From the pdf found here:
On the Car Diagnosis. Your Volvo's engine uses port fuel injection firing all injectors simultaneously with some cross-over occurring within the plenum. The injection pulses can vary considerably, and Volvo FI set-ups fire the injector twice per revolution (keeps the valve wet between intake cycles) to prevent hardened deposits from forming on the valves. This often results in enough fuel being present for near-normal engine operation on three injectors. All four injectors fire at the same time on +12V current supplied from the radio suppression relay (later cars) or the car's power supply. The injectors fire when the FI ECU grounds the circuit. Basic diagnostic tests for injectors include:
On the Car Diagnosis. Your Volvo's engine uses port fuel injection firing all injectors simultaneously with some cross-over occurring within the plenum. The injection pulses can vary considerably, and Volvo FI set-ups fire the injector twice per revolution (keeps the valve wet between intake cycles) to prevent hardened deposits from forming on the valves. This often results in enough fuel being present for near-normal engine operation on three injectors. All four injectors fire at the same time on +12V current supplied from the radio suppression relay (later cars) or the car's power supply. The injectors fire when the FI ECU grounds the circuit. Basic diagnostic tests for injectors include: