Abs light on and brake issues

[220bhp]

Took the s2 out today and the abs light came on after 20miles . The brakes then at first where hard and harsh only to almost fail on and of . Any ideas . Would be appreciated . Cars a 3b with Porsche brakes up front.

[Crasher]

Do you have VCDS?

[Tractor Dave]

Interesting. Abs light normally indicates a wheel speed sensor issue but shouldn't affect brakes from operating albeit without abs. I would suspect the brake booster. There is a pressure switch under it which may trigger the abs light? I thought that was a separate red exclamation mark light but not familiar with 3b set up. Before you start the engine, press down on the pedal a few times until it goes hard. Keep your foot on the pedal as you start the engine and it should move down a bit.

[220bhp]

I tried what you said Dave and it seems ok . pumped it several times ,kept my foot on , started the car , and after a second or so the pedal dropped a bit .

[220bhp]

Just topped up the power steering fluid which was low And took it for drive . No abs light , no brake issues until it was up to temperature the light came on again but brakes seem ok . Turned ignition of then started the car light back on .
is it possible it’s heat related in any way ,calliper sticking or something ?

[Tractor Dave]

If the calipers stick they get very hot very quickly. There is a known issue with the master cylinder getting hot and causing the brakes to bind on but your symptoms sound quite different. If your pas fluid was low have you checked for leaks? The original system takes high pressure pas fluid into a block with internal valves which directs fluid to the accumulator (bomb) and booster. The booster uses the pressure to act on the master cylinder when the brakes are operated. This pressure is converted to brake fluid pressure to operate your brakes via the abs pump. The fact that the abs light is coming on would suggested a fault there. I'm not sure but believe all the valve block and booster sensors trigger the red brake warning light, not the abs fault light. However, failure of the abs system should still give full power brakes, just no abs.
As your fault is intermittent, it might suggest something sticking internally. When was the brake fluid last changed? If it hasn't been done in the last three years i would flush it thoroughly with new dot 4 and see what comes out.

[220bhp]

Thanks for that . I have not had the fluid change in a while , so I think it’s a good call.
I have a small leak from the pump which I have to address, but no warning lights other than the abs light
which I have to say often comes on under full boost then goes of soon as you lift . It’s done that for quite a while
it flickers so I just assumed it was some sort of loose connection

[Ian Simmonds]

ABS light. Silly question, it’s not overcharging is it? A few years back the alternator voltage regulator packed up on ours and the ABS went nuts.it seemed to be the first thing that was voltage sensitive. Your description of the brakes failing certainly wasn’t a symptom mind.

[220bhp]

That’s a thought .when I first start the car the voltage meter only registers about 10 until I touch the accelerator , then it is around 13 .I also have a parasitic drain on the battery for which I suspect the alternator . I had no idea voltage could affect the Abs .
I still have an intermittent brake issue but the ABS light no longer comes on ,it’s more like a sudden loose of power assistance .I still have brakes but you have to push harder . That’s what catches you out , suddenly getting a different feel from the brake peddle .
I have it booked in to do a fluid change to see if that helps .

[newsh]

Thanks for that . I have not had the fluid change in a while , so I think it's a good call.
I have a small leak from the pump which I have to address, but no warning lights other than the abs light
which I have to say often comes on under full boost then goes of soon as you lift . It's done that for quite a while
it flickers so I just assumed it was some sort of loose connection

Here's a theory.. completely off the top of my head

The one way valve in the vac line to the rear diff lock (under the rear seat) has gone bad and boost pressure is getting to the diff lock actuator and triggering the ABS off?

As I say, worth thinking about and checking.

[220bhp]

Good call John I will take a look