ABS light comes on

[orggti]

Anybody got any ideas? It is ok for the first 100 meters and then comes on. Im guessing its a wheel sensor and after 100 meters it works out the numbers dont add up and puts the light on. Anybody else had to clean their sensors before?

[Pushbutton_auto]

Yeah don't the computer check all the sensors and connections and if anything is off it lights up.

from audiworld

.I THINK that the ABS computer, when it first receives power, runs a simple test to confirm:

1. No AC signal from any of the 4 sensors

2. DC resistance of each of the 4 sensors is approx 1200 Ohms.

If these two conditions are discovered at start up, the ABS computer waits for AC signals from the 4 sensors. If not, it complains by disabling ABS.

Once the car is rolling above some minimum speed (8 MPH? 20? I don't know.) the ABS requires two things of all four sensors:

1. DC resistance approx 1200 Ohms

2. AC voltage from all four sensors is approx the same.

If either one of these requirements isn't met, the ABS computer disables ABS.

Finding ABS sensor electrical problems:
I've found that I can save time by temporarily removing the rear seat cushion and removing the large harness connector from the ABS computer (left side of car, beneath rear cushion). You'll need a small (nbr 0) Phillips head screwdriver to remove the connector's hold-down clamp. I use my DMM's sharp test probes to push into the female contact cavities of the harness connector for the ABS computer.
Then I use the DMM in Ohms position to measure the DC resistance of each of the four sensors with the car stationary. Consult the Bentley shop manual's schematic diagrams for connector pin outs. I discovered a couple incorrect pins on the schematic for the 1988 5000CS TQ, which I confirmed by checking other schematics' ABS computer pin outs for other model years.
In this case, all 4 sensors measured approx 1100 to 1200 Ohms with the car stationary. If a sensor measures significantly higher resistance, either the sensor or the wires to it is broken. If the resistance is significantly lower, there's a short in this sensor's circuit.

If all seems okay, set the DMM to measure AC volts and test drive the car at say 40 MPH, noting the AC voltage that's produced by each of the 4 sensors. (I rested the DMM face up on the front centre arm rest.) Again, refer to the shop manual's schematics for connector pin out. In my case, three sensors produced between 1.5 and 2.5 Volts AC. One (left rear) produced almost nothing. I measured its DC resistance while test driving, and saw its resistance generally hovering around infinity, with twitches to some lower value. Ah-hah!
I next raised the left rear corner off the ground, removed the wheel, slid under the car, and began tugging on the sensor wiring, while I had the DMM set in DC ohmmeter with a buzz sound when it encountered a low resistance. I located the intermittent break within six inches of the sensor.

[Duncan]

Mine was a broken wire in the run from the wheel sensor to the first connector up by the firewall. DMM showed this up. The sensors themselves are fairly robust, and need to be positioned extremely close to the toothed wheel to work correctly.
I used an oscilloscope to check the teeth were all okay, and to position the sensor in the hub (matched signal amplitude to the sensor on the other side.

Duncan

[wilco]

I wonder if my problem in the Coupe quattro is related??  The ABS kicks in at very low car speed, like drifting to a stop in my carport.  Still works fine at higher speeds but odd when crawling to a halt on dry cobbles.
PB_a, Duncan any ideas?

Wilco

[orggti]

Thanks guys

[Duncan]

Wilco, sounds like yours is a different symptom. Maybe check the condition of the toothed wheel  inside the hub..?
At lower speeds the signal amplitude (strength) from the sensors will be lower, resulting in more effect from dodgy wiring or dirty / rusty teeth.

Duncan

[wilco]

Good points thanks, guess I'll have to get under it at some stage.

Cheers.