ABS-OK at high speed, Light on at low speeds?

[dome]

Hi guys

It's been a frustrating first few weeks with my new S2 as weather and Christmas are getting in the way of me actually doing anything to it despite a growing list of small niggles but I thought I'd ask on here about my ABS problem to see if anyone has any ideas.

Start up and the light goes out, however at low speeds around town it kicks in really early and within a mile or so usually the light comes on. However, I drove all the way from just outside Ayr last night to Edinburgh(80 miles) the light stayed off until I got to below 40mph.

I'm going to go through all the trouble shooting I've found on tinternet(when the weather allows) but wondered if anyone had an idea what might be causing this?

Cheers

Brian

 

[George Elliott]

If you find a solution, post it please Brian,

I pulled a 944T out of hibernation last week, and have the same issue. Can you hear the ABS pump self test approx 5 metres after you move off.....mine is silent......very silent

Has yours been in regular use?

and Good Luck

George
944t

 

[964RS]

Just a few ideas:

Inflate the tyres to the correct pressures.

Have you got the original wheels and correct profile tyres fitted?

Bleed the brakes.

Replace the brake fluid.

 

[George Elliott]

Paul,
Tyres, done
Forged CS originals - dead straight
Bleed Done
Replace Fluid Done
Bleed Redone
Check Wheel Sensor Pulse Done
ABS Pump removed & Check Done (but pump did not operate on the 12v supply it was tested on, but I thought it may require multiple inputs)

Next Replace ABS Control unit, and ABS Pump if required

First ABS problem ever for me

By the way, Kermit is lovely []

 

[T3rra]

What's your battery volts reading when the light is on? Have u tried turning on and off the engine.

 

[964RS]

Thanks George. []

Let's hope you both get the issue resolved. Be systematic. Keep us up to date.

 

[dome]

Thats for the ideas guys.

Brake pedal feels solid so I've not bled it yet
I believe I have the correct offset, it's a 1991 car with genuine Porsche wheels, offset on the front is 55.

I have just purchased new front tyres, will be getting them fitted when I get the chance. I have new 245/40 on the rear and 2mm 205/50 on the front-I have some new 225 45 to go on the front. I had initially thought that this could be the issue but surely high speeds would show up the difference in rolling radii and cause an error moreso than at low speeds?

Alternator is my other port of call-I need to check the voltage across the battery as the volt gauge varies between just over 12 and 13.5ish. However where the voltmeter is has no impact on whether or not the light comes on. I rebuilt the alternator on my old Jag so would be happy to do this one if need be.

I'd just love a clear day or so to faff about and fix all the niggles-pesky Christmas and crap weather getting in the way! I need to go now and brave the shops. Humbug!

 

[edh]

Brian I bet your issue is a duff sensor (or two) - even though they may all check out for resistance, they can still be faulty. Not easy to remove either...good luck I had a similar problem my S2 which was fixed by swapping sensors.

The ABS system only has a few components (pump, ECU, sensors, relay) but seems to cause lots of hard to fix problems. George have you tested the ABS relay? ISTR there's a simple test for it. I'm assuming your pump earth is fine.

 

[eddieedmo]

the ABS system seems to cause alot of problems now as cars are getting older.
Systematic approach needed
check sensors are all ok - resistance and osiloscope test.
check all earths are good with a voltmeter , double check the barrel connectors are good these go from sensors to main ABS loom - the earth wires can break up inside - I replaced all mine last year.
check pump - most do not get used and swell and sieze up = activate pump atleast once a week to keep working - loose gravel or any surface were you can brake hard and get the ABS operating.
check relay.
I have spare good tested sensors and ECU if you want to borrow to test.
check all connections in system are good.
Good look it can be frustrating - might be worth an hour at a Bosch diagnostic specialist.

Nick []

 

[George Elliott]

Earth is good Ed, did all the wheel sensor ones too,

Relay not tested, will look into that.

Thx all and have a good Christmas
George
944t

 

[George Elliott]

Nick,
agree there needs to be some method to fault finding.

I think ABS (or many low voltage circuit) fault finding is perhaps more difficult if earths are less than 99% perfect.
Therefore my first move has been to do every "MP" location earth point on the body shell.

I also removed the copper flexi-strap and soldered the P Clip on the end. (this requires a 4mm drill x 8mm deep hole into the side of the connector at each end, then heat the P Clip with a torch and flow solder into the copper braiding until it emerges from the Clip.
I did this at the battery-body and at the bulkhead-bellhousing earth straps.
Then cleaned the Front Struts & Strut tops x4 points, Trailing Arms x 2 rear, Chassis legs x2 front, Rear Panel x1, Central Elec (Fuse Box) x2 large / x2 medium.

I am now confident of a constant ground / earth everywhere on the car. (unless some-one tells me there are more points [])

I damaged one rear sensor so progress is on hold

George
944t

 

[edh]

Agree although 944's don't seem to have as many earth problems as 924's in my experience.
I remember discussing ABS problems with Simon at PH (before the split). He said that the system usually kept on working even when the earths on the sensors themselves were almost shot.

have you tested your relay?

 

[George Elliott]

Brian,

I have at last fixed this problem.

I also have a car with a "new" starter motor....the effects of cleaning every MP (electrical ground to body) location on the car.

I replaced the rear O/S wheel sensor which I damaged when checking if it was clean.

Good luck with your repair, I would be of the view it is an earth problem as it sounds intermittent.....

The wheel sensors are incredible bits of engineering. Firstly they are all stainless and look like new despite being over 20 years old. Pity the whole car was not made to the same standard. I calculated each sensor provides about 3 billion inputs in 100,000 miles...[8|]

 

[eddieedmo]

well done.

Nick

 

[George Elliott]

Thankyou Nick,

but I cheated 'cus I used a calculator []

George
944t

 

[edh]

George - what was the fix? Was it the sensor that was faulty?

ORIGINAL: George Elliott

Brian,

I have at last fixed this problem.

I also have a car with a "new" starter motor....the effects of cleaning every MP (electrical ground to body) location on the car.

I replaced the rear O/S wheel sensor which I damaged when checking if it was clean.

Good luck with your repair, I would be of the view it is an earth problem as it sounds intermittent.....

The wheel sensors are incredible bits of engineering. Firstly they are all stainless and look like new despite being over 20 years old. Pity the whole car was not made to the same standard. I calculated each sensor provides about 3 billion inputs in 100,000 miles...[8|]

 

[George Elliott]

edh
will never know, the sensor resistance was OK but I wanted to check it was clean and I broke it. (I dont think foreign matter affects the sensor operation,) but I wanted to know it was clean and the teeth on the pulse/encoder wheel were also clean, and you cant see the teeth without removing the sensor.

I replaced it with a used one, did all the earths and it no longer throws up the fault light.

I am 99% certain it was the earths, the sensor inputs are electrically delicate IMO. There are lots of earths and when I checked various parts of the car on Ohms I was surprised at the results.

EG Inlet manifold to body shell 85 ohms.....? I know, gaskets etc etc,but there is a pathway thru the studs........and its 0.2 ohms now since I did the MP earth locations.

I was quite surprised to get poor readings at the Main Electrical Panel earth points behind the dash

George
944t