I have a 1999 Ford F-250 with a 7.3 diesel. My batteries will not hold a charge. I just replaced my alternator, battery cables and cylenoid (sp.

Most chances one of your two batteries is bad. If one is bad it will kill the other one. Did you disconnect both of them and test them independently of each other?

No i didn't test them independently. If that is the problem that would cause the alternator not charge the batteries? When I replaced the alternator I charged the batteries last night and started my truck and let it run for about 10 minutes I turned the headlights on and the blower motor the longer the both were on the amps would just keep dropping? I appreciate any other help.

I tested the batteries and you were right one was bad and the other wasn't in the greatest shape as well. So I replaced them both. When the motor is running and there is no load on it, the alternator is charging only 12.2 volts, and when there is a load on it the volts go down to 11.6-11.7 which it would always charge at 14.2-14.4. Any thoughts are appreciated..Thanks a bunch..

Sounds like the alternator exciter is not working. To check while starting watch the panel for the battery light. If no battery light comes on it is confirmed. You can run a wire from the alternator plug to the starter solenoid that is hot only while cranking. The new circuit will polarize the alternator and it should work, charge should rise to 14+ volts. Larry