Did you have the opportunity to run the OBD test on the injectors? Takes about a minute, and will tell you if they're working or not. If they're clicking, they're working, which means that the ECU is working, at least for that function, and that the RSR is working. If no clicks, you can test for voltage at the injectors.

The wires to the injectors are either GREEN/RED(+ from the RSR) and GREY (- from the ECU) in the 2.4 system, or GREEN (+ from the RSR) and GREY (- from the ECU) for the 2.2 system. With the KEY in KP II, test for voltage on the GREEN/RED or GREEN side. With a good battery, should be fairly consistent 11-12.5 volts. If no volts, in the 2.4 system, there is likely a break in the wiring between the RSR and the injectors. In the 2.2 system, there is a ballast resistor between the RSR and injector wiring. In an earlier post, you mentioned fluctuating voltages at the injectors. If you have the 2.2 system, it might be the ballast resistor. Check the connection.

You can find wiring diagrams for all 1989 740 FI and ignition systems at

http://www.autoelectric.ru/auto/volvo/740/1989/740-89.htm

Chuck