I have a 96 850R wagon that I bought new. It currently has about 91K miles, but I have the ABS control module replaced 3 times (once when it was only a few months old and a 2nd one about 3 years ago and now is the 3rd one). Does anyone out there having this same problem occuring so many times? I don't understand why the module breaks down so often. Thanks.

All the ABS control units (Installed on 96 and newer 850, 70 series and S80 models) will eventually fail after they reach 65,000 miles or 5 years. The average is 85,000 miles and/or 6 years for failures.
The erratic ABS light and/or no trouble codes is an indication of a defective ABS unit.
A dealer will charge from $60.0 to $120.00 to diagnose the problem and about 95% of the cases the mechanic will recommend to replace the ABS control unit.

Here's more information on this issue and a replacement alternative.
Vitor's Volvo ABS website

Thanks, Vitor and bl.

Both the TRACS and ABS light come on intermittently. The dealer said that the sensor on one of the wheels has been sending the wrong signal to the ABS control (this is where I don't get it. I thought if it's the sensor that has been sending the wrong signal, shouldn't that part be replaced instead of the control module?).

The number 1 symptom of a defective ABS unit is a lack of wheel signal or an intermittent signal from the right rear wheel.
Lots of rear wheel speed sensors have been replaced because of this. Replacing the sensors (the rear sensors are replaced together because of the way they are wired). This does not solve the problem. On the contrary, I have seen two cases where more problems were created by replacing the wheel sensors.

Will it be necessary to disconnect the battery before removing the module to avoid short circuiting any electrical system? Thanks.

No, just the ABS fuse.
Just follow the instructions on my website.
http://home.earthlink.net/~vicrocha/

ABS control unit failure on the 850 is virtually 100%. Contact Victor Rocha who will rebuild and upgrade your ABS unit for ~$140. vs the $800+ you'll pay at a dealer who will simply replace it with another defective unit that will also fail. Victor is top of the line and highly respected on this forum. DOn't have his website handy but is easy to find here.

bl