my uncle has a 01 automatic that i put a new lift pump on for him, on the way back to deliver his truck to him the "check gauges" light came on. i checked and the voltage was down to below 8V so i took the alternator off and it tested bad, after i put the new one on i started the truck and it hung around 10-11ish for about 3-4 min and then it dropped right down to below 8 again. had the new alternator tested and it tested good, i was also told the the voltage regulator was in the PCM (JTEC) and it might be a bad computer. does this sound right to anyone or has anyone had the same problem? any info would be helpful. THANKS!

 

As a starting point, I would disconnect both batteries, check voltage while disconnected, and charge them before spending any more money. After charging, check the voltage before reconnecting them. If either is under 12.6 volts, it's likely bad, and troubleshooting charging problems with a bad battery is tough.

 

Yes, check both batteries separatelly.
A quick test of elecrtonics for charging is with engine running, the alt pulley should be a magnet. The reason. IF PCM is working regulator circuit the rotor will be a magnet and the alt pulley is also a magnet. Carefully hold a screwdriver near front of pulley and should pull to pulley.
A FSM would help clear the issues of testing.

 

yes, pcm controls voltage regulation, many have had your prob. u can bypass the pcm control and add an external voltage regulator.use the search function on this board and subject u will find what u need.