ABS performs a continuity check with the dash lights on or maybe after engine start.

If ABS light on before movement, system continuity error at any connection, usually.

If ABS light on after you begin to take up drive, may indeed be:
A. one or both front wheel sensors loaded with brake ferrous particles lowering impulse threshold the sensor generates as the ABS hub tone ring turns,
B. continuity at any connection in the ABS system, and
C. signal processing between the differential speedo sensor that can correspond with a Vehicle Speed Sensor signal processing error. Yet am unsure on the different 900 series instrument clusters and how these receive, process, and direct the processed signal to engine control (injection. ignition).

May forget a cause or two.

As David Stevens mentions using the under-hood OBD-1 to check for fault codes as here in the Volvo 700-900 series FAQ.

Hope that helps.

Duffed
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Jonathan Harshman Winters III: The Mightiest, Greatest, & Most Powerful North American Comedian & Comedic Actor in Perpetuity!

Thanks to all.
I haven't had the problem other than the first time starting the car after cleaning the 3 wires to the positive terminal. I cleaned the positive terminal wires per Abe Crombie's post in the ABS Brakes FAQs. Maybe it was a residual code for the one time after the terminal cleaning. It was very cold this morning and that is when the ABS light previously came (expansion/contraction of dirty wire connection on the terminal) on so maybe cleaning the terminal wires fixed the problem.
I'll re-post if the problem comes back.
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Mine:3-940s running; 1-740, 3-940 parts; dtrs:3-940s running

The ABS sensors on the front wheels should be easy to find - they will be in a bracket and have an electric wire attached to them,

In our '90 744 TI the thick wire to the right front wheel was worn half away because it was in contact with the tire.

Besides stuff at each wheel in our 740, there is stuff under the driver's side of the dash and inside the trunk.

In my case, the light was always out upon start-up and came on about one time out of 10. trips when I hit a bump

As others mention it's most likely a dirty ABS reluctor ring at one of the front wheels. If you want to confirm that and even know which wheel then you can use the OBD-I diagnostic socket under the hood to get the ABS trouble code from the ECU.

The procedure and codes are here in the FAQ at
https://www.brickboard.com/FAQ/700-900/BrakesABS.htm#ABSDiagnosticCodeRetrieval

You will see codes like
1-2-1 Left side front wheel sensor: faulty signal at speed less than 40km/h
1-2-2 Right side front wheel sensor: faulty signal at speed less than 40km/h

If you can't understand the code, at least it tells you which wheel to look at.

I'm trying to recall if for transient conditions like dirty sensors that some of the codes clear every time the ignition is switched off. If necessary and all you get is a 1-1-1 code then codes can be read with the engine left running while the ABS indicator is still lit.

If the ABS indicator is lit as soon as you start the engine as you indicated then it may be more than a leftover code from a previous condition and there may be something else wrong, a damaged sensor, damaged wiring to the sensor or a dirty/loose connector. Read the rest of the ABS FAQ article for more information.

In any case, if you haven't done it in recent years, do as suggested and use a small wire brush (and maybe a small screwdriver) to clean the notches in the rings and as well clean around the sensor tip. This should be done at least with every brake pad change, although it's easy to forget when you're in a hurry. Run something like a thick rag dampened with solvent between the sensor tip and the ring to clear any oily debris there (avoid using air where there is brake pad dust). If you try to remove the sensor for damage inspection or replacement, be very careful as they can break when being twisted to loosen them -maybe soak with penetrating oil first, but so far I've always been able to carefully crack them loose. There is generally no cleaning or inspection required at the trans output shaft sensor, it's almost always at the front wheels.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now

The intermittent nature of your problem leads me to believe it may be dirty ABS sensors. Common problem. You could try cleaning the front wheel sensors. They tend to collect rust and grime with age. They are somewhere near the front wheels. Having never owned one, I don’t know exactly where they are on 940’s.
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Will I buy another Volvo??? We'll see....