Check the voltage on the terminal of the solenoid that goes to the starter switch when you engage start position on the switch. It is possible that the contact in your ignition switch has failed. You want to make sure that the ignition switch is not the cause of your starter problems before you purchase a replacement.

If the voltage is good, then the starter solenoid is likely toast. Sometimes you can do a temporary fix by tapping the solenoid / starter with a hammer and it may resume operation; however, the writing is on the wall. If the problem is just the solenoid, I think you can get rebuild parts; but, it may be just as easy to get a total rebuild.

If you are going to replace the starter, consider the later version Bosch SR437X which followed the Bosch SR37X on your car. I think Volvo started using the SR437X sometime in the 80s. The SR437X is a permanent magnet gear drive starter. Much smaller and lighter, draws less current and spins the motor faster. Its pretty much a drop in replacement for the SR37X. The only problem is that the mounting ears on the SR437X are 12 mm metric threaded and the bolts are meant to be installed from the back of the bell housing going forward. You can't do that on a 140 if the starter is in the car - not enough clearance with the firewall. I used some 10 mm bolts inserted from the front with retaining nuts on the back of the bell housing. 100 mm long bolts are marginal length wise. Not long enough for a nut, flat and lock washer so you need to use a nyloc style nut. 110 mm long would be the ideal length if you can get it.

Last year I got an SR437X from Rockauto dropped off on my doorstep for around $160 Cdn. It was a genuine Bosch reman. If you check Rockauto, don't search by your model and year - it will come up with the SR37X. Do a part number search on Bosch SR437X