I’ve been really enjoying the extra capacity of our Northstar 27F AGM batteries that we run in both my 4th Gen 4Runner, and my wife’s 5th Gen 4Runner. I definitely haven’t taken full advantage of it’s capacity, but my wife runs our Costway 54 Qt portable refrigerator three times a week from 8AM to about 4PM without any issues.
As I monitored the voltage over time, I noticed that it wasn’t always getting to a full charge at night. Sometimes it was reading 12.9V and other nights it would read 12.7V. In an attempt to keep a healthier charge in our batteries, I went for a quick and cheap fix that really works.
I ordered GM part 12135037 which is a diode fuse that allows our alternator to output more voltage. Unfortunately, it won’t just plug in as the fuse is a little too big to fit all the way in. In order to swap it with the 7.5 amp alternator fuse in your fuse box, you will have to file down the knobby edges seen in the pictures. The plastic is soft and only took me an extra minute or so.
After the mod, it worked great! I’m sure there’s a scientific explanation behind why it give more voltage output, but I’m not here to tell you about it because simply put, I don’t know. I have been running without any issues for a couple of months now, but do this mod at your own risk.
Below are before and after pictures of my voltage output.
Before 
After
Good luck and let me know if you have any questions!
