You are extremely lucky the generator is not working. Removing the battery cable was a trick done many years ago by mechanics who didn't understand how these systems work and how to test them. Since the 1987 model year, GM generators are an extremely poor design that produce huge voltage spikes. Without a good battery to absorb and dampen those spikes, you run the risk if destroying every computer on the car and rendering it not worth repair. It's one of many reasons GM has ten times more trouble related to their computers than all other manufacturers combined.
With any brand of car, the battery is the key component in controlling system voltage. The voltage regulator will lose control when it gets confused by the dips in output voltage caused by the normal AC ripple. In response to those dips, it will try to make the generator work harder. The battery holds the system voltage steady to hide those voltage dips. Without the battery, it is possible for the generator to reach 30 volts or more although you usually have to speed up the engine for that to occur. Besides all of the complicated, unreliable computers, the diodes in the generator have a reverse blocking voltage capability of only around 30 volts which is plenty under normal circumstances, but they will be destroyed with voltages higher than that. A working generator could be instantly damaged from those high voltages.
If the engine still runs with a charged battery and everything else appears to be working, measure the voltage on the output wire bolted to the back of the generator. With the engine off, you should find full battery voltage there. If it is 0 volts, there is a fuse or fuse link blown between there and the battery. On older cars that would be a wire in the wiring harness. On newer cars it is often a large fuse bolted in the under-hood fuse box.
If you remeasure that voltage with the engine running, it should be a fairly steady 13.75 to 14.75. If it is pulsating, temporarily run a new wire from the output terminal directly to the positive terminal of the battery. A rare and difficult-to-find problem can occur if the fuse link wire burns open but leaves a carbon track behind inside its insulation. That carbon will pass enough current to be picked up by a digital voltmeter giving the incorrect impression that you do have full battery voltage at the generator's output terminal. That carbon will not be able to pass enough current to power a test light, so in that instance, an inexpensive test light is more accurate than a voltmeter.
Next, measure the voltage on the small wire plugged into the generator. You should expect to see around 2.0 volts when the battery light on the dash is on. When the generator is working, it will put 12 volts back onto that wire to turn the light off. If that voltage is pulsing, the internal voltage regulator is defective. It is darn near impossible to get it apart to replace it without damaging the tabs on the diode block so you are better off just replacing the entire generator, ... Again.
It is common to go through four to six of these generators in the life of the car. What many professionals are finding out is the chance of a repeat failure can be reduced by replacing the perfectly good battery at the same time. That battery will work fine in pre-1987 cars, but as they age, they lose their ability to absorb the voltage spikes. Their "internal resistance" goes up as the lead normally flakes off the plates. By the way, a battery charger produces pulsating DC current that can cause the plates in the battery to vibrate. For that reason, it is smart to always use a low charge rate on older batteries. At a high charge rate, the plates vibrate harder and get hotter which can accelerate the speed at which the lead flakes off. You might not notice the loss in performance right now, but the high-rate charging could result in a dead cell in a few weeks. You don't have to worry so much about this with newer batteries, but they can be overheated from repeated high-rate charging. To be safe, it is better to charge a battery longer at lower rates. This applies to any brand of car and battery.
Sunday, December 5th, 2010 AT 5:06 PM