The most likely place would be the ballast resistors, in a square pack
mounted to the body in front of the left fender well, right beside the
battery (at least that's where they are in my '85). You can unfasten the
connector and remove the resistor pack if necessary for testing or
replacement - it may be just the connector anyway. There is one resistor per
injector, in series with the 12 volt supply. Wrecking yards would be a good
source if you have to replace it since they aren't a common failure.

Mike

You have probably got one of the four ballast resistors gone duff. You may
have to replace the whole four resistor unit. Follow the wiring back from
the injectors
to the resistor unit. That is what needs replacing. You should be able to
find one in a scrap/wreckers yard.

All the best, Peter.

700/900/90 Register Keeper,
Volvo Owners Club (UK).