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This info pertains to a 1992 Volvo 740. It may be applicable to other models/years; I'm not sure. If your ABS light is on, and the DLC procedure for obtaining the codes from the ABS ECU produces no codes (no blinks after holding the button for >1sec with the DLC probe attached to #3 in the "A" unit, yet the DLC LED lights when the button is pushed), the over-voltage protection relay may be defective. This relay is located high under the kick-panel (you'll need to remove this), behind the instrument panel, to the left of the steering wheel. It is on the other side of metal bracket that holds the ABS computer (the large metal box with many wires going into it through a sturdy connector, very close to the left fender). It looks like a regular relay, except that there is a 10amp fuse pushed into the top of it. If you follow the wires coming from it, you'll see they lead to the main ABS computer. The relay can be removed from the bracket by pushing up and twisting the bottom towards the footpedals. Now first remove the fuse and check that it is OK. If it is, put the fuse back in the relay and obtain a voltmeter. Leave the wiring connector on the relay and switch the ignition on, start the engine and leave it running. The pins are labled 1, 2, 3, 4 on the connector (as well as inside the relay, but the numbers are hard to see). Pin 1 (red wire) should show +12V. Pin 3 (black wire) should show continuity to ground. Pin 2 (yellow/red wire)is the switched circuit, and should show +12V with the engine running, and 0V with the car switched completely off. If you are not observing this behavior from Pin2, the relay is bad, and needs to be replaced. The relay is essentially impossible to dissasemble without destroying it, so a replacement unit will be required. My dealership charges $85 for the part, and it is listed in their catalog as an "ABS over-voltage protection relay."
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