A 1990 740 GLE will have the B234F DOHC engine. The alternator mounting is high up and uses a different mounting bracket from the standard B230F and B230FT engines. It also has a belt guard on it -not something to be ignored when working on a running engine. My documentation indicates there were both 80 and 100 amp Bosch N1 type alternators available for the B234F, however I'm only familiar with the 80 amp version and I only see 80 amp units in the aftermarket. Unless you know the 100 amp alt was out of B234F it's not likely to fit. If it does happen to fit then just drop it in -no problem.

If you want to choke, just ask Volvo the price for a replacement (it will be a factory reconditioned unit). If yours is in really sad shape then a good used one would certainly be an improvement, especially if it's been properly checked for output, bearing play, slip ring wear and brush length. Normally, rebuilt alternators are the best way to go, but off the shelf or good used ones for the B234F can be difficult to find and/or expensive.

I presume you've done appropriate testing and that it's not just a worn regulator/brush pack -those are easy to replace. Many times I've found the best solution is to take the alternator to a starter/alternator shop. They usually can do an entire overhaul for a relatively reasonable price. If the shop is familiar with Bosch alternators and has a good parts supply chain then it should only take a few days. If your budget is tight you can have just the slip rings, diodes or bearings replaced as needed. The diodes tend to last unless exposed to a lot of heat. The slip rings will wear with age. The bearings also wear with age and unfortunately the B234F alternators with their long drive belts are very susceptible to this.

As mentioned, the B234F alternator is up high on a long belt. There is a great tendency to overtighten this belt, especially using the crude 1/4" deflection rule. For the sake of extended alternator bearing life (and your water pump bearing) do not overtighten this belt -just tighten it enough to keep it from slipping when turned by hand and from squealing during cold, damp weather. For a new belt, readjustment will be needed after a break-in period. Also make sure the rubber accessory mounting bushings aren't worn (compressed) which will put the belt out of alignment and encourage squealing. These do need occasional replacement and are relatively cheap. With the belt mildly tightened, check the alignment of the alternator pulley face with the pulley faces on the crank and water pump.
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Dave -own 940's, prev 740/240/140/120's & quasi-expert only on a good day