The later '93-on 940 gear shift knobs with the OD button part way down on the back are a known problem area. Years of heat and UV sun cause the black rubber compound to degrade and go soft, eventually a bit gummy. Pieces eventually start breaking off, normally starting with an unsupported chunk above the park/shift release tab.

I've now had two of these go. On the first, I tried various flexible glues to put the pieces back in. I first attempted black RTV so it would match and fill gaps. That lasted just over a year. My most recent attempt was the supposedly industrial strength E6000 flexible adhesive I had on hand. It didn't last a week in the hot sun. That shift knob is now too far gone to warrant further attempts. I finally broke down and ordered a new one, p/n 6843471 with a current Volvo list price of $141.56 USD before dealer/re-seller discounts. You can find it online for less from a few discount sellers, but by the time cross-border shipping was factored in I couldn't beat one of the main Volvo dealers in Toronto offering an online 15% dealer discount and free shipping, netting to an equivalent $114 USD. My regional dealer would have been over $150 USD equivalent after shipping. It's special order through my local dealer, no savings. Next stop will be carefully prying out and disconnecting the OD button and getting the old one off and the new knob on with the least amount of effort, grunting and groaning -cutting it off being a viable option once the OD button is safely out. Hopefully there will be enough slack in the wire so I don't have to remove the shifter assembly and take it apart as I've done a few times, last time to remove the troublesome shift lock solenoid they added in '94/'95.

Now as for my second one that just had the first chunk fall off, I'm seeking advice. I want to make another attempt to glue it after the E6000 not lasting. I'm sure we've all got lots of ideas of what might work best. What I'm really hoping for is someone who's actually succeeded in getting it to last for more than a year. I'm at the stage of thinking that once the rubber compound starts to degrade, it's too late for almost anything and is destined to fail again in the sun and heat.
--
Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now