Many instrument clusters include built-in voltmeters, and some vehicles include dash mounted warning lights that illuminate when alternator output drops below a set level.
These are great. Convenient. Easy.
And it’s almost as easy to check the voltage output of your alternator with a handheld voltmeter.
But voltage doesn’t tell the whole story.
Any responsible and thorough alternator testing procedure must also include a measure of the amperage output.
A clamp-on ammeter is a great tool for completing an accurate test.
Testing the actual output voltage.
Dashboard warnings and voltmeters are convenient, but convenience comes by sacrificing completeness.
Instrument cluster voltmeters can provide faulty or inaccurate readings.
Our best advice is:
Don’t rely on built in monitors.
Your business stands to lose more money if an alternator fails on the road than it does by implementing a regular, end-of-use voltage output test.
Check alternator output voltage with the engine running and all accessories powered off.
You can check the alternator output at the battery only as part of completing a thorough test.
The positive lead of the voltmeter should connect to the positive output terminal on the alternator.
The negative lead should be grounded. This can be any metal component with proper grounding, with the exception of the alternator housing itself.
Output voltage should be around 14 volts (13.8 to 14.5 depending upon the application) with the engine running.
If you check the voltage at the battery under the same parameters, you should not have more than a half volt drop between your result at the alternator and the result at the battery.
If the voltage is higher by more than 4 volts, the regulator may need repairs or replacing. If the voltage is low, the alternator may be on its last legs.
Low voltage also comes from poor electrical connections, a loose drive belt, low engine idle speed, or poor ground.
Alternator amp output test.
When you test the output voltage of your alternator, always include an amperage output test.
You can purchase an electronic alternator amperage tester for your shop use a clamp-on ammeter.
Do this properly:
Clamp the ammeter around the positive output wire on the alternator.
Connect the carbon pile to the battery.
Zero out the ammeter (connect as above and perform the test with the vehicle off).
Start the vehicle and hold the engine at around 1,500 rpm.
With the engine held, the carbon pile can simulate an electrical load.
Alternator output should increase, and the ammeter reading should register close to the alternator amp rating.
Bad habits happen, and it’s important to refresh your training and methods annually.
If you run a shop or employ a shop of workers to maintain your fleet, keep on your toes and reevaluate your alternator testing methods every year.
