My wife 97 850 sportswagon had the abs/tracs light popping on and off,
after reading many posts on this topic , I assumed the abs was off when light is on and it was safe to drive.

I did a test on wet parking lot , abs seems fine, it kicked on like it should.
Last night under normal heavy braking on dry pavement, I felt the abs popping on, there was no way the tires where about to slip, the abs should not
have kicked on. My wife describe the same thing a couple of weeks ago,
I thought she was nuts.

I hope this indicates a wheel sensor and not the dreaded abs modual.

needless to say I need to get the car to my local volvo expert to
see what the codes tell us.