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.

There are very good chances your controller reached the end of its life. Specially if your car has over 75k miles.
You still have to have the system properly diagnosed to make sure there is not something else causing your problems; however, the activation of the pump and solenoids by themselves is the most recognizable symptom of the failure of your unit.

This is my collection of information on this subject.

Victor's page

the car just reached 70k