I thought I had a faulty ABS module which was causing some warning lights to come on, but now I'm not so sure. Symptoms are:

Brake/ABS/STC/Service lights randomly come on and stay on for anywhere between 1 minute and several hours.

When the brake light is on, the trip meter goes to "---.--" and the mileage counter stops counting! They both resume from where they left off after the brake warning light goes off.

A few times I've been gently braking (like when coming up to traffic lights) when the brake warning light came on... As soon as it does, the ABS kicks in for a few seconds! (you hear and feel it on the pedal)

And I've noticed that sometimes driving over a bump in the road, or accelerating HARD, will make the brake warning light go out if its on....

So, any ideas? The fact that the ABS kicks in when the light goes on suggests to me that maybe theres a dodgy sensor somewhere or other....? How hard is it to get access to the ABS sensors to see if they're dirty?

My 99 XC70 w/85k just started the same problem...volvo diagnosed as a bad abs sensor. Rebuilding sounds like a great option - another thought is that i've found lots of used units on car-parts.com

one caveat is that you better get the exact part number off the bottom of your unit - you'll need a flashlight and a telescoping auto mirror. When you call around to the various salvage yards, make sure the #'s match.

prices from $150 - $250

dave

vovlo will cahrge 8-900 to replace. Mine went at 102000 miles. Bitch a lot and they will drop off a few hundred.

I've had S80's with this problem in my shop. It was not easy to find. Whenever you see brake light in addition to abs/stc lights and trip meter "---" it is not abs module. On CEM central electronic module driver side behind dash you have X-supply relay and 15- supply overload relay. Replace them both, this will fix it . e-mail me for exact position of the relays. Good luck.

PS. We repair all 96-up abs modules, all types in stock ready for shipping.
Chris, Crosscountry Automotive 905-712-3700

 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

I'm having the same problem and searching for an answer.