Many times an alternator is not working correctly because of poor electrical connections in the charging circuit. If the alternator isn’t putting out enough current to keep up with the electrical loads that are placed upon it, or the charging voltage is low, don’t automatically assume the starter or alternator is bad and needs to be replaced (unless you’re bench testing the unit out of the vehicle).
Loose or corroded connections on the back of the unit can increase resistance and restrict the current flowing through the circuit.
The same goes for broken or frayed wires inside a connector or the alternator wiring harness. The connectors and wires may appear to be OK visually, but unless they are tested, there is no way to know if they’re clean, tight, undamaged and making good electrical contact.
If the wires and connectors are not checked, you may replace the alternator only to discover the new unit you just installed is “no good.” Now you get to replace it again, and that can be a lot of lost time and labour on vehicles where the alternator is buried under a lot of other stuff.
What Signs Indicate a Weak Alternator?
Before you test your alternator, here are some signs that your alternator might be weak:
- Your battery light is illuminated on the dashboard.
- Your engine is cranking slowly or won’t crank at all.
- The battery, headlights, and interior lights seem weak.
- The alternator is making rumbling or squeaking sounds.
- You’ve tried and failed to jump-start your car.
The above symptoms could also be signs of a weak car battery.
Every alternator supplier we’ve ever talked to about this issue says the same thing: Most of the alternators that are returned under warranty have nothing wrong with them whatsoever. The alternator isn’t charging the battery because of other vehicle problems like bad wiring connections, bad battery cables, a bad battery or a bad PCM.
Save yourself the embarrassment and hassle of a comeback and test the alternator wiring connectors and wiring harness.
You can do this by using your voltmeter to run “voltage drop” checks across the connections when the engine is running. To perform a voltage drop test, set the voltmeter on the 2-volt scale and touch the positive and negative test leads on opposite sides of a connection.
If there is resistance in the connection, some of the voltage will bypass the resistance by flowing through the voltmeter. A reading of more than 0.2 volts means trouble. Ideally, the voltage drop across any connection should be zero or less than 0.1 volts.
Poor ground connections are an often-overlooked cause of low charging output and alternator failure. Check for voltage drops at the positive and negative battery cable connections, the alternator BAT+ power connection and the engine ground strap(s).
Voltage drops on the negative side can cause overcharging. Voltage drops on the positive side of the charging circuit can cause undercharging.
Another approach for reducing comebacks and unnecessary warranty returns is to ask your parts supplier to bench test your customer’s old alternator and bench test the new alternator before installing it.
If the old alternator passes the tests, the problem is not the alternator, and you’ve missed something. It’s time to get out your voltmeter and check for voltage drop in the battery and charging circuit.
When a new alternator is installed, check the battery voltage and use a battery charger to bring the battery up to full charge before returning the vehicle to your customer. Also, start the engine and use your DVOM to check the charging output of the alternator. Don’t assume everything is working OK just because you bolted in a new alternator.
Testing Alternator with Multimeter
With the engine on:
- Step 1: Open your hood, and so you have clear access to the car battery. Locate your alternator and check to be sure the alternator belt isn’t loose.
- Step 2: Turn your multimeter to the 20 V setting.
- Step 3: Start the engine. At this point, check to make sure the alternator pulley/belt is spinning correctly without slipping.
- Step 4: There are two ways to test the voltage coming from the alternator. If you have easy access to the alternator without getting your body parts/clothing / etc. tangled in the spinning pulley’s, then place the positive multimeter probe (red) and touch it to the red terminal connector coming out of the alternator. Now take the negative (black) multimeter probe and touch it to some metal part of your car frame (like a bolt head nearby or even the negative terminal on the car battery). Don’t ground to the alternator itself. That would be potentially bad. You should now be getting a reading on your multimeter display. Suppose you don’t have good access to your alternator. In that case, you can test the alternator by touching the positive multimeter probe to the positive terminal on your battery and likewise the negative probe to the negative terminal on your battery.
Testing an AlternatorIf the alternator is working well, your multimeter should read somewhere in the vicinity of 14 volts (typically 13.8-14.2). If it is reading excessively higher than 14 volts (greater than 15 volts), it is possible that the voltage regulator on your alternator is faulty or going bad.
If it is reading lower than 13-14 volts, there are several possibilities as to the reason. First, your engine idle speed may be too low for the alternator to put out sufficient voltage/power. Try revving up the engine to 2000 RPM or higher and take a reading.
If the voltage is still too low, check to be sure all the connectors on your alternator are tight and that the alternator belt is not slipping and is spinning on the pulley correctly. If it is still not putting out sufficient power, then the alternator’s voltage regulator could be bad, or the alternator itself may need to be replaced.
A car alternator needs to be putting out at least 13-14 volts (ideally between 13.8 and 14.2 volts) to charge a 12-volt car battery effectively. If the alternator is putting out too much voltage (15+ volts), your battery acid will likely boil over out of the battery. When a battery is near fully charged, the alternator will cease to put out sufficient voltage to charge the battery.
The reading you will get from the battery probing method will be the voltage coming from the battery itself. In this case, you can leave your lights on with the car off for ten or fifteen minutes to drain your battery a little bit.
It can also be helpful to test at the battery terminals with the engine off and then turn the car on and test again at the terminals. If your battery is more or less fully charged, it should read at around 12-13 volts with the car off.
If you’ve run your car for a long time and while the vehicle is running, the voltage reading is in the 13-14 volt range, but then you shut the car off, and the battery voltage instantly drops to well below 12 volts (10 volts or under; 9 volts or under in freezing weather conditions), it is likely that your battery needs maintenance or replaced.
If your tests result in normal voltage readings, but you still have issues with battery warning lights, a discharging battery, or your car’s electrical systems, further testing is needed. You may want to enlist the help of a professional mechanic who can test or replace the alternator for you.