Reddit mentions of Volvo 240 Service Manual: 1983, 1984, 1985, 1986, 1987, 1988, 1989, 1990, 1991, 1992, 1993

That odometer is fixable with a replacement gear from ipd or Ebay, some clown put copper grease in mine though so I had to replace the whole speedometer. :(

Also, I absolutely recommend the Bentley service manual for this car, it's not perfect but it's the best of the "home manuals" and great for common procedures and torque specs. You can find everything online but having the book is super handy.

The wiring diagrams aren't great (that's its weak point) so I recommend downloading the PDF(s) from here and using them for any wiring woes you may have, they're much more readable.

'92 is a great year that said, water pooling on the dash is really not good. The ECU for the fuel injection,the EZK for the ignition and the fuel pump relay are all located above the passenger side foot well. You need to address that pronto, probably a poorly installed windshield or rust under the seal allowing water ingress. Get a Bentley manual and check the fuses, fuel pump relay, see if it looks like water is getting into the ECU or EZK. Get the manual, these cars are easy to work on but even if you don't it'll give you a better understanding of your 28 year old car. Lastly have your mechanic check out the crank position sensor (CPS for short). Located on the back of the engine, subject to high heat so the wire insulation gets cracked. If it shorts out that'll cause the problem you're seeing. Oh yeah, find a Volvo shop in your area if you can't do the work. Where are you?