As a mechanic, you should have been taught to never, ever remove a battery cable while the engine is running.
All AC generators put out three-phase output which is rather stable and steady. One failed diode is a common failure, and even more-so on 1987 and newer GM vehicles. That will cause a loss of one phase of output current. Voltage drops real low during the missing phase, and that difference between the highest voltage and the lowest voltage is called "ripple" voltage.
Every year I did a demonstration on the generator test bench for my students to show what can happen when you do that. It was real easy for the voltage to reach over 35 volts. That will destroy any computer on the vehicle, the generator's internal diodes and built-in voltage regulator, and any light bulbs that are turned on.
The thinking is that if you disconnect either cable and the engine stays running, the generator must be working but a lot of them will stop working due to the voltage regulator responding to the dips in the "ripple" voltage being produced. That will make a perfectly good generator appear to be bad so that test is not valid.
If a mechanic is caught pulling this stunt he will typically get one verbal warning. For the second offense he will be fired. It's that big a deal.
Some generators respond to the high points in the ripple. That momentary higher voltage goes right back to the field winding and creates a stronger magnetic field. That stronger electromagnet creates a higher output voltage which again creates a stronger electromagnet. It's a vicious circle and voltage can keep on rising until something gives out. The main thing that smooths out that ripple so it doesn't affect the voltage regulator or the generator is the battery.
Three things are needed to generate the output current. They are a magnet, (electromagnet, in this case), a coil of wire, and most importantly, movement between them. That's why the belt needs to make it spin. One thing that can save you from doing damage by removing a battery cable is not raising engine speed. Generators are relatively inefficient at low engine speeds and their output voltage is less likely to rise to dangerous levels, ... As long as you don't raise engine speed.
One other thing to keep in mind is batteries give off explosive hydrogen gas. Regardless if your generator is working or not there is going to be a big spark when you remove a battery cable with the engine running. Either the generator's current will be recharging the battery, and that can be up to 20 amps, or the battery is going to be supplying the car's electrical systems, and that can easily be over 30 amps. That kind of current is going to create a big spark when a connection is broken or reconnected. Small arc welders run as low as 40 - 60 amps and look at the sparks they create. The reason we don't hear about more battery explosions is because people are careful to not disconnect the cables when there is current flowing through them. It's also why there are huge warning labels on all battery chargers to be sure they are turned off before connecting or disconnecting them from the battery.
Another common generator problem is one defective diode out of the six. You will lose exactly two thirds of the generator's capacity but system voltage will remain normal or it could even be just a little high from the voltage regulator responding to the greatly increased dips in the ripple voltage.
It's always a good idea to wear safety glasses when working around car batteries, but if you still insist on removing a cable while the engine is running, a face shield makes more sense, and have plenty of water on hand to wash any acid off the vehicle's paint.
Ford used to have a really nice generator design that allowed testing right on the back of the unit. Only Chrysler alternators are easier to diagnose. Unfortunately the engineers don't really care about ease of service on GMs and many other brands.
The way you tell if the charging system is working is to measure the battery voltage while the engine is running. It must be between 13.75 and 14.75 volts. There still could be a bad diode though. You need a professional load tester to test for that. Ripple will be very high and the most output current you will get will be one third of the generator's design value. That is not enough to meet the demands of the electrical system under all conditions so the battery will have to make up the difference, until it runs down.
If you find charging voltage is low, suspect the generator. Before you replace it, be sure the "Battery" warning light turns on when the ignition switch is on and the engine is not running. That light circuit is the turn-on circuit for the generator's internal voltage regulator. If the light doesn't turn on, there is a break in that wire or the bulb is burned out. The turn-on circuit will still work if the bulb is burned out because there's also a 510 ohm resistor across the bulb for that purpose.
Once the generator starts producing its output current, the voltage regulator puts 12 volts back onto the dash light wire to turn the bulb off. Typically that is the brown wire in the small connector at the rear / side of the generator. Measure the voltage on that wire if the bulb never turns on. With the ignition switch on and engine off, you should find about 2 volts on that brown wire. With the engine running, you should have over 12 volts on it.
If the warning light turns off with the engine running, and you find between 13.75 and 14.75 volts at the battery, that only means it is okay to do the rest of the tests, and that requires a professional load tester. Of particular interest besides charging voltage, is full-load output current, and "ripple" voltage. There is no way to sugar-coat this. GM had, in my opinion, the world's second best generator design through 1986, but starting with the '87 models, they have the worst pile ever. It is not practical to try to take it apart in the field. If you could get it apart without damaging the diode block, you can't test the critical parts, mainly the voltage regulator. You'd want to replace everything that might be causing the problem, and that can cost as much as buying a rebuilt unit.
Besides the miserable repair procedure, these generators develop huge voltage spikes that can damage the internal diodes, voltage regulator, and interfere with computer sensor signals. Its the battery that absorbs and dampens those harmful spikes, but they lose their ability to do that as they age. The lead flakes off the plates, and while the battery will still crank the engine okay, its "internal resistance" goes up and makes it much less effective at damping those spikes. It is common to go through four to six replacement generators in the life of a GM vehicle. To reduce that number of repeat failures, always replace the battery at the same time as you replace the generator, unless it is less than about two years old.
When testing shows you can only get one third of the current the generator is rated for, and ripple voltage is high, one of the six diodes has failed. If you can't get any output current, and battery voltage stays at 12.6 volts or less with the engine running, the voltage regulator has failed. Both conditions can be caused, in part, by an old battery. Related to this, if your "Battery" warning light is working properly, and it turns off with the engine running, the regulator did that simply because it saw some output current. If you have a separate "Volts" gauge on the dash, and that is reading low, while you may be getting some output current, it is likely too low to keep up with the demands of the electrical system, and the current flow from the battery is drawing its voltage down. You'll need the load tester's results to figure out where to go next. Post the numbers here if you need me to interpret them for you.
I should mention too to check that the serpentine belt isn't slipping. That can cause a perfectly good generator to not keep up with demand. The warning light will be off, because the system is working, but voltage will be low because the battery is being drawn down. If your generator has an external fan, push on one of the blades with your thumb. If you can spin the fan, (do that with the engine off!), The belt is too loose.
For your specific truck, you are already done with the diagnosis. You found a light was left on and it ran the battery down. Slow cranking is the obvious symptom and is to be expected, so nothing else needs to be done other than charging or jump-starting the battery.
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Saturday, December 29th, 2018 AT 4:01 PM
