1996 cougar charging problems

I have a 1996 4.6l cougar that is showing a bad alternator when checked on the vehicle. when I take it off and have it checked, they tell me the alternator test good. I have changed the battery, alternator connecter and voltage regulator. When I checked it on the car they tell me the tester says start the vehicle , but its already on. any help on where to look?? thanks

Probably the only sure way to check an alternator is to read the battery voltage with the engine running and the accessories on.
To do this:

get a digital voltmeter that will read to 2 decimal places (i.e 12.34 volts)

put the volmeter directly on the + and - posts of the battery.

if the voltmeter reads below 12.5 volts, you either need to charge the battery or you have a bad battery. If you charge the battery and it is dead the next day (with no use of the battery) you have a bad battery.

if the battery is good:

Start the engine, turn on the headlights (high beam), air conditioner (if you have one), turn the heater fan on high and run the windshield wipers at high speed.

run the engine at 2000 RPM by pressing the gas pedal slightly

Your voltmeter on the battery should read between 14.2 and 14.5 volts.

Below 14.00 volts is a bad alternator or a loose alternator belt.

Above 14.6 volts is a bad regulator in the alternator.

14.00-14.2 volts is a questionable alternator
14.21 volts is a weak alternator
14.50 volts is a good strong alternator
readings above 14.6 volts can burn out headlamps and other electrical stuff (bad alternator)

Bob

Also,

You should get the same voltage at idle as you got at 2000 RPM, but to get that voltage at idle, turn off the accessories but leave the headlights on.
If the headlights dim at idle, you have a loose belt or a weak alternator