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[ritzen]

Vehicle = 1988 5.0
Symptom  = Disconnect battery while engine is running and it stops running.

Question = Is this always an indication of a faulty alternator?

The reason I ask is, I’ve had experience with other vehicles where there is a fuse that, when blown, gives the same problem.

[66GTKFB]

Quite the contrary. An alternator needs a 12V source to create a magnetic field to 'generate' electricity. The best check is with a voltmeter. With the car off, check the voltage of the battery to ground. It should read somewhere between 11.5V and 12.5V. Then do the same check with the engine running. It should read between 14V and 18V. If you read on the low side you might have a regulator problem.
Jim

[Soaring]

Jim is exactly right.  If you don't have a meter, then just take your car to Autozone or any other main chain and have them check your battery and alternator.  Simple.......

[ritzen]

I had the alternator tested. It is, according to Autozone, working fine. Tossed in a new, fully charged, battery. 

Started it up and the voltmeter showed a 16V output from the alternator. With the engine running I disconnected the positive cable from the battery and the car died.

Is there a "hidden" fuse somewhere that could be blown?

[Bossnutt]

In my experiences, when we've unhooked the battery and the car died the alternator was bad.  At 16V it sounds like it's over charging which can burn up your alternator and your battery.

[Soaring]

If your alternator is putting out 16 volts, your voltage regulator is not working properly.  I learned a lesson from an old mechanic one time.  If you ever replace the alternator, also replace the regulator at the same time.  They are cheap, so try that next. 

[Bossnutt]

If your alternator is putting out 16 volts, your voltage regulator is not working properly.  I learned a lesson from an old mechanic one time.  If you ever replace the alternator, also replace the regulator at the same time.  They are cheap, so try that next. 

THis is very true.  I didn't think about that.  The same thing happened with my escort when I was married.

[ritzen]

Okay - I tossed in a new regulator. Had the Alternator retested to make sure I did not burn it out.

I connected the battery and the car started just fine. I then removed the positive terminal, from the battery, and it died.

Any other ideas.

My question again is; is there some hidden fuse or breaker I should be looking for?

[66GTKFB]

There's an old saying - 'if it ain't broke, don't fix it'. If you car starts, runs (at least up to the 711 for a six pack), the battery charges and the lights work - then don't disconnect the positive lead.
Jim

[Soaring]

Now that you have the new regulator in there, check out the voltage from the alternator now.   And, something else.  Never disconnect the battery cables with the car running.  I doesn't happen very often, but it has happened, and that is the battery blows up in your face.  It's the same reason why you don't smoke around a battery.