I have a 2008 150 hd, 8200 gvr 4x4, new battery last year, new motor at 57000, last night while driving home the battery light came on and voltage dropped to 6 volts and lights all went off, figured the battery was shot or alt was junk, put on charger over night and truck runs fine but votlage at battery with truck running is 12.3 across terms, getting code p0265 low field output, what should I check before assuming the alternator is shot?

 

Pull the alternater and have it tested [many parts stores will do it free] Im sure you will find its bad then either buy a reman or hit the salvage yard.

 

It's the alt....should read way more that 12.3 at the batt with the truck running...

 

It's your altenator...should read 13-14 VDC at the Batt while truck is running...this is confirmed by the P0265 code.

 

I agree the alt output is not reaching the battery, but before assuming its the alternator, could it be the terminals from altewrnator or somewhere in the harness that was removed and reinstalled during the engine replacement procedure?

 

^^^^agree^^^^

 

I took truck hunting after putting the charger on battery, on the way i turned off the overdrive and reved the truck to around 4000 rpm and all of a sudden the voltage kicked up to 14.4, there were some bumps in the road so I dont know if the alternator kicked on because of rpm's or bumps from a bad connection. Is it normal for a bad alternator to turn on then off again when battery voltage is 12.5? I would like to know what controls the charging circuit in this truck, what sensors or parameter's tell the pcm to call for the charging circuit to charge?

 

As suggested, have the alternator and battery tested! Sitting here on the internet guessing what the problem is will not help you. I for one would not be driving the truck until its figured out, if it is the alternator it could very quickly destroy the battery if you put it off. Most auto parts stores will test your charging system for free.