The regulator isn't regulating properly.
Easiest test is to use a voltmeter. Engine stopped , voltage about 12 volts Engine running, voltage about 14.7 volts.
Maybe because your alternator is not working. While the engine is running, test the battery with a volt meter, anything less than 13 volts is a problem Should have min of 14 volts. If its at 12 volts, then replace the alternator.
Most likely the alternator, voltage should be approximately 14 volts with the engine running.
The alternator is on the top left corner of the engine as you are looking at it from the front.
The alternator is belt driven off the front of the engine.
1st there must be some reason as to why you are doing this. I don't feel it is ever a good idea to remove the battery cable while the engine is running. But if you must know, your engine can only keep running while it has 12 volts going to certain areas... such as coil and distributor. If it dies when you remove the battery then I would guess that the alternator is not working, which may be why you were testing this in the 1st place. A good test for your alternator is to put a volt meter across the battery and see the volts, then start the engine and see if the volts increase. If they dont and the volts are dropping then the alternator needs to be checked out.
The alternator is on the passenger side and between the engine and the compartment wall.
Could have a blown fusible link in the charging system. Start with attaching a volt meter to the battery. If the battery is fully charged it should read 12.68 volts (engine off). With engine running, 14.2 volts.
The voltage regulator is on the alternator, you'll have to take the alternator off to get at it.
Check the voltage in the battery with the engine off. Now, start the engine and watch the volt meter. The voltage should start climbing, if it does not, then the alternator is not working. Should read between 13.5 and 15.5 volts with the engine running at idle.
Battery keeps running down when you are driving. Your battery light should be on if the alternator is bad. Check the voltage at the battery with a digital DC volt meter set to the 20 volt scale. You should read from 13.5 to 15.5 volts with the engine running at idle. If not the alternator is suspect.
The1999 Ford F1 50 pickup truck 5.4 liter engine alternator field fuse can be found in the fuse box. The alternator field fuse will be in the second column, second from the top.