The only thing that saved you from destroying thousands of dollars worth of computers is you car doesn't have them. If a mechanic was caught doing that trick, he might get one written warning, but the second time he would be fired.
There's a couple of things wrong with that do-it-yourselfer test. First of all, all AC generators put out three-phase output which means there's a lot of "ripple" voltage. The battery is the single component that smooths that out. Without the battery in the circuit, the voltage regulator may respond to the peek voltage, which will be too high, and in response it will try to cut back on the generator's output. There's a limit to how much control the regulator has, and if those peek voltages are all it responds to, it can cut back so much that the generator won't develop enough current to run the entire electrical system. The result is the engine will stall with a perfectly fine generator. That incorrectly tells you the generator is bad.
More commonly the voltage regulator will respond to the dips in the ripple voltage, and will try to raise the output voltage. I did a demonstration for my students every year to show WHY they can be fired for pulling this trick. All generators are very inefficient at low rpm, but when you raise engine speed, it is real easy to hit more than 30 volts. The regulator doesn't have enough control to keep the voltage down to a safe level, and it has no control whatsoever when there's no battery in the circuit. This is where you can destroy every computer module on the car, and shop owners know it. Imagine the repair bill the shop would have to pay for on a late '90s Cadillac with 47 computer modules.
Usually the diodes will short first on the older cars. Those are one-way valves for electrical current, and all AC generators have them. They are designed to handle the maximum current that unit can develop, but when blocking reverse current, the maximum voltage they can block is typically around 18 - 20 volts. With more voltage than that, they become shorted, like a piece of wire, and no longer produce usable DC current that can be stored in the battery and run the electrical system. That means you turned a good generator into a bad one. Really old cars with breaker point ignition can tolerate a lot because there's no electronic circuitry to get damaged. The only thing that will be destroyed on those is the radio and any light bulbs that are turned on. Of course those diodes will be damaged too if the voltage goes high enough.
The test you did was done many years ago by uneducated mechanics who didn't understand how these extremely simple circuits worked. Now that we know better, to do a proper test, just stick a voltmeter on the battery. With the engine not running, you will find 12.6 volts if the battery is fully-charged. It will be closer to 12.2 volts if it is good but discharged. If it's around 11 volts, it has a shorted cell and must be replaced.
With the engine running you must find between 13.75 and 14.75 volts. If it's real high, the voltage regulator is shorted. If it's a little high, the generator MAY have a bad diode. If it's low, but higher than when the engine is off, there may also be a bad diode. If there's no change between engine off and running, the regulator could be defective or there could be worn brushes in the generator. There's a real simple test to determine which it is.
Also consider that the wire between the output terminal and the battery could be corroded apart. Think of a water pump feeding a pipe with a valve that is closed. It will build a lot of pressure, but no water will flow. Voltage is electrical pressure, and a break in the wire will allow the output voltage to go real high, just like when you disconnected the battery, only this time it won't feed the rest of the car either. This is where the diodes short, you replace the unit, and the diodes short on that one too as soon as you run the engine and raise its speed. Fortunately that doesn't happen real often. To check for that, just measure the voltage on the output terminal with the engine off. You must find full battery voltage there all the time. There is one way to get a false reading, so for more accuracy, a test light is a better choice than a voltmeter. If the voltmeter shows 0 volts, there's a break in the wire, period. If it shows battery voltage, THAT'S when you want to double-check with a test light.
Even if you find the voltage is okay with the engine running, that's only half of the test. You also want to know the maximum output current the generator can deliver. You need a professional load-tester for that. A typical generator for an '83 model would be in the 60 - 70 amp range. If the load-test shows it's only able to produce exactly one third of its current rating, it has a bad diode. It's not practical to replace them. There are six, and they usually are in two blocks of three. Early '80s generators are relatively inexpensive, so with the cost of the parts, and the time involved, it's most common to just replace it with a rebuilt unit with a warranty.
Monday, December 9th, 2013 AT 6:09 PM