Johnny G Pipe

Hi, I am stuck with an ABS fault wondering if anyone had any ideas.

From starting I get orange light on at the diff lock switch for about 10-30 seconds of driving (it varies), with ABS activated at any touch of the brakes. Then I get a the differential warning light, the brake pressure light and the gong. The lights stay on, the brakes act normally but with ABS now disabled. I sometimes get a second warning gong a few seconds later.

My local specialist has run a hammer test, which identifies front left sensor, but new sensor made no difference. On further investigation he says he's getting different readings from the 2 front sensors, and is scratching his head. We already tried a spare ABS pump unit with no improvement. The teeth in the hub that the sensor reads all seem fine etc. All the sensors and earths are clean, plenty of fresh brake fluid etc.

The car is a mainly stock C4 on d90's, engine's never been out in 120k miles, original wires, coils etc..but my guy doesnt think its an electrical noise problem. Thanks in advance..its been a couple of weeks now, and a need the car working as its my daily..

Computamedic

Johnny,

It sounds like a stuck accelerometer to me. Get both of them out, prise off the top cover with a knife and clean out the grot from beneath the pendulam so it can freely move back to the central position.

The two are screwed to a metal plate attached to the centre console just in front of the gearstick. Access is through a small carpetted panel on the right side of the tunnel. It's not easy to get them out (or put them back).

Regards

Dave

Johnny G Pipe

Thanks Dave..how come youre thinking accelerometer for these particular symptoms?

Computamedic

Johnny,

It's the bit about the ABS activating as soon as you touch the brakes. The longditudinal accelerometer (the one that detects fore/aft movement) detects braking by the pendulum moving forward. If it's jammed in the forward position the ABS system thinks you are constantly decelerating - and, hence, will activate the ABS.

The lateral accelerometer in not significant to ABS but if' you're going to clean one you might as well clean both.

It was also one of the things you hadn't mentioned so I'm reckoning that it's worth a punt!!

Regards

Dave

Indycam

"with ABS activated at any touch of the brakes."
The computer looks at the accelerometer and "sees" that the car is braking hard and that the wheel speed is low , it switches on the abs to keep the wheel from locking up ?

Indycam

When I wrote my comment I had not read yours .
We were just thinking along the same lines .