Alternator heating up

My car was converted to negative earth many years ago and an alternator fitted (not by me) apparently without any problems. However today while working on the engine I discovered something very odd. Normally I have the battery isolator activated when not actually driving the car but today forgot to do so. The back of my hand touched the alternator and it was very hot - this is with the ignition off. When I swithed the isolator switch to off the alternator slowly cooled down.
The battery pole of the alternator is connected to the fusebox and the fusebox is connected to the battery pole of the starter solenoid. What is going on? Alternator works fine according to voltmeter, and all wiring seems ok. Is it a fault in the electronic control in the alternator - and if so will I need a new alternator? or can it be fixed?
Thanks in advance for any help

Hi ABH,

Responding to your call is a nice distraction while waiting for Hurricane Irene to pass over.

The heat is caused by a grounded or shorted circuit.

Assuming you have a built-in regulator, the regulator will go to ground when the alternator is not creating currant. The dash indicator charge light is activated when the ignition switch passes a positive 12 volt current through the lamp and the regulator is at ground. The indicator lamp goes out when the alternator is charging and the regulator goes from ground to also passing a 12 volt current on the indicator line. With no ground, the lamp goes off.

In most alternator installations, the Batt terminal is usually connected to the battery side of the starter solenoid with 10-gauge wire. From the starter solenoid, the standard wiring harness would then send power to the B-connector of the external regulator and from there to the light switch, ignition switch and fuse block. Since most alternations try to maintain standard wiring, the external regulator is maintained as the power distribution point but is first gutted to eliminate any electrical conflicts with its original regulating function.

Since your alternator Batt terminal is connected to the constant voltage side (I assume terminal 1) of your fuse block and then to other parts of the electrical system, it is important to know how your power distribution is implemented. Do you still have the external regulator mounted on your firewall? If so, how are the wires connected and to what?

Although you could have a faulty regulator within the alternator, it is very difficult to identify the source of your issue without more information.

Good luck,
Ray (64BJ8P1)