I would try disconnecting the speed sensor first.This is in the rear axle,but the connector can be found in the back of the boot area.However it is more likely to be a speedo fault,not uncommon on these.I am sure they are repairable,or available s/hand.Iirc,the problematic speedo was made by Yazaki instruments.

Hi there my h reg 1991 Volvo 940 GLE speedometer has failed did you get yours fixed and if so how? Thanks. Phil.