DIY: Testing and Replacing an Alternator
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DIY: Testing and Replacing an Alternator

What do you do when the “Battery” idiot light comes on when you are driving down the highway? What does it mean? The simple answer is that there is something wrong within the charging system. That is the simplistic answer. The real problem may be the battery. It may have shorted cells, or open cells, or a broken connection internally. The bottom line is that the battery is not accepting a charge and will need to be replaced. The other very real possibility is that you have a bad alternator and will need to replace it. If you were to take your car to a service center they will use test equipment designed to test the charging system components. The good news is that you can check your car's charging system with a simple and inexpensive Digital Multimeter and save some big bucks that a mechanic would charge you.

Diagnosing the cause of problems with your car's charging system is relatively easy and, if you do have a defective alternator, you can save a $100 or more labor charge by replacing it yourself. The most difficult part of this project will be getting the serpentine drive belt back on correctly. They call it a serpentine belt for a very good reason. That single belt winds itself between and over many pulleys in a serpentine manner. Most vehicles have a belt routing diagram under the hood,on the fender-well or near the radiator, but if the diagram is missing and you do not have a service manual for your car, it will behoove you to make a careful sketch of the belt's routing before you remove it.

Tools and Supplies

  • A battery keeper

  • Digital Multimeter

  • Set of combination wrenches

  • Belt-tension wrench

  • Screwdrivers

  • Fender cover

  • Shop rags

  • Mechanic's Gloves

  • Replacement alternator

What is a Battery-Keeper?

A Battery-Keeper is nothing more than a 9-Volt battery that plugs into your cigarette lighter socket or auxiliary power outlet. Today's high-tech vehicles contain many on-board computers and other devices that contain programmed setting, which lose their programming when the storage battery is disconnected. The Battery-Keeper provides enough voltage and current foe these devices to maintain their programmed settings while the car's battery is disconnected. With some vehicles, the use of a Battery-Keeper is mandatory because many of their on-board computers can be irreparably damaged if the car battery is disconnected with a Battery-Keeper in-place. Every DIY auto mechanic needs to have a Battery-Keeper in his tool box and needs to use it faithfully.

A typical Battery-Keeper, like the one shown here, sells for less than $10, so there is no good reason not to have one in your tool box. That street price includes the 9-Volt battery.

The Digital Multimeter

The street price for digital multimeter run from less than $10 to several hundred dollars but a DMM (Digital Multimeter) like the Cen-Tech 11-Function DMM sold by Harbor Freight for less than $25 will do everything that the DIY auto mechanic needs and will also perform equally well for the DIY electrician.

Test the condition of the battery first.

The first thing that you will need to do is to check the charge on the battery with the car's ignition switch in the off position. Set the function switch on your DMM to a 20-volts DC or higher DC range, then connect the meter to the battery as shown in this photo.

Connect the “Red”, positive test lead to the “Positive” battery post and the “Black” test lead to the “Negative” battery post. The positive and negative battery terminals can be identified by the “+” and “-”signs embossed on them or on the battery case next to them. The positive battery post also has a larger diameter than the negative battery post. In some cases the positive post may be covered with a red rubber cover and/or have a red felt washer around it. Compare the reading on the DMM with the reading in the chart shown here, being careful to use the column for the current ambient temperature.

If the voltage reading indicates that the battery is at less than 50-percent charge, you must bring the battery up to full-charge before testing the alternator. If you do not have a battery charger in your shop, take the battery to any Auto Zone or Advance Auto Parts store and have it charged for free. Many other auto parts stores that deal with the DIY auto mechanics also offer this service for free.

If you remove the battery from your car and take it in for a charge, follow this procedure.

  • Plug the Battery-Keeper in to the cigarette lighter socket or the auxiliary power socket.

  • Disconnect the negative, ground cable from the battery first. Always disconnect the ground cable first when removing a battery and reconnect it last when installing a battery.

  • Disconnect the positive cable.

  • Remove the battery hold-down strap.

  • Lift the battery from your car and take it in for a charge.

While the battery is being charged the technician will also check the battery's plate conditions and will be able to tell you whether the battery should be replaced or whether it is still serviceable.

Remove the surface charge from the battery.

Once you have installed the fully charged battery in your car, you must discharge the surface-charge that has built-up on the battery's plates. Here is how to do it.

  • Disconnect the Battery-Keeper.

  • With the ignition switch still in the off position, turn the headlights on for four minutes to drain away the battery's surface charge.

Testing the alternator.

Connect the DMM to battery again just as you connected it when testing the charge on the battery. With the DMM in place, start the engine and note the reading displayed on the DMM. A alternator in good condition should put out between 13.5 and 14.5 Volts. If the initial reading is below 13.5 volts, place a load on the electrical system by turning on the headlights and the blower motor while increasing the engine's RPMs to approximately 2,000 RPM. The DMM should now display 13.5 to 14.5 Volts for a good alternator.

If the voltage output is still below the 13.5 to 14.5 Volt range, there is one more test to perform before replacing the alternator. Let the engine run at idle speed for at least five more minutes and then repeat the above test. If the alternator's output still falls below the 13.5 to 14.5 volt range, replace the alternator.

Removing the old alternator.

  • Reconnect the Battery-Keeper and disconnect the car battery as you did before when removing the battery for charging.

  • Disconnect the wiring from the alternator's integral voltage regulator by depressing the clip on the connector and pulling the connector out of the alternator.

  • Remove the nut holding the battery cable on the terminal post,

  • release the tension on the serpentine belt, and slip the belt from the pulley. Now is a good time to inspect the condition of the belt and replace if needed.

  • Remove the old alternator from the engine.

Installing the new alternator.

Simply reverse the steps you took when removing the old, defective alternator.

Tips on buying a replacement alternator.

Quite often you can buy a brand new alternator online for 30 to 50 percent less than what a rebuilt unit will cost you at a brick and mortar parts store, so if you can do without your car for a few days, it would behoove to check the online parts suppliers. A good place to start your search is Visteon Alternator.



Cen-Tech model 37772 DMM Manual

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Comments (4)

Well illustrated and explained.

Well done Jerry, although I pay to have someone take a look at this stuff.

very interesting, I don't understand that stuff but you do a good job explaining it.

Brilliant pictures and explanation, although it's still all Dutch to me. -)