Alternator Troubleshooting
TROUBLESHOOTING ALTERNATORS

By National Quick Start Salesubois (alternatorman)

What is an Alternator, How does it Work and what does it do,
How do you tell if it's working and what to do?


The alternator is the source of power for your car's or truck's electrical system, this system includes the battery, alternator, lighting, controls, accessories and wiring. The Alternator controls may include a voltage regulator , on older and some other vehicles the voltage regulator is separate from the alternator, in newer vehicles the regulator is inside the alternator, or on Chrysler and related makes and some other vehicles the voltage regulator is in the on-board computer. In some newer vehicles the voltage regulator in the alternator or computer communicate with each other for the alternator to work properly. If you do not have the correct communication between the alternator and on-board computer the alternator may not work properly or the vehicles computer will shift to default mode and the vehicle will run the vehicle rich on fuel Setting and you will use more fuel. With all these different possibilities your first step is for you to determine what type of system you have.

The alternator's job is to generate electricity to replace in the battery what was used to start the vehicle, and to run all of the electrical accessories that are in use as you drive. Newer vehicles are loaded with electrical demanding devices including lights, A/C fan, radiator fan, electric and heated seats, power windows, stereo, snow plow, inverters and whatever other gadgets you have.

The vehicles alternator is not a battery charger like the one you plug into the wall. If you run your battery dead and jump-start the vehicle to "drive around" to build up the battery voltage you must be very careful. I know in the old days you could do that with no problem but back then they had 35 to 60 amp alternators you could easily get away with it. Today, most alternators are capable of making over 100 amps or more, but only for short periods of time. With greater out put and demand on the same physical size alternator can cause early failure.
If you drive on a dead battery you will overheat your alternator so you must be very careful. Charging on a low battery can damage your Alternators Bridge Rectifier . Excessive use will surely cause damaged and shorten your alternators life. Jump starting your vehicle to charge the battery should be for emergencies only. As the amperage of the alternator goes up, so has the price and problems with them.

Also, do not disconnect the battery wire with the engine running and the alternator working, this may damage the vehicle electronics. In the old days you could do this when there were no sensitive electronics to be damaged. Of course, if the alternator is not working, this will rarely damage a thing. If the alternator is charging, you could send a voltage spike through the electronic equipment on the car and possibly cause all kinds of damage.


There is a simple way to check the alternator. All you need is an inexpensive voltmeter and some knowledge of how the charging system works on your vehicle.

The first thing to do read the voltage across the battery terminals with the motor shut off . It should read about 12.6 volts. If it is lower than 12.2, you must charge the battery.

Start the engine and check the voltage at the battery. Ob high idle, around 800 to 1100 RPMs it should build up to 13.5 to 15 volts with everything shut off.

If the voltage builds up then you should turn on the lights and A/C-heater fan on high. These are a couple of the biggest electrical loads on a vehicle. Now again raise the engine speed to a above idle and check the voltage. If the alternator is working properly, it should maintain voltage around or above 13 volts with everything turned on.

If the alternator is not charging after the testing above you should move on to the next phase of trouble shooting alternator problems or charging system troubles.

How vehicles activate the alternator may different between different vehicle makes and models.

For the next step you need knowledge of the alternator wiring. You must have a wiring diagram or knowledge of the wiring configuration to activate and regulate your alternator.

The next step is to check the activation wire that plugs into the alternator.

Every alternator has an ignition wire to the alternator plug, this is to activate or make the alternator start charging. This ignition wire is fused, this fuse can fail, check the wiring diagram for you vehicle, find this wire and check it to make sure it is not broken or the fuse is blown The ignition wire to the alternator should be hot with the key in the on position and off, no power, when the key is turned off.

The above holds true unless you have a special self exciting or one-wire alternator. One wire alternators only have the main battery wire to the back of the alternator, no other wires. Some people confuse the situation when you have one wire to the plug then this is a one wire alternator, this is incorrect. One wire to the plug is a two wire alternator in that you have one wire to the plug and one to the battery post, two wires total.

If you have checked the ignition wire, battery cables and all other possibilities the next step is to take the alternator off and have it checked off the vehicle.

Good Luck
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