Disconnecting The Battery With The Engine Running?

The other day, while looking up a procedure for testing a fuel injector fault code, I noticed this warning about disconnecting the battery with the engine running, and it brought back memories from years ago.

Disconnecting The Engine With The Battery Running

In the first shop I ever worked at, disconnecting the battery with the engine running was something the shop owner did quite regularly. Of course, much of what we worked on in that shop was old trucks and farm tractors equipped with generators. It seemed to work for him in a pinch, although, he usually tested the system with the testers we had in the shop.

Also, the extent of the electrical system in those vehicles rarely went beyond the starter, generator, a few lights, an ignitions system and gas models—not a whole lot to damage. Still, it is questionable if disconnecting the battery with the engine running was ever a valid test even on those now antique vehicles. Rest assured, I am not the only one who has seen this done or remember hearing about it as a testing procedure. And just because I do remember it does not make it a valid practice.

Today, with computer controls and the vast array of electrical components, any kind of voltage spikes or shorts in the system can cause damage that can potentially cost thousands of dollars. Disconnecting the battery with the engine running could easily cause a spike. It is simply the best practice to follow published testing guidelines in every instance.

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About the Author

Jake Schell

Jake Schell is an editorial consultant with Mitchell 1. Previously, he served as Product Manager for the Commercial Vehicle Group from 2002 to 2023. Prior to joining Mitchell 1, Jake spent 20 years as a technician. He holds a Chevrolet Master certification in the transmission category as well as ASE certifications in both cars and trucks.