DO NOT DISCONNECT ANY CABLE WITH THE ENGINE RUNNING!
Every year I did a demonstration on the alternator 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 alternator's internal diodes, the 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 alternator 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 alternator 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 alternators 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 smoothes out that ripple so it doesn't affect the voltage regulator or the alternator 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 alternator'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 alternator problem is one defective diode out of the six. You will lose exactly two thirds of the alternator'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.
To test your charging system, there's one larger output wire bolted to the back of the alternator and two smaller terminals. You can take these three readings when the system is working properly so you can see what "normal" is, but the real testing has to be done when the problem is acting up. Measure the battery voltage with the engine running. Next, you must find the same voltage on the output terminal on the alternator, within perhaps a tenth of a volt. If it's a lot higher, there's a break in that wire, usually at the bullet connector near the back of the battery. You'll also find 0 volts there with the engine off. You SHOULD have full battery voltage on that terminal all the time.
Next, measure the two voltages on the smaller terminals. This has to be done with the engine running. With the engine off, the automatic shutdown (ASD) relay will be turned off, and that's what feeds the alternator field, among a lot of other things. One terminal will have full battery voltage. The secret is the other terminal. It will also have the same full battery voltage, 0 volts, or ideally, something in between. 4 - 11 volts is what you can expect to find.
If you find battery voltage on one terminal and 0 volts on the other, there's worn brushes inside and those always start out being intermittent long before they fail completely. The brush assembly costs about twelve bucks and is fairly easy to replace. This is the most common failure.
The second most common failure is the main suspect when you have what appears to be multiple alternator failures. That is a break in the wire from that second smaller terminal to the voltage regulator inside the Engine Computer. For this you'll find exactly the same voltage on both smaller wires, typically around 12.2 to 12.6 volts. That means the difference in voltage between the two wires is 0.0 volts, and that means no electromagnet is being developed. This is almost always caused by corroded or spread terminals in the connector between the engine and body.
If you do find 4 - 11 volts on one terminal but output voltage is also low, as in 12.2 to 12.6 volts, suspect a defective diode inside the alternator, but those are rarely intermittent. It would be more likely there's a bad connection inside, but that would be really rare.
Monday, October 6th, 2014 AT 2:06 AM