Runs fine w/o battery?

[Rocketman]

The field coils generate the magnetic field in the alternator, which is used to generate the alternator's output from the armature. When starting, the battery's current is used to energize the field coils.

Once the car is started & running, if you disconnect the battery, the load from the electronics in the car helps sustain a current thru the field coils. This keeps the magnetic field going & the car will continue to run.

I'm told this is a way to check for a good alternator, but also a really good way to burn an alt out.

-Rocketman

[JPT]

...as for it running when the negative battery cable is taken off that's how i was taught to test a alternator

Another old school way to check for a dead alternator is to take a screw driver and touch the rear bearing cap on the alt. If you get a magnetic pull, the alt is good. No pull? Dead alt.

[blkfordsedan]

Ahh..........the good old days. I remember when converting a generator to an alternator was "trick".

[FestYboy]

without the battery to act as a buffer, any and all AC voltage will be pushed into the rest of the cars electronics (this is bad). also, without a battery, the alt has no referance voltage as feedback and subsequently goes full field and can reach upwards of 16VDC (also bad). never test an alt by removing the battery from the curcuit. if you want to test an alt, grab a good battery, and check the voltage output when running at idle (between 13.6 and 14.4 is good) and at 2500 rpm. then turn on EVERYTHING in the car and check voltage again at idle, anything lower thean 13.4 is a bad sign. then also check for AC voltage at the battery (no more than 100mV is acceptible). all this should be done with a DVOM

[BrianFestiva]

We used to run and old car with an alti with no battery. It was used to do a run around the property. It never had a battery because we hill started it everytime. Battery blew up and we never replaced it. Had it like that for a year. Never had any problems. Car still goes today and still same alti and that was 5 years ago lol. Maybe it depends on the brand?

[scitzz]

I used to test alternators by pulling the negative terminal off, until I heard stories about them going full field and frying electronics

So, yes, Nate, it is obvious now that it is normal for it to keep running