Measure the voltage right at the large starter terminal while a helper tries to crank the engine for you. Put the negative meter probe right on the engine block, not the battery cable. The lowest acceptable reading is 9.6 volts during cranking. If it is lower than that you'll need to move the meter probes around to find where the voltage is being dropped. I can walk you through that.
Keep in mind that testing a starter off the engine is not a worthwhile test. With no load on it, the motor will only draw about 50 amps. There are actually two identical high-current circuits inside the motor and each one has two brushes. If one brush is worn and not making contact, the motor will still work fine on a bench-test but in the car, it will draw half the normal current, (at first), and spin way too slowly. Without going into all the electrical theory, when the motor spins slower than normal, current flow through it will go way up. That makes diagnosis difficult. A properly working starter will draw around 150 amps. When half of it isn't working, logic dictates it will draw half of that, but in fact, actual current will go up to almost a normal 150 amps. The difference is it will be spinning much too slowly. The secret is to measure the voltage at the motor during cranking.
Once you get the engine started, measure battery voltage again. It must be between 13.75 and 14.75 volts. Since the 1987 model year, GM has the worst design generator by far. It is common to go through four to six of them in the life of the car. Due to their design, they develop huge voltage spikes that interfere with engine sensor signals and they can destroy the diodes and the voltage regulator that are built in. What many professionals are finding out is to reduce the number of repeat failures, you must replace the perfectly good battery at the same time. As they age, they lose their ability to absorb and dampen those voltage spikes. The old battery will work fine in a 1986 or older model.
Thursday, May 5th, 2011 AT 8:35 AM