Are you telling me you removed a battery cable while the engine was running? If you'd like me to, I can find a copy / paste version of a previous reply about why that can destroy a lot of stuff on the car, and why it is not a valid test, but for now, understand that if a mechanic is caught doing this, he might get one verbal warning, ... The second time he will be fired. It's really that big of a deal. Uneducated mechanics pulled that stunt decades ago when they didn't understand how these simple system worked. The thinking was if the engine stayed running, the generator must be working, but there's a lot of misinformation related to that.
To tell if the generator is working, just measure the battery voltage while the engine is running. It must be between 13.75 and 14.75 volts. If it is low, there's a couple of voltage measurements that will tell us why. If it is okay, something may be draining the battery after the engine is stopped.
That's the first part of the test. For the next part you need a professional load tester to measure output current. If one of the six internal diodes is bad, you'll get exactly one third of the generator's rated maximum output current, and that's not enough to run the entire electrical system under all conditions. The battery will have to make up the difference until it slowly runs down over hours or days. One of the common symptoms of that is a voltage gauge on the dash dropping when a real small load, like brake lights, is turned on.
Sunday, September 14th, 2014 AT 7:11 PM