Tuner1

I should just be patient and have my tech check over the car but instead I am throwing this out into the world of Rennlist

964 turbo (IIRC same ABS set-up as a C4)....the ABS doesn't work but everything up front looks fine and the dash warning light isn't on.

Are there any common failure points on this ABS system that would cause the ABS to stop working without showing the warning light?

Makmov

how do you know the ABS doesn't work?

campoj2

Agree with Makmov, if the light is not on how do you know the ABS is not working?

swmic

The ABS in my old C4 did not work because the PDAS controller was dead.

Makmov

There are lots of reasons why the ABS stops working. Pumps are pretty common, speed sensors. However, why there is no light is a bit of a mystery.

I guess, it could be someone removed the bulb.

FWIW, I have found the ABS to be very mild-mannered. The way it should be, barely noticeable.

Tuner1

The ABS warning light illuminates with the ignition turned on.

I know the ABS doesn't work because the brakes lock up at will (read tire flat spots )!

boxsey911

I had the same symptoms as you and agree that it's pretty easy to know that you're locking up and there is no ABS kicking in. I first cleaned the ABS sensors on each wheel - took them out, cleaned off all the iron filings stuck to them and vacuumed out the holes in the hubs where they fit. The ABS worked much better then but I still got the occasional lock up. I later found a tip on here about undoing the positive lead on the battery terminal, cleaning it all up with sand paper and shining up the tail lead that goes to the ABS controller. The ABS has worked fine since then.

nidon

Hi,
Had similar on my C4, no warning lights but ABS was not working. Cleaned sensors made no difference.
Car was due a service so it went to my indie where the fluid was changed during service , also front pads where changed.
Problem went away.
He commented may have been pads as had EBC reds on front and standards on rear. Standard all round at present

Tuner1

Thanks for the tips. The braking system is currently 100% factory stock but I will check all the connections, grounds and wheel speed sensors.

993MAN

Did you get to the bottom of this? I am also having the same issues. I even pulled the pump out and had it rebuilt, thinking (hoping) it would solve it, but it never.
I have cleaned all the sensors and poked and prodded everything, bled the car 5 times, 4 with a power bleeder and once the old fashioned way, ran a litre of new fluid through the system, read threads, Streathers book, you name it.
I would love to get to the bottom of this before driving for 4 hours to the dealer.

alexjc4

If u have the hydraulic assisted brakes, there are three big ticket failures. 1 the pdas controller which can get corroded contacts, 2 hydraulic pump, 3 abs modulator which produces the abs pulses (some call this the abs pump). Any of the electrics can suffer if you have damp in the front pan.

There is a specific bosch abs diagnostic tool thats mentioned in adrians book that makes life a lot easier.

In my case my local indy borrowed the tool after a month of trial and error and found the modulator was at fault, her also found two other c4 in his shop with similar issues; no warning light but non functioning abs. Basically i think the brakes are so good that abs comes on rarely and the tiny valves in the modulator get gummed up.

alexjc4

The high pressure hydraulic pump is for servo assistance and 4wd only and tends to cause warning lights when it fails. So if u have checked sensors and bled fluid, and have proper servo assist, just no abs then you need the bosch tool, or swap out pdas control or modulator for known good units (not forgetting that unless you have seen them working on the donor car either may look ok but be non functioning). This was the mistake my indy made, he had already swapped the modulator but the "tested" s/h replacement was also bad.

993MAN

Thanks for that info Alex. At least it gives me something else to try. I will have a look at Streathers book and see where the PDAS controller is now!
It would be awesome if all this came down to a corroded terminal!
I will let you know of it works.
Cheers

Tuner1

Great info guys. It would be nice if it was something as simple as cleaning a few bits

Turbohead

I have these parts if you need them. From 3.6 T
Elliot