Hi There,

You have two belts because of the water pump located directly above your alternator. You want two belts unless you just love living on the edge. Back up so you do not loose a belt, loose water circulation, and warp an Al Head from overheating.

Most Volvos have a Bosch Alt by the way.

Easy stuff first:

The two belts are supposed to be matched. Getting two belts from the same batch run does the trick most all the time. If by some chance these are not matches and one belt is a longer than the other that will cause squeaking belts.

See if you can tighten up the belts Take a couple of 13mm wrenches or a socket and wrench and loosen (Do NOT REMOVE) the long bolt that holds the Alt to the Engine frame. Then loosen the 13mm nut (Just get it loose) and bolt that holds the rectangular block to the Curved arm. Take a 10mm socket and crank on the 10mm adjustment nut CW and start feeling the belt tension. You should be able to get both to snug up buy turning that nut. If you can get this to snug up the belts, take you time and retighten the 13mm nuts and bolts. Start it up and see if the incessant singing is gone or not. a lot of folks from the old school of pry over and tighten the bolt school of belt tightening have trouble with just tuning the 10mm nut to snug up a belt. I know because I used to be one of them. Just loosen the 13's and crank the 10mm is the trick to volvo belts.

Check and make sure that this is not the water pump bearing singing to you. It can sound just like the alternator and sing just as loud or louder. That you want to fix right away.

If belts are matched and still one belt is tight and the other is looser, you problem is most likely a well know one on the 240 and 740, worn accessory bushings.. There are three bushing on the alternator. The one that catches the worst of it is located under the adjustment arm . There is a 13mm bolt that holds this on the block, you will have to take the 13mm nut and bolt off the adjustment bolt block to remove it.. The other two are what the long bolt goes through on the bottom side of the alternator. They get oil soaked and loose shape over time. That can allow the Alt to lean inwards and one of the belts most ofter the back one can sing to you.

Hope this helps,

Paul