DO NOT EVER DO THIS
The newer the car is, and the more electronics it has on board, the greater the risk that you'll damage something.
Years ago, as a Navy ET (electronics technician) I won a sizable bet about this very thing. At the time, we wired up my '64 Dodge with a points dist for testing, it had no solid state "anything" in the car, and hooked the battery buss to an oscilloscope with a camera attachment.
Pulling off a battery cable can generate a VOLTAGE SPIKE MORE OF MORE THAN 100 VOLTS!!!
You will never, ever find this so called procedure in ANY mechanics class, textbook, repair or shop manual of any kind.
This is nothing but an old wives tale that has been passed around and around, and now that we have the internet, is just will not die.
Now this WILL start a flame war. There WILL be some who will get on here and claim "I've done this hundreds of times." Well you MIGHT have done it that many, and you MIGHT have damaged something. MAYBE, next month, the clock mysteriously quit. Or the radio. Or the delay wipers. The point is if you do this often enough---one day you'll be standing there wondering whuuu......happened?
The other point about this so called test is that it proves NOTHING. Alternators are three phase devices, meaning they have three sets of output windings (stator) with three sets of diodes.
Let's say that two of the stators have lost their connection, etc, or that the diode pairs for two of them are open. What you have is, instead of a 60A alternator, you probably have less than 1/3 of that. Maybe enough to run the car, but not enough to keep up with the lights and heater.
IN THIS CASE if you tried this test, you would believe that the alternator was working!!!
