Reasons for Replacement

A majority of the time when an alternator fails, it stops charging and the battery goes dead. Other times the alternator over charges and cooks the battery. For testing and evaluating of the battery, starting, and charging system refer to the Battery, Starter, and Alternator Testing post to help with diagnosis.

Difficulty Scale

This job may not be time consuming, but there are a few steps that are critical when replacing an alternator. This skill level is more for the advanced level technician. Follow along and learn these important steps.

Tools for the Job

• 1/4″ ratchet
• 5/16″ socket
• 3/8″ ratchet
• 13mm socket
• 15mm socket
• 3/8″ extension 6″ long

Prep for Alternator Removal

Always evaluate the job before starting. Here is a view of the alternator on the left. There are 2 mounting bolts, 1 corner bracket, 1 plugin style connector, 1 nut holding the B+ wire from the battery, and the belt.

Considering that we are working with the heart of the electrical system, the first and most important thing to do is to disconnect the battery before servicing the alternator.

All side post batteries have a 5/16″ or 8mm socket size to remove the battery cable.

Important Electrical Caution!!!

Always remember, when disconnecting the battery remove the negative terminal/ground cable first!!! This is to prevent any possible contact with surrounding grounds.

If you touch a ground no problems can occur if your removing the ground cable. On the other hand, if you contact a ground while removing the positive of a live battery you can short circuit the battery and do significant damage to the vehicles electrical system.

Note: Refer to the Replacing a Serpentine Belt and Tensioner post for how to remove the belt prior to servicing the alternator.

Alternator Removal

Using your ratchet and 15mm socket, remove the upper alternator mounting bolt.

Using the 13mm socket with extension, remove the nut and the alternator support bracket from the corner of the intake manifold.

Remove the lower alternator mounting bolt.

Pull back the protective boot and remove the nut for the battery cable. This is the cable that supplies voltage to the battery when the vehicle is running.

Remove the battery cable. This is why you should disconnect the battery. Eliminate the thought of your wrench touching a ground when removing the B+ cable from the alternator.

Lift out the alternator for removal, but be aware that there is still one more wire to disconnect. The only way to reach the connector is to lift out the alternator for access.

Unplug the 2 terminal connector. This is the power and ground wires that supply the alternators voltage regulator. (More on how the alternator works later)

Install the new alternator in reverse order and retest the charging system to make sure the new alternator is working correctly.
Refer to the Battery, Starter, and Alternator Testing post for a recap on how to check for proper operation of the alternator.

Estimate for Repair

(check with your local parts stores for current prices)
Parts to replace
Using aftermarket parts you can purchase a remanufactured alternator for around $80 and a new alternator for approx. $120.
Labor to replace
This job should cost approx. 2 hr. labor to test the charging system and replace the alternator. (Example: 2hrs.@ $80)
Parts and Labor
Re-man price/ $240
New price/ $280
Note: Factory parts and shop labor rates will be higher depending on type of repair shop and location.
Time to replace
This job should take even the experienced technician approx. 1 to 1.5 hours to complete

Purchasing a Tester

For those who continually invest money in tools for their job, here’s a piece of equipment that is a must have item. There are several different types of electronic charging system testers out there, this one is from Mac tools. These testers vary in price from $199-$399. The great thing about electronic charging system testers, is the ability to test and have results within 2 minutes.

Having the correct tools for the job is the most important part of diagnosing and repairing automobiles. Testing the battery, starter, and alternator is an area where improper diagnosis can be costly. With the correct tester this job can be diagnosed within a couple of minutes.

Before testing the battery or any other electrical issues, making sure there is voltage present with no corrosion or poor connections is the first step into diagnosis. For more battery and electrical maintenance tips refer to the following posts:

• Battery Knowledge
• Battery Cleaning
• Check, Test, and Replace a bad car Battery

Here is a step by step procedure on how to perform the 3 individual component tests that makeup your charging system. The tester used in this article is the Accuracy Plus tester from Mac tools.

1-The Battery Test

Hook up the battery tester color coded clamps. Red to the positive terminal of the battery and black to the negative terminal of the battery.

Once powered up, the tester will guide you through each step of testing.

The first thing the tester will ask is what the CCA or cold cranking amps are.

Input the amount indicated on the battery and press go. (Battery rating – 550 CCA)

This is the amount that the tester will “load” or take away from the battery to see if it meets the rated capacity.

After pressing go, the tester will load the battery two times within 20 seconds. Then it will show “battery good” or “battery bad” on the display and the actual amount of voltage and CCA’s tested. It’s that easy.

Battery condition good.

Test results, 12.6 volts and 510 cold cranking amps.

Battery Test Results

If this battery was bad, it would show low cranking amps. 510/550 is a good test result. If the voltage tests lower than 12.0 volts, the battery should be charged and re-tested.

2-Starter Test

Immediately following the battery test, the tester will instruct you to start the vehicle.

At this point the starter current draw will automatically be tested. (The starter results will show up at the end of the last test)

Starter Test Results

Good test results should be no more than 150 amps of current draw. Any amount over 200 amps could put excess drain on the battery. This could cause problems during the winter months and may even shorted the life of the battery over time.

3-Alternator Test

This is the final of the 3 tests for the complete charging system.

With the vehicle running, you will be asked to press go and increase the throttle to 1500 RPM. During this time (approx. 15 seconds) either jump in the car and rev up the engine…

…or look for the throttle cable on the engine. Increase the RPM slowly until the tester tells you to maintain the RPM.

The alternator will automatically be tested by simulating the maximum amount of load devices that could be turned on from your vehicle and record the output. It will also test the internal voltage regulator functions as well.

Here is the measurement of output from the alternator under a load test. (14.2 volts)

Alternator Test Results

Good results would be an increase in voltage by approximately 2.0 volts. (12.5 volts before starting, 14.5 volts running) The tester will show “diodes good” or “diodes bad” as a result also.

Final Results

When the test is finished, you will be instructed to turn the vehicle off. The complete evaluation of the charging system will scroll through for you to read. Here are the items tested:

• Battery load test – with voltage and CCA results
• Starter current draw test – with amount of amperage and voltage used
• Alternator output test – results with load and no load voltage and amperage output
• Alternator/Regulator – internal function test of the voltage regulator diodes

Final Thought

This tester is one of the most user friendly testers available. It’s also one of my favorite pieces of test equipment to use. If there are any electrical problems found on the vehicle being tested, the tester will tell you and stop with any further tests. This allows you to replace the faulty part and retest after the problem is fixed.