Hi there, my 1995 940 has an erratic rough idle.It seemed to be random ,but i noticed when i hit the power window up button, with the window already up the rpm would dip.It does this with all window buttons, it is also noticable if you turn on the heater fan really quick,could this be a faulty alternator. thanks

 

Just put a meter on the battery and see what voltage you are getting. Check it at idle and when you run the window up. It should read around 13-14 volts.

 

Thanks for your reply jagtoes , I have a battery charger that has a alternator tester.I hooked it up, first it shows 100% battery -13-14 volts- 90% alternator. When i hit the window button the volts drop to approx 12%,and the alternator drops 90%-60%-30%-0% rapidly,then jumps back to 90%,This seems like my alternator is bad but before i change it does anybody know of any sensor or fault or ? that may cause this .Oh and by the way this is with the car running ,and any window switch or the fan turned to high will do this, thanks

 

I've never been fond of combo tester/chargers like that. I'd spend a few bucks on a digital multi-meter; they're a necessity w/ old Volvos anyway. Then you can check your alternator output independently from the battery charge. Common failure issues? The alternator excitor wire breaking, folks driving around without a dash volt gauge, poor case ground. If it is indeed the alternator not outputting, you don't have long to drive once the engine starts breaking up.