Arby,

I am currently having the same problem with my ABS control module on my 1998 S70 T5 (By coincidence I also own a 1990 740 turbo) My module had been blinking intermittently for almost a year. Now Volvo tells me that the module has failed, although the light still goes on and off. They also quoted me a price of $600.
Before you install the part you should know that VCS(volvo customer service )told me that unless volvo installed the part there was NO warranty on the part. and my volvo dealer would not even install the part if I suppled it, they insisted on supplying the part themselves.
My contention is that this module is defective and dangerous, because a person could be on ice, hit the brake expecting the ABS system to kick in, and go into a dangerous skid, because the techniques for braking on ice with and without the ABS are radically different. I have spoken to other Volvo owners with the same defective module, going on and off . When the light is on, your ABS system is NOT working. I contacted Volvo(with no effect of course) and wrote online to the National traffic safety organization, which I suggest you and any other owners with this problem do as well.