I bought a 2005 Volvo S40 T5 AWD on Saturday. The battery was bad, so we replaced it. The alternator isn't charging the battery. 12 volts without load, 11 under load. ABS light is on, a whole mess of lights come on whenever I do something like turn the radio on or anything like that. Now I thought obviously, alternator is bad, need to replace it. I called a place that rebuilds alternators and he said on some cars you need to reset the computer with a scan tool because the car is still reading the old battery. Is there any truth to this? I really don't feel like spending $1000 replacing the alternator. Thanks.

 

there's two common ways the alternator can fail - the voltage regulator can have a component (like a diode) fail or the brushes/bearing on the generator can fail. On some years/models the regulator is available separately for about $50. So you have a few choices here 1) get yours rebuilt or have the VR replaced 2) buy an aftermarket rebuilt unit (I'd estimate those to run $250) 3) buy genuine volvo @ ~$500 - try Tasca Parts etc 4) buy a used part (try car-part.com to search). It appears there may be a parts number break for the 2005s, so you should research part numbers for your VIN's chassis number or copy the part number off the existing unit.

As to the codes, low system voltage can set some false positive DTCs that need to be cleared to turn off the check engine light (disconnecting the battery won't clear codes). Many big box parts stores like Autozone will loan an OBD2 scan tool to view/clear the codes - assuming you do the install. Otherwise a pro would simply do that as part of the job. Since you need to remove the accessory belt, its a good time to replace the belt and inspect the idlers for any play.

 

This model is infamous for not accepting any old alternator you put on it. This summer mine went out for the 2nd time. This particular alternator was a reman unit from one of the big chain parts stores. Since it had a limited lifetime warranty I went back and got another. This "new" one only lasted a week before the car refused to charge. In desperation I bought one off Amazon that was listed as new. It turned out to be a reman unit but it was done very well. All the plastic parts were new (not just cleaned) and there wasn't a speck of dirt. It has been working for months now without issue.

Now I still had my original alternator so I took it to the local rebuild shop and had them put in good quality parts. It was still cheaper than a reman unit. Now I have a spare should my car decide it doesn't like the installed unit again.

You shouldn't have to do anything but install it and start the car. If the alternator is outputting the desired voltage then all will be well.