1990 Volvo 240 broken odometer

I'm new to the Forum. Just replied to your question but lost it somehow so I'll try again. Go to hyperocity.com. They have a detailed step-by-step pictorial as to how to replace a small plastic gear within the odometer. That was the trouble on mine which I just repaired. Took about one hour. Cost is $29 including priority shipping. I assume yours has the same problem mine had. I just responded to a similar question re a 1986 model so you can check that response on the Forum, too, although it is the same info I am passing along to you. Good luck.