!990 745 A/T 185k mi. It might be orig alt. Good battery, checked electrolyte , had it on an automatic charger. Belt is proper. Alt ground to engine block is good. I've owned it for 10k mi. Within a few days it went from reliable and smooth start and run, to loss of power, hesitation, would only stay running in P & N while goosing the gas pedal to find a fast idle (2500 - 3000 rpm) where it wouldn't quit. Shift into any gear and the load is too much for the engine to move the car, it bogs down or stalls. No indicator/warning lts on across instrument panel in KP-II. I installed a regulator w good long brushes after removing Reg w very short brushes. In KP-II only the red lt for Bat on row of dash lights was on. I took the instr panel out and cleaned all flux off the elec paths and connections w alcohol and Q-tips. Didn't see any damaged solder connections. I reinstalled and now in KP-II only the red lt for Bat on row of dash lights is on. Maybe it's brighter than before.
Looking at FCPEuro for parts it seems both cars can use the same Bosch alt #5002899 Lots of redundant & sometimes contradictory cross referencing of "fitment" of Regs w Alts.
My '88 245 is off the road, but runs and the alt charges. I could swap its alt w reg into the '90 745. Advice, info, your experience would help me. I have 2 people interested to buy the 745, if it runs.

I have a 55-Amp Bosch from a 1981 240 and a 80-Amp from an unknown year 700 on my work bench. The front frame mounting lugs are identical and look to be spaced the same distance from the front pulley. Both units have 2-sheave pulleys although I thought the 700's only used a single drive belt. The B+ and D+ connectors on the rear frame of the units are very close to being in the same place.

Based on this, I would say that the units are externally identical and can be swapped. I expect this applies to your 1988 240 alternator. The output rating of the 240 might be a bit lower, but not enough to affect normal operation.
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Bob: Son's XC70, my 83 244DL, 89 745 (V8), 98 S90, and XC60. Also '77 MGB and some old motorcycles