Question for any electrics experts out there

[Johnny C]

I had the ABS system replaced a year or two ago. Sensors and 'brain'. Since then, if I press the brake pedal slightly the ABS kicks in and pulses the pedal. 9M have looked at it several times and are still scratching their heads. Also when I've been doing low, constant revs for a while (say in a traffic queue) the ABS light comes on. Turn the engine off and back on and it goes out.

When the alternator went recently (and battery got flat) the pulsing problem with the pedal went away for about the first 20 miles. It came back, but yesterday after the engine loom was replaced, the pulsing problem also disappeared temporarily.

Now I'm only a simple soul with little technical knowledge, but it seems to me there could be some capacitance building up, and when all the electrics are disconnected that drains off. It doesn't drain off when the ignition is turned off.

Is there any way of earthing the ABS system? Would a Bosch specialist be able to tell me what's wrong?

 

[Mark Elder]

Sounds a bit worrying. If the ABS is pulsing when it does not need to, then it's not letting you press the brake pads onto the disks as had as you sometimes might want to.

How well do the brakes work?

Given it started when you got a new system, I wonder if the new system needs to be told something about the expected roll rate of each wheel (given front and back are different sizes, the ABS brain must need to know this in order to determine if a wheel pair is revolving at the correct rate). On the other hand, as the problem goes away with an ECU reset (both the low battery and changed loom are likely to have caused this) it implies to me the ABS brain is being feed some wrong information and "learning" wrong roll rates???

Just speculation.

What you need is a tool like Maurice's OBD reader and the "Scan tool" software that can feed actual ABS signal values to a laptop as you drive along, to get an understanding of what is going on! I don't think the Bosch Hammer gives the codes - just the fault summaries. Probably a Bosch Hammer that 9M would be using.

 

[Johnny C]

Would a proper Bosch service centre be a better place to get that tested? The light coming on doesn't bother me particularly, but the brakes pulse does need sorting. I forgot to add, if it does pulse and I let the brake go and depress it again there's no pulsing, it works as it should.

It's not a problem above 20 mph; and only when the brake is lightly depressed; hard braking doesn't cause it.

 

[Guards Red]

I had a similar problem with mine after an OPC had worked on the ABS system. Another OPC had the car for a week before they finally figured it out - on one one of the front wheels, a technician had leaned on the ABS toothed wheel with a lever trying to get the ABS sensor out. These toothed wheels are made from light steel and are easily bent but mechanics insist on sticking a screwdriver between them and the sensor. Anyway, one tooth was bent which was suficient to confuse the ABS. Porsche changed the computer plus other components before figuring it out as these are a none wear item and are hardly ever suspected. Not sure this applies to you, but the fact that it happened after work on the system makes me suspicious....