ABS light is stuck on even though there’s nothing wrong with the ABS system – e.g. works fine when backing down a very steep wet driveway.
If the light is on, there IS a problem.
Backing down a driveway will unlikely generate forces to engage the ABS.
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+1
In fact, I really doubt if an ABS system that was designed 2+ decades ago even had the capability to work at the low speeds involved when driving in reverse.
Is the red brake warning light also on . . . ?!
Anyways . . . on many vehicles, something as simple as a defective brake lamp switch will cause the amber abs warning light to illuminate
Just something to think about
I will toss out a idea that it could be a speed sensor. I was surprised that they actually had them in 1990 for a Volvo. If there is a bed speed sensor, dirty connectors, or the wire going to the sensor that has broken somewhere, it can cause the ABS light to go on. Any other troubles, like with the speedometer.
Other issues to cause a ABS light on can be a weak battery or a bad fuse and relay.
Why are you surprised?
Volvo has long been known for safety
Benz had abs since the early 80s . . . maybe the late 70s, if you include european-spec cars . . . so it’s not surprising that a 1990 Volvo 740 has abs
I know Volvos are known for safety. I just found it surprising as it was what close to mid 90’s that it was standard equipment for American manufactures.
European car manufactures are different though. Cause what anything prior to mid 80’s that had ABS was usually foreign made and not on every car.
Hmm … Thx … but
backing down our very steep driveway does engage the ABS ?
i.e. they don’t lock.
Thx. No … the only
light that’s on is the yellow/orange ABS light
Ha ! … now we might
be into something. Yes. The speedometer is also flakey (as is
the mileage counter). Come to think of it, the ABS light issue
probably arose soon after the speedo issue – a while back in
each case.
My local mechanic
reckoned that the only cost effective fix was to replace the
entire instrument panel, but I assume that’s no longer an option
c/o unavailability of even used parts from wreckers.
re: battery – the
duration of the ABS light issue is longer than the duration of
battery changes. 
Someone here may know of a good Volvo owners forum. Good luck!
Well start cheap like with electrical cleaner and check the wires. Since it is a 1990 you won’t be able to pin point what wheel to start looking.
Glad it gives you some direction to start.
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ABS was not standard for US cars in the 90s. It was an option on most. There were a few models that had ABS standard in the mid 90s. Primarily the first ABS systems came from Bosch and others from ATE… both European companies. German cars very heavily used Bosch.
Kelsey Hays and Delco Moraine both made ABS systems in the US. Both businesses were sold to other companies. The old Delco no longer makes units in the US
The may be a speed sensor on the transmission, as well as at each wheel. This might be easy.
Well see I don’t know sh#t. I will keep my mouth shut.
Check the simple things before replacing expensive stuff. Volvos of that era had problems with printed circuit board failures - sometimes burned traces (A/C controllers were infamous for this), sometimes cracking at through-holes and such. Dealers would replace whole modules, like instrument panel wiring boards and A/C modules when they suspected this, but others had success with re-soldering component attachment points and adding jumper wires around cracked or blasted traces. I recall getting some sort of fault light on our early 90’s Volvos when the bulb fail sensor relay went bad (it also caused the brake lights and cruise control to not function) - it’s a large round can on the relay board under the radio, accessed by a secret screw hiding behind the cigar lighter bezel, fixed by resoldering all the joints (use an ESD protected soldering iron or surface mount soldering tool to avoid frying semiconductors).
The forum swedishbricks.net has useful information.
The Chrysler “Sure-Brake” 4-wheel anti-skid system is described in their 1972 shop manual, believe it was introduced a bit before that.