The "Battery" light hasn't changed since the 1960s. It means the generator isn't recharging the battery while you're driving. What HAS changed is GM redesigned their generators for the '87 model year and it's a very poor design. They develop huge voltage spikes that can damage the internal diodes and voltage regulator, and can interfere with computer sensor signals. It is real common to go through four to six replacement generators in the life of the vehicle. To reduce the number of repeat failures, replace the battery at the same time unless it's less than about two years old. As they age, the lead flakes off the plates and they lose their ability to absorb those voltage spikes.
You already noticed the charging voltage is too low. Measured at the battery with the engine running, the voltage must be between 13.75 to 14.75 volts. If it is low, suspect the generator. If the voltage is okay, you need a professional load tester for the second part of the test. That will test for maximum output current and "ripple" voltage. If one of the six diodes has been damaged, you'll only be able to get exactly one third of the generator's rated maximum current, and ripple voltage will be very high.
Wednesday, September 23rd, 2015 AT 11:17 PM