Pulling Battery Cables To Check The Alternator

What I’m talking about is the old practice of pulling a battery cable off the battery while the engine is running to see if the engine dies due to the alternator not charging.

This is another old-school no-no! First and foremost, this was an unsafe procedure at anytime, let alone being used years ago. If you should be the unfortunate one who had a leaky battery or one that was being overcharged, building up gasses above the battery and then when removing the battery cables, you created a spark, it would cause the battery to explode in your face, throwing acid in your eyes and all over you and everything else. This of course is the worst-case scenario and is rare, but it has happened several times!

Doing this on older vehicles (1975 & older) did not result in much damage to the vehicle but it could blow a diode or a regulator if the conditions were right. Doing this on newer vehicles could potentially damage a lot of electronic components and may not even properly work because of the electrical system design, causing you to buy a new alternator when something else was the problem. Doing this on 2000 and later vehicles will damage expensive electronic components and render the car useless until you take out a second mortgage on your home to pay for the repairs!

The proper and inexpensive way to check the alternator on the vehicle is to use a voltmeter or Multimeter. Set the voltmeter on the 20 or 50-volt range (depending on the meters range selection) and measure the battery voltage across the battery terminals. A fully charged battery should read about 12.5 volts. Start the vehicle and check it again and it should read about 1 volt higher or 13.5 volts. If the battery is low due to cranking the engine and not being charged, then put a battery charger on it to build it back up to full charge, then conduct the test. If the battery won’t build up, then the battery may be your problem and will need replacing. If the battery is good but won’t show the 1 volt increase after starting the engine, then the problem is likely the alternator and you’ll need to pull it and take it to get tested and repaired or replaced.

You can find cheap voltmeters or ‘multi-meters’ just about everywhere, like part store chains, the large variety stores like WalMart Target and Sears and the electronic stores. Prices range from about $10.00 on up, depending on features and quality. You don’t need much, just any small meter will do. A Digital Multimeter is a little more expensive but easier to read. Most mechanics will buy a heavy-duty digital multi-meters unit that costs around $200. As you can see, you can spend as much as you want to on a meter, so go one and check your alternator the right way!

To sum this up, my advice on pulling the battery cable when the engine is running is ‘don’t do it!’ If you won’t heed my advice and you are going to do it anyway on older vehicles, then for your own safety, wear safety glasses, a face shield and a rubber apron to protect yourself, just in case the battery explodes!

BOOM! Get the picture?