had a seized pump that happily primed when it should and never caused the ABS light to come on. The only symptoms I got was inop ABS! A replacement pump (nice clean one from the US of A) sorted my issue.

Stuart