Car Radio Fires Up Check Engine Light

My son and his new wife were preparing to drive away for a cross-country honeymoon in their 2003 Saturn Vue. About a week before they were to leave, they noticed that the engine was running rough and the check engine light had come on. They checked the car with an OBD2 code scanner and it indicated that the engine knock sensor was faulty. They replaced the knock sensor and also installed new spark plugs to be sure the car would be ready for the trip.

The car ran well for a short time. Then it started running rough and the check engine light came on again. They soon discovered that when they turned the radio on, the check engine light would come on, the engine would run rough, and when they put the car in drive, the car would not move. However, turning the radio off caused the light to go out, the engine to run smoothly again, and the transmission to work properly.

I was wondering how the factory-installed radio could possibly affect the engine and transmission control systems. I checked all of the grounds on the engine control module and could find nothing wrong. I then removed a cover under the dash and noticed a heavy copper ground braid connected between the radio and the under-dash support frame.

The braid was attached to the steel frame through a ring-type terminal lug, and the fastener that secured it was slightly loose. I removed the fastener and cleaned the steel frame under the lug -- it had a very slight film of rust. After reconnecting the ground strap, everything worked perfectly, and the newlyweds were able to make their nearly 2000-mile trip without any car problems. Apparently, this vehicle has some interconnections between sensors that depend upon the integrity of the radio ground strap for proper operation of the engine and transmission control computers.

This entry was submitted by Bill Mars and edited by Rob Spiegel.

Bill Mars is the director of research and development for DipTech Systems. He has worked in the business of designing and building dipping and coating systems for more than 40 years.

Tell us your experience in solving a knotty engineering problem. Send to Rob Spiegel for Sherlock Ohms.

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