Likely ABS Module.
Located next to the ABS hydraulic pump.
Need Torx E5 socket to remove it:
http://www.modulemaster.com/volvoabsremoval.html

There are many advertising the fix for this (Rocha, Module Master, BBA, ebay etc and etc.) but I strongly recommend BBA for the following reasons:

- They are authorized BOSCH repair facility in the US
- The rebuilt ABS Module is warrantied for life.
- I replaced mine and a friend's ABS Module with units from BBA, so far so good.
- BBA is Inexpensive....2 options:
a. Remove your own unit, send it in for repair, they will ship it back, you install it.
Cost: $69.00 ABS Module + $20.00 shipping.

Pro: You save $30.00 (see below)
Con: you will be driving WITHOUT ABS Module, liability issue: NO ABS, No Speedometer.

b. Get a mirror and read off your existing ABS Module Part Number, Call BBA for a rebuilt Unit, they send you a rebuilt Unit, you ship them your defective Unit.
Cost: $99.00 ABS Module + $20.00 shipping.
Pro/Con: Opposite of the above.

HTH

As little as you use abs in Oklahoma, I would not worry about it too much. Victor: http://home.earthlink.net/~vicrocha/
Will fix the module for about $150. He removes the solder and replaces it with stuff that won't crack in the heat.
Remember, ABS is not required, it just keeps the wheels from locking up. The old 164 had pressure valves for the rear brakes to keep them from locking. ABS now does that function. So, if you don't get the ABS module fixed, pay attention to the wear on the rear pads.

Klaus
--
The 164 has a new home, all I am left with are a 95 854T and a 98 V70R :)

George:

Could be your ABS module is going out. A guy named Victor rebuilds them better than new at a fraction of the price and it is a do it yourself thing. I am sure others will be commenting. This converting to FWD from RWD is another world but fun. These turbos know how to move.

Fron CS TX, dick