systematic approach to ABS fault finding ?

[eddieedmo]

Since I got the car the ABS has sometimes shown a fault but now the light is on all the time so time to sort out.
Is there a posting which details a systematic way to find ABS faults.And also how to do electrics tests on various components and connections etc.
I have heard the ABS system on S2's has to be reset after work is carried out ? is this true and if so how ?
Thanks for any help.

Nick []

 

[DavidL]

I gather it isn't that easy.
A search should show up various options but I think:
The system checks the wheel sensors at 7mph and the light comes on if there is a failure. Exactly how it checks I don't know.
There is a test with an oscilloscope and spinning wheels but that's beyond me.

So light will come on with ignition, go off and then come back on once moving.

If the light comes on and stays on it tends to be the pump or ecu. Ecu is behind the carpet by your right knee when sat in the drivers seat - by the hatch release. The ecu is easy enough to change but the pump is rather more of a fiddle (or just more expensive if someone does it for you.)

 

[bertelli_1]

There is a diagnostic procedure but it uses a very old bit of test equipment - I've never seen one. If the light comes on at walking pace check the wheel sensors. If the light is on all the time there are a lot of potential faults, I'd suggest starting with checking the 12v supplys and earths - hasn't your can been in bits recently? It does not need to be reset after repair.

 

[Cater_Racer]

Same problem, my car has never had it working in my ownership.

I need a much more ground up approach, I've no ABS, but the light never shows, so I'll need to start at the fuse box, i suppose.


I raced it quite successfully last year without, but it would be nice to know if there was a reasonably simple fix.

Or maybe I just invest in brake bias lever in the cockpit.

 

[eddieedmo]

yep light comes on at 8mph ish and stays on.Will check supply and earths to start.I have tested another ABS relay ( know to be working ) in main fuse box and no difference.
Will let you know how I sort it.

Nick []

 

[chrisg]

Nick

I have to confess, diagnostics and fixing things isn't my forte. However, as previously stated on this forum, in simplistic terms the ABS system on the 944s is pretty basic with four components:

ABS pump
ECU or brain
Four wheel sensors
Wiring and loom (inc warning light)

If at tickover and whilst stationery, the light stays off I would conclude that the pump and 'brain' are working ok. If the light comes on at speed, it would indicate to me it's a sensor or wiring that nots feeding a signal into the system.

If you have the facility to work on the car at home, putting it on four axle stands then running the rear wheels up gently in 1st or 2nd gear should activate the system - after all it's looking for a differential between front wheel and rear wheel speed. Doing this procedure you should get a noticeable vibration through the brake pedal as the front wheel are effectively locked.

My suggestion as a first step, would be double check all the wiring to and from the sensors. In particular the rear wiring system - there's a series of black barrel shaped clips that hold the wiring onto the body (and at the front the front strut). At the rear the white block or join has a small earth wire coming out of it - this gets very rusty and can snap where it connects to the body. Similarly at the front, it earths to the front strut.

After checking and confirming this is all in working order, try cleaning up the sensors which are embedded in the hub.

Next step, remove the ABS brain hidden underneath the dash board, drivers side - held onto the body with 2 x 10mm bolts and connects to the pump with a big multipin connector. Once you've taken this off, you should get it testes by a Bosch service centre. It's not uncommon for them to get fried if the pump fails.

If having checked all of the above are working correctly = replacement pump required. Removing and fitting a 'new' is a right PITA, but doable on the drive or garage at home ( I know.... as I've done it myself....)

I have a spare pump and ECU from an S2 if you need, failing that, give me a text or ring for phone help :

07540 193874

Chris

 

[eddieedmo]

Thanks for that Chris, will make a start on Thursday and will let you know how I get on.
Cheers

Nick []

 

[robwright]

I would suggest that that method wouldn't work. The ECU is looking for pulses from every corner of the car. If it does not see these pulses when the car starts moving it assumes that one of the sensors is faulty and the light comes on. If all is OK and it suddenly loses pulses whilst at speed it assumes that one of the wheels is locked and the pump does it's thing. The ABS will not operate under 7 mph and it should perform as self test after you start moving i.e. it operates the pump. You can unplug each sensor at the wheel and test ther resistance. They are just mag pick ups so should have a resistance of around 1 kohm just like the flywheel sensors. If the sensors are OK assume the wiring is shot. The pump is behind the wheel arch liner on the drivers side and the ECU is behind the carpet on the right of the drivers footwell near where the boot release switch is. Hope this helps a bit mate.

 

[rcsalmons]

I had the same fault and went about it by looking directly at the connector to the abs control unit.

I cant remember the connector numbers but if you connect a digital volt meter set to Vac to each pair and get someone to spin each wheel individually you will see a voltage.

In my case 3 of them measured 0.5Vac and the other 0.3Vac. This turned out to be the sensor.

Checking resistance alone is not enough.

This method you are checking the wiring back to the control unit as well.

Rich

 

[robwright]

You are of course correct Rich the resistance is purely a rudimentary check but easy to do if you have basic knowledge and method B does require a glamourous assistant. The pins at the ABS control unit are 21 & 11, 6 & 4, 9 & 8 and 26 & 24. These are the pairs of wires for each sensor. Like Rich said you should see AC voltage at the plug measured across each pair of wires. Polarity not essential as it is AC you are measuring.

 

[eddieedmo]

Thanks for that guys , the pump used to test when I set off but it has stopped doing that now but warning light only comes on at set off when about 7mph.Lots to think about and test.I will have to let my imagination run riot to get the help of a glamorous assistant.

Nick []