How many times has someone just grabbed an old relay and swapped it to check and see if the one in the vehicle was bad? Nearly all students are taught during an automotive or truck diagnostic competition to replace one with another to shorten the diagnostic time. Many of technicians have a drawer full of relays in their toolbox and use them every day.
The problem is, not all relays are the same. Many relays, if installed for the wrong application, could and will cause a short (Internal Relay Circuit) and most likely cause functionality issues or even damage to the vehicle computer systems. Just because it has the same number/location of terminals, doesn’t mean the relay works for that application.
Some relays, when used for the wrong application, can generate a voltage spike over 100V on a traditional 12V system.
Understand electrical circuits
Diagnosing electrical concerns can be challenging. But, with a good understanding of the electrical circuit operation and a solid troubleshooting plan, most faults can be fixed accurately the first time. Components do fail, but normally something helps or causes the failure. The challenge lies in knowing how to find these items that have a direct or indirect effect on failed parts.
In over 30 years of diagnosing, I have noticed that relays seem to be misdiagnosed, overlooked and misunderstood. Most technicians have a drawer in their toolbox filled with “known good parts.” Technicians will replace relays as an “easy” way to determine if a relay has failed.
The problem with “easy” is it can backfire on you. Just because a relay fits does not mean it should be used as a known-good part. I have diagnosed behind many technicians to find a trail of unintended damage to fasteners, circuitry, connector terminals and many high-dollar components.
Determining different relays
There are notable concerns with this dangerous method of swapping relays to troubleshoot circuitry. It is important to know how to accurately test for a damaged or incorrect relay in a system. The two common types of suppressed relays (relays that have little or no voltage spike) are resistor and diode. Both types can fail and should not be interchanged.
To clarify, swapping relays is not the only item that can cause voltage spikes, but it is a shortcut practice that can lead to avoidable damage. Some circuitry/components have suppression devices inside them and others are more sensitive to voltage spikes, but if a low-voltage circuit is continually exposed to high-voltage spikes, damage will most likely occur.
Technicians that are well-trained understand it is much easier to take the time to make a few measurements to reveal the root cause instead of being misled by swapping parts that may lead you down the wrong diagnostic path. When parts are replaced and they fail repeatedly because the root cause was never found, it can increase and extend vehicle downtime.
Make sure if you are replacing with a known good part that it is the correct one. If you suspect voltage spike damage, check for incorrect components or damaged suppression circuits.
Keith E. Littleton is president of K&D Technical Innovations, a service provider offering training solutions for industry and education. Littleton specializes in CAN communication issues and lab scope diagnostics, and is the current Station Chair for TMC SuperTech’s electrical test station. Littleton holds numerous ASE certifications, as well as nine Toyota certifications and 11 GM certifications.
