The early LH 2.4 units used in '89/'90 (in both the 240 & 740) are well known for fatal brain seizures. The most common is the inability to provide an electrical ground for the fuel pump relay. A secondary symptom is the loss of diagnostic capability on socket #2, which by itself doesn't seem to affect LH operation. When it's really dead you'll get a 1-4-4 diagnostic code on the #6 (ignition) socket saying there's a missing load signal (from the LH ECU). When mine suddenly died I had all those. There may be other less common failure modes.

The best solution is to get a reman LH 2.4 unit (either from Volvo or a Bosch distributor) which will have an upgraded design and give you some kind of warranty. Second choice is to get a used one from a yard, but the odds of finding a working one, especially one with the updated part number, may not be good (as you may have discovered). Third choice is to send yours in for repair. There are two companies (one US, one UK) claiming they can turn the unit around in a few days (plus shipping), but they're not all that cheap. A fourth option I've seen, but don't recommend, is to hard wire a ground for the fuel pump relay bypassing the ECU altogether. There would be serious risks in going that route, like battery drain and uncontrolled fuel spillage in a crash.