As a mechanical guy, I yield to those with better training and experience and heartily recommend against removing the battery as a system check. The potential for damage far outweighs the simplicity of the test and any information gained therefrom.
As we got into before, you cannot disconnect the battery, as this phenomenon is called “load dump” in EE , on cars—a well known issue..look it up. ; Motorola makes a big surge diode clamp (Zener) fitted to many cars to hold volts at like 15 max or so, due to this V surge taking out the car electronics. . In a new car , all the computer stuff can be wiped out by doing this. (BMW has 90 small computers) The reason is that the field coil ( controlled magnet in the alternator) has a time constant about 10-20 milliseconds, so even if the VR sends a “reduce” signal to control it , it takes that long for magnetic field to change, During that time the volts go through the roof, as nothing is connected anymore to soak up the 5 amps or 30 it was putting out before lifting the cable ..so 30 or more volts ! Spike. Your meter will show 13 or 14 , as it is not fast enough to see this ; and it does usually get it under control , but it takes time. In our cars, any electronic ignition are toast, as are the VR and the radio if on . Why “mysterious failures” of this stuff like Pertronix happens. Loose battery cables. Also the battery dampens the servo response of VR..it (VR) can be therefore made fast, to avoid a somewhat normal volt upswing when you suddenly shut off your 2 35A AC fans , leaving the alternator full on momentarily , just like a portable gas generator VR does . So it may overshoot pull back overshoot pull back (called oscillation) without the battery . This is even worse, can blow all your light bulbs too.
If someone got away with it, buy a few lottery tickets…
The alternator can self excite if already running, or even if off ,as the rotor is slightly magnetized anyway..some cars, GM and Ford , use the idiot light current drain for this too,, a bad idiot light, it will not start charging ; note one wire alternators will still start up. Called residual magnetism in rotor . like happens to your screwdriver tip….
You must have a battery in the car and connected with the cables to excite the alternate or it will not put out any current.
Posted by: "Rich Barber" <c300@xxxxxxx>
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