More than likely it is dying. Most wiper motors either fail due to the brush assembly or bearing failure. Volvo seems to have neither of these faults but there is always the exception. In your case it might not be either one. A lot of the motors that will still turn but not under pressure is due to a magnet that has come loose from the housing. They will also make a grinding noise. Some times just glueing them back will work. JB Weld or something epoxy based will work. Yours being a 91 model, it has a ground strap attachment near the contact housing. A lot of people fail to reattach the ground. Check it. If you look at the motor you will see a threaded hole near where the electrical connector attaches. Make sure there is a ground running from there to firewall just before the upper strut brace hole on some models. Do not know what area you are in but I just replaced my daughters wiper motor with a $40.00 junk yard motor. Works great. They usually want $50.00 for their motors but I talked him down some..Oh yea while you have the glovebox out replacng the motor, you will want to greese all the attaching rods. Good Luck...Max:>)
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Max..1989 244 DL 5 Spd., V15 Phase II Cam Bilstein HD, Turbo Swaybars, Poly Bushings all round, Turbo Wheels, Black leather interior, Electric mirrors, LED dash and gauge lights and now NEW ECODES with the turn signals, 1992 black 244 next project