Yes, you can diagnose your car’s charging system with a pocketknife

Kyle Smith

Certain systems of our beloved old cars require faith to understand how they work. You can’t see electricity be generated. Physically impossible. If you know how and where to look though, there are signs. I know this because a stranger in a parking lot told me so. I have been using the trick he taught me for years now.

It all started after driving my 1964 Chevrolet Corvair to lunch at Stacy’s Restaurant in Grandview Plaza, Kansas. After lunch with the family it was time to leave. I turned the ignition key to start the engine … and nothing happened. Dead battery. We collectively stood and stared, wondering what was wrong, while Dad pulled the family car around to give me a jump start.

This was my first drive after I had switched out the factory-fit generator for an alternator. The conversation was easy and one of my first projects on the car as a 17-year-old, budding mechanic. With only two wires connecting to the back of the alternator, it was an easy swap. How could I have gotten it wrong?

While we blankly stared at the engine, one of the local farmers—a regular at the diner—walked over to glance down into the engine compartment.

“What’s wrong?”

The battery died.

“Well, is it charging?”

It should be. Don’t know how it couldn’t be, the alternator is brand new.

He gave me a look up and down and asked if my pocket knife was steel. It was. He then instructed me to jump-start the car, which it easily did. Then he took my pocket knife and held it to the back housing of the alternator, after which he confidently declared: “Nope, not charging. There’s your problem.”

knife on back of Corvair alternator
Kyle Smith

The first-generation Chevrolet Corvair was like most early-’60s cars in that it had a generator rather than an alternator. There’s only a small difference between the two: A generator has a rotating armature whereas an alternator has a stationary one. Either works thanks to the Faraday law of electromagnetic induction that states “the magnitude of voltage is directly proportional to the rate of change of flux.” Sounds complicated, but it basically means that a magnetic field moving relative to the conductor creates current. This current can charge a battery or power an ignition system.

wiring on back of Corvair alternator
Two simple wires that, if installed incorrectly, make the whole thing a paperweight. Kyle Smith

The exciting parts comes when you, well, have to excite the components so that they work. Without a starter current—whose job is to “excite” a system—an alternator is just a spinning magnet. An alternator or generator is unable to build voltage without that little kick to get things working. Once excited and spinning, the current will rise with engine revolutions. If an alternator is unexcited, it will do nothing but spin. No current. More or less, it becomes a fancy belt tensioner.

The small crowd that had gathered around the Corvair gazed back at our impromptu prophet blankly. It was simple, he said: The back of the alternator will be magnetic if it is excited and, therefore creating current. He wished us a good day, hopped in his flatbed pickup, and went off to tend to the fields. At least, that’s what I assume. For a person who can conduct diagnostic tests on electrical systems with a closed pocketknife, I’m sure the world is his oyster.

Click below for more about
Read next Up next: 5 potential Pontiac project cars under $20K


    I don’t know if it could tell it was excited before you jumped it but after he could tell it was generating a magnetic field in the coils to show it was generating a current.

    Pocket knife, screwdriver, pliers, anything magnetic will do the same. I’ve known about this quick alternator test for many years. It is not a strong magnetic pull, but certainly enough to tell if it is present or not.

Leave a Reply

Your email address will not be published. Required fields are marked *