i have a 1996 150 wth 4.9 ...alternator shows 13.7 volts output...after starting engine and running fo 10 to 15 mimutes the idiot gauge shows a significant drop in voltage and when lights are turned on the drop is more ....the battery is new and checks out ok the alternator checks out as ok i've checked ground points engine frame etc.....any ideas???/

 

The alternator is bad. Voltage should be between 13.8 volts and 14.8 volts when running even with a load. If it is less than this the alternator/regulator is not working correctly.

The alternator is internally regulated, so just replace the alternator.

The only exception to this is a very dead battery draining all the volts.

An easy check is to remove the neg. terminal of the battery while it is running. If the engine dies, the alternator is bad for sure. If the voltage across the battery terminals is ever below 13.8 volts the alternator is still bad.

I hope this helps!.....

 

DO NOT do this with a computer controlled vehicle! That worked in the days of contact points and generators, but never, ever, ever do this with an EEC III or newer controlled vehicle! The subsequent voltage spike can damage the PCM.