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ABS Light On

M

Mystique

New member
Hi All,

The ABS light on my '89 S2 is permanently on. The car has been standing for 2 years and I'm in the process of restoring/ recommissioning. I'm reliably informed that the ABS was fine when the car was laid up (with the battery removed). I've checked the wiring & cleaned the sensors but the light stays on (even when driving on my private road [;)]). Can leaving the battery disconnected cause the light to come on? (or is this an urban myth...), if so, is there any way I can reset the light??

Thanks,

Graham.
 
Not that Im aware of... The sensors are the most likely cause though and if the worst comes to the worst a new ABS ECU will be required. If youre canny this shouldnt be expensive, although some eBay chancers will try for an excessive price. The part is shared with the 928 and carries a 928 part number.


Simon
 
Corrosion can trash the sensors if left for a long time. Worth disconnecting them and checking the resistance and also cleaning the earth points for the screens. Could also be a knackered pump. If the ECU is duff, I have some serviceable ones in stock.
Alasdair
 
Thanks guys - all good pointers. Once I've isolated & fixed the fault, will the light go out as it should or does it need to be reset by a specialist??

Cheers.
 
If its the sensors the light normally comes on around 10mph.

The light will go out if all is OK it wont need to be reset by a specialist.

Rich
 
As Rich says if its a sensor then it fails its initial test on moving - around 8mph I believe. If its on permanently its either the ecu the pump itself or a poor connection somewhere in between.
It doesn't mean it can't be a sensor as well unfortunately but you won't know that until the initial problem is fixed.
 
Alasdair is spot on about the sensors, I would also suspect a frozen/ stuck ABS pump if the car has stood for a long time.

I've said it before and will repeat myself, I'm no expert on the ABS system ( as per the previous post many parts are shared with 928) but it is pretty component ised:

4 x sensors
Lots of wiring and earth straps
1 ECU
1 Pump
light on dash board

My understanding is that if the light is on from start its unlikely to be the sensors as these only 'kick' in at > 8mph,

I have a pump and ECU if needed - be warned check the part numbers very carefully for both, including the Bosch AND porsche numbers,

Chris
 
I have a knackered pump and my light is never on. My ABS does everything it should (pump whirring on take away etc.) except stop my brakes from locking! [&:] From what I know a duff ECU or relay or fuse will cause the ABS light to be on at start-up and then the self test at around 8mph will trigger a duff sensor. I am not sure if the sensors can trigger the light whilst not in self test mode (stationary)? Don't think they can.

It may also be a bad earth to the sensor or a damaged sensor wire. Again not sure when that would show though. I don't think the ECU talks to the sensors at all unless moving.

Stuart
 
The sensors can't trigger the light when stationary as they are mag pick ups working off teeth on the hubs. When stationary they produce no output as no EMF is induced and hence no pulses generated. The ECU will look for pulses from each sensor shortly after moving off. If it sees no pulses from one or more sensors it will trigger a fault. Whilst motoring it sees pulses constantly. If it suddenly sees no pulses from a sensor it will presume a wheel is locked and not turning and trigger the ABS pump to do its thing. Simples really but seen as such complicated thing.
 
OK, so I've managed to find the ABS pump (not the easiest access...), and am ready to carry out some investigations. The exact symptom is that the ABS light comes on with the ignition, then goes off for a split second before coming back onand staying on. Are there any simple tests I can carry out on the pump in-situ, or do I need to remove it to do anything worthwhile??

Thanks for all the help & advice [:)]
 
There are 2 12v feeds to the abs pump, check them both with the engine running. Should be terminal 6 & 12 of the pump connector. Also check the earth wire next to the pump.
 

ORIGINAL: sulzeruk

Corrosion can trash the sensors if left for a long time. Worth disconnecting them and checking the resistance and also cleaning the earth points for the screens.  Could also be a knackered pump. If the ECU is duff, I have some serviceable ones in stock.
Alasdair
Click to expand...

Can you tell me what the resistance should be please, I know the red pointed thing and the black pointed thing and the dial to ohms but no idea what it should read. Cheers
 

Approx 1k Ohm, worth checking resistance at the plug to the ECU so you will also be checking wiring. When mine was faulty all the resistances were correct.
Also worth checking voltage. Mine read about 0.5Vac when spinning the wheel by hand but the faulty one was 0.3Vac.

Rich
 
Hi,

My old posting might provide some insights - I tried to record everything I learnt, including the expected resitances and pin numbers:

http://www.porscheclubgbforum.com/tm.asp?m=440895&mpage=2&key=

From what I've read in your thread so far, it doesn't sound like a problem with the sensors because these are checked once the car reaches about 10 mph. Probably worth testing the resistance of the solenoids in the pump unit - you can get at almost everything from the plug to the ECU in the driver's footwell.

Hope this helps - it took me over a year an a lot of wasted time and money with garages before I diagnosed the problem with my ABS myself.

Tom
 
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