Need some help on a 08 g37s coupe. I had an alternator go out recently due to a leaking power steering pump. I replaced both and at first the alternator read 14.6 volts but as I kept driving the voltage kept dropping down to 13, 12, then to 11.4 when I finally parked. I took the alternator out and took it to the parts store to get it warrantied and they denied me because it tested good at 14.6 volts 3 times. I put it back in the car and checked if there was anything loose after installing it and there wasn’t and once again the voltage kept dropping after driving. Don’t know what else it could be.

 

Perfectly normal. These cars, and modern cars in general, use a variable charging voltage system. As the battery gets fully charged the ECU reduces the actual charging voltage to both protect the battery from overcharging and to help improve fuel efficiency (so they say).

 

So car broke down alternator dropped to 10 volts before car shut off battery is currently at 11 volts car still turns on but when i put it in gear it doesn’t do anything its basically in limp mode now. Gonna get a new battery tomorrow just in case its that as well if not then I’ll buy an oem alternator after that idk what to do

 

If the alternator is not charging you should have the BATTERY and BRAKE lights on the cluster turn ON. This indicates that the car is running on battery and falls in-line with your symptoms described above ("shut off at 10V").

Are there any codes?

Where did you get the alternator? New or "rebuilt?" It has been proven that these cars are super sensitive to voltage changes. It has also been shown that these cars do not like "new" Chinese knockoff alternators. Several users have reported issues with "new" units. If it were me I'd get a used unit off eBay and go from there if it is determined the alternator is the issue which, at this point, it likely is. That, or the battery current sensor.

Important to note, your car is a 2008, therefore, the alternator is rated for 150amps. From 2008-2010 alternators were rated for 150amps whereas from 2011-on the alternators were rated for 130amps. Probably not the issue but a point of fact to keep in mind.

Also, obviously you've checked all of the connections, but it would not hurt to check the two connections at the battery current sensor- just to make sure everything is clean and tight.