Before you start twisting knobs and turning screws I would run some diagnostics on the charging system first.
Do you have a volt meter? If the answer is yes then you can test the alt. to see if it is charging. If the answer is no go to the hardware store or your local Wal Mart and buy a $10 volt meter. Make sure it is AC and DC capable.
The Alt. has two basic outputs. Volts and Amps. Amps are needed to cover the total load used by the truck. Lights, heater blower motor etc. For the cursory testing it is less important to know what the amps output is. For the purposes of this test we need to know what the voltage output is.
If the batteries are bad, sulfated or have seperated plates you will need to disconnect them to do the testing. If they charge up fine leave them connected but make sure they are well charged before testing. DO NOT do the testing with a charger hooked up. It will alter the test results.
The truck has a 24 volt system. This means the batteries combine to make 24 volts. In order for us to charge the 24 volts worth of batteries we have to have a voltage output from the Alt. of about 28 volts. Set your volt meter to a scale where it reads above 24 volts DC. If the voltage output is around the 24 volt mark or below the alt. may be making power enough to cover the load from the truck while running but not enough to charge the batteries. If it is putting out 28 volts then we need to do some load testing.
If you want to load test the alt. to see if it is making amps. turn on all the electrical accessories you can. Heater blower motor on high, Headlights on bright etc. Set the meter to the DC amps side. You should have an alt. that puts out 30-60 amps so set the scale accordingly. Then with all the accessories on see if the alternator is making enough amps to cover the accessories. Then turn them all off and see if the amps output lowers.
Outside of that you will need a load tester tool and do some diagnostics on the batteries and cables.
If the alt. does not act as described it will need rebuilt or replaced.
