Automotive Bypass Relays Made Simple: What They Do in Your Car

Over my almost 60 years in the auto repair industry as an ASE and GM Master Technician, and vocational education automotive technology instructor, I have seen firsthand how vehicles have evolved from basic transportation to super complex high-tech wonders. Sophisticated electronic systems have made cars inherently safer, drastically increased fuel mileage, and made them more fun to drive. Despite all of these benefits, these advancements have also made modern cars extremely challenging to diagnose and repair.

One of my trusted experts, ASE and Mazda technician Tom Diamond, recently introduced me to automotive bypass relays. These are inexpensive tools every DIYer (and technician) should have in their toolbox; and had me saying, “Why didn’t I think of this!” Read along as ASE Master Technicians help explain the benefits—and limits—of automotive bypass relays.

What Is An Automotive Relay, and What Does It Do?

In the simplest terms, a relay permits a small, independent, low-current circuit to control an independent, high-current circuit. For example, when you turn on the rear window defogger switch, low current from a small five- or ten-amp fused curvy energizes a coil or electromagnet inside the relay. When the coil becomes energized, it “pulls” the contact on the high-current side of the relay “closed.”

Because the other side, or the high current circuit, protected by its own 25 or 30 amp fuse, is always “hot” or “on” once the relay contact closes the circuit is now complete. High current then flows out of the relay to the rear window defogger grid wires to heat up the glass. This allows manufacturers to install smaller gauge wires inside the dash, steering column and fuse/relay block(s).

A popped or blown fuse in either circuit, a broken relay contact, or a defective coil will keep the circuit from functioning. Stuck or corroded contacts or coil can keep your window defogger grid from turning OFF and drain the battery. Use your owner’s manual to locate the in-car and engine compartment fuse/relay block(s) to identify the relay for the system that is not functioning properly. And that’s the beauty of having a bypass relay to help you discover the problem.

• Note: Most rear window defogger relays have a built-in timer that will turn the relay off after a certain time to keep the glass from overheating.

What Is An Automotive Bypass Relay?

For diagnostic purposes, automotive bypass relays allow you to “bypass” or override a control circuit independently of its input/control system. “Automotive bypass relays are used in place of the relay in the circuit that is going to be tested. The relay can be turned off and on by hand and helps protect the system that is being worked on,” says ASE Master Technician Frank Roeder. Roeder added, “Automotive bypass relays can be used to help diagnose complex climate control systems, trailer wiring or something as simple as a horn circuit.”

How Does An Automotive Bypass Relay Help Diagnose An Electrical Problem?

An automotive bypass relay can test if a component is functioning correctly, as well as test the relay at the same time. Diamond says, “Automotive bypass relays are really helpful when a fuse [or] circuit breaker keeps popping intermittently for no obvious reason. You can place a clamp amp probe/meter on main feed wire in the circuit, then power the circuit on using a bypass relay to observe if the component is drawing too much amperage after running for a while.”

When Would I Need an Automotive Bypass Relay?

Automotive bypass relays were developed to help troubleshoot relay-controlled circuits in your vehicle’s electrical system. Bypass relays temporarily override the control system side of a circuit. By directly powering on a fuel pump, radiator cooling fans, air conditioning compressor clutch, blower fan motor (especially if the high blower resistor has failed more than once), or other components, you can quickly verify if they’re functioning correctly.

Benefits Of An Automotive Bypass Relay

“You won’t have to carefully pry open a relay’s housing to [visually] check the mechanical contacts, or springs, to see they are broken or welded together,” says Diamond of the benefits of using an automotive bypass relay. My biggest pro is that you won’t have to use jumper wires to make a connection between the input and output sides of the circuit. Bypass relays eliminate the danger of jumping the wrong terminals in the fuse/relay block(s).

Another trick from Diamond: “[Installing [a bypass relay] in place of the fuel pump relay can make emptying the gas tank easier and safer.”

Roeder adds, “Bypass relays are useful when modifying or making custom modifications [such as installing high-powered sound systems, light packages, or a winch] to help keep existing wiring systems from overloading.”

Precautions Of Using An Automotive Bypass Relay

Everything has its limits. Don’t mistake these devices for fixing the problem. They are made to diagnose a circuit, not replace a defective relay, and can be dangerous if permanently left in place.

In my experience, using the wrong test light (only use a high impedance test light) or connecting the wrong relay terminals in the fuse/relay block(s) using a jumper wire can instantly ruin solid state/low current components, for example, the Body Control Module (BCM), that powers the low amp side of a circuit. Sometimes, shorting out a module or control circuit component will damage a perfectly good relay after you reinstall it. That is the reason most automotive parts stores and dealership parts departments will not take back a relay if it’s been used.

My inexpensive set of bypass relays are not fused, and a few are rated at 10 amps, so if you have a really high draw circuit, the switch, coil or contacts inside the bypass relay can fail.

FAQ

What is the difference between an automotive relay, fuse and circuit breaker?

Relays control devices; fuses and circuit breakers are circuit and device protectors. To prevent damage to a circuit from short circuits (when a wire carrying voltage goes to ground before reaching the intended device) or overloads, fuses and circuit breakers are designed to “open” or “break” a circuit if the current (amperage) flowing through the circuit exceeds their load limits. Once a fuse “pops” it must be replaced with a new fuse having the same amp rating.

Unlike the circuit breakers in your home’s electrical service panel, automotive circuit breakers cannot be reset once they “trip.” If the load on the breaker returns to normal levels, the circuit breaker will reset itself, and fingers crossed—it was a one-time glitch—not a part starting to fail. The automotive aftermarket does offer manual resettable circuit breakers to help protect expensive, high-powered sound systems, light packages, etc.

Experts

• Frank Roeder has 46 years’ experience in the automotive repair and custom car industry. He is an ASE Master Technician, General Motors Master Technician, PA Department of Transportation (PennDoT) inspection and emission inspector and is currently employed at J&E Performance and Fabrication located in Chalfont, PA. Interview Apr 18, 2025.
• Tom Diamond has over 35 years of experience in the automotive repair industry as a technician, certified original equipment manufacturer (Mazda) trainer and shop supervisor. He has been an ASE Master Technician for over 25 years, including ASE L1 advanced automotive engine performance certification and is a PennDoT certified state safety, emissions and reconstructed vehicle inspector as well as a certified welder. Interview Apr 14, 2025.

Resources

• Bill Kirkpatrick, ASE and Volkswagen Master Technician; original interview for Heater Blower Motor Replacement, Explained By an Expert
• Dorman Products: “Shop Press” (Apr 3, 2025)
• ASE Webinar: EV Contactors and Interlock Circuits” (Oct 8, 2024)
• Special thanks to the administration, staff and students from the Faulkner Automotive Institute, at the Career Pathways Academy, Father Judge High School, Phila., PA. for all their assistance preparing this story.

The post Automotive Bypass Relays Made Simple: What They Do in Your Car appeared first on Family Handyman.

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