BustedKnuckles is correct that module may be faulty, but I would expect to find it inside the cabin on a '91, around the glove box or behind the passenger side kick panel. Also, if the module has failed, it is unlikely that you have lost spark. The much more common failure is that the transistor in the module that supplies ground to the fuel pump relay has failed. Rebuilt modules are available on line, and generally all they did to rebuild it was to replace that transistor. You have to be careful to get the correct module for your engine, transmission, and whether your car is a California or 49 state car.
The most likely failure, however, is a bit less expensive. The fuel pump relays in these old Volvos fail so routinely that some owners carry a spare fuel pump relay in the glove box. I replaced the fuel pump relays in both my '90 and '91 Volvos. I replaced one of the engine control modules.
Finally, there is the crank position sensor. On the back of the engine, stuck in the top of the bell housing, is a sensor. The insulated lead from that sensor runs up the narrow gap between the back of the engine and the firewall. The insulation on that lead gets old and flakes off after a few thousand heat cycles. When the shielding is gone, EMF from the starter and ignition interfere with the signal in that wire and the car starts acting up. Eventually the wire shorts against the engine and the car stops running. Replacement crank position sensors have better insulation that does not fail. If yours has not been replaced yet, it is about 10 years overdue. The reason this is my third suggestion is that yours has likely already been replaced.