You must have an aftermarket replacement radio. If so, disregard my comment about the display failing to turn off. If you have an original system, GM used a lot of three-piece systems to prevent people from replacing them with something better. If you cut the wires on the control box with the switches and display, you still have the receiver box in the dash that could be causing the current drain.
Off-engine testing of the generator is not a good test of the unit. You can do the first part of the test with an inexpensive digital voltmeter. The second part requires a professional load tester to measure full-load maximum current output. That will require from five to eight horsepower to run the generator. In-store test benches rarely have motors larger than one horsepower. Also, they will run the generator at roughly idle speed which can show if there's some output, but not how much. All generators are very inefficient at slow speeds, so all on-car testers require engine speed to be increased to 2,000 rpm to get accurate results.
We also run into plenty of wiring problems that cause the charging system to be dead. Naturally, those won't show up with off-car testing. The other thing that doesn't show up is when the generator has one bad diode of the six. You will lose exactly two thirds of the generator's output. 30 amps from the common 90 amp generator is not enough to run the entire electrical system under all conditions. The battery will have to make up the difference until it runs down over hours or days.
As a side note, GM has had a huge problem with their generators since they redesigned them for the '87 model year. They develop large voltage spikes that can damage the internal diodes and voltage regulator. To reduce the number of repeat failures, replace the battery at the same time as you replace the generator, unless it is less than about two years old.
Holler when you have the amp meter and clip leads. To test for the current drain you'll need to use one clip lead to connect one meter lead to the battery's negative post, (you may need a bolt and nut to attach to the battery), another clip lead to attach the other meter probe to the cable, (you'll need a nut there too), and most importantly, a third clip lead to temporarily connect the two meter leads together.
Connecting to GM battery terminals and cables is a miserable and time-consuming job. For this low-current testing it is acceptable to clip a clip lead to the threads inside the battery's post, but contact on the cable is not made through the 5/16" bolt. Contact has to be made to the metal disc recessed in the end of the cable. That's where the nut comes in because there's nothing else to clip to.
Saturday, January 18th, 2014 AT 1:05 AM