So, kinda a long story, but I'm going to try to be detailed so I get decent ideas. I have a 2007 volvo xc90 3.2 awd. About 3 months ago, I was driving along and the dash randomly went crazy saying over voltage, and since I was close to my sisters house I drove about 2 blocks and parked it. The over voltage message on the dash instantly clued me in to the fact that the voltage regulator on the alternator probably died and I needed a new one. Out of desperation since I need the car running and theres no volvo dealers nearby, i went to napa and ordered a remanufactured alternator. I install it, charge the battery, and the car starts right up, though the battery light wont go off. However, I test the voltage output and all is good.

Despite the napa alternator working, I didnt trust it especially with the battery light on, so I sent in the old one to be rebuilt at a trusted shop. A week or so later, I hop in the car and the battery is dead. Rather then messing around, i throw in a brand new agm battery and the car runs fine for another week or so and then yet again wont start. Obviously the alternator isnt charging, so I go pick up and install the rebuilt alternator. Yet again I test the voltage and everything is reading good, though the battery light is still on.

Fast forward a month and a half and as I'm idling in the gym parking lot, the radio cuts out, abs light and traction control light kick on, and the car wont start after I shut it off.. until a few minutes later where it starts fine and all the lights are gone.

When i installed the alternator i checked to make sure the pulley that the alternator couples to was tight. I also had the clutch pulley on the alternator replaced. I've checked every fuse in the car as far as I know (though I could be wrong, theres a lot of them). The battery cables are tight and free of corrosion. I feel like I've covered pretty much everything, which is why I'm desperate for ideas. My last thought is maybe the alternator cooked the ecm when the voltage regulator fried, but I dont have a way of testing it. Any ideas are greatly appreciated.

 

Figure it out? I'm having something similar going on.

 

I did a voltage drop test on the positive battery cable and its losing over half a volt. After relentlessly researching, I found that the battery cables had problems with corrosion in the terminals and that theres an updated cable for them. I'm installing it this weekend and I'll let you know if it fixes the problem.

 

I'm not finished yet, but I just wanted to share that replacing the battery cables is absolute hell. Pretty much the entire interior has to come out other than the front passenger seat. To do it right, the dash would need to cone out, but I cut the insulation instead

 

It didnt solve the problem. I noticed today that the alternator output is bouncing around about half a volt a few times every second, despite a new regulator and all the connections being clean. I have no idea what's going on at this point