How many ohms should an automotive relay have?

[MacDerek]

A standard automotive relay coil should have a resistance between 50 and 120 ohms. This is the industry-accepted range for most common 12-volt relays used in vehicles. If your multimeter reads outside this range, or shows an open circuit (infinite resistance/O.L.), the relay coil is faulty and the relay needs replacement.

The coil resistance is critical because it determines the current draw. For a 12V system, a coil with 70-80 ohms resistance draws about 150-170 milliamps, which is safe for switches and control modules. This specification is defined by standards from organizations like the Society of Automotive Engineers (SAE) and individual manufacturer engineering specifications to ensure reliability and compatibility across vehicle systems.

To measure it correctly, set your digital multimeter to the ohms (Ω) setting. Touch the probes to the two coil terminals—typically numbered 85 and 86. The reading should be stable and fall within the expected range. A reading of 0 ohms indicates a shorted coil, which can blow fuses or damage the control circuit. A reading of O.L. (over limit) means the coil wire is broken internally.

It's useful to know typical values for different relay types:

Relay Type / Common Application	Typical Coil Resistance Range	Notes
Standard ISO Mini Relay (e.g., headlights, fuel pump)	70 - 90 Ω	The most common range found in vehicles.
Micro Relay / Low Profile Relay	80 - 120 Ω	Often used in newer, compact fuse boxes.
High-Current Relay (e.g., starter solenoid)	50 - 70 Ω	May have a lower resistance for stronger magnetic pull.
Faulty Relay Diagnosis	Reading	Implication
Shorted Coil	0 - 10 Ω	Will cause excessive current draw, potentially blowing a fuse.
Open Coil	O.L. (No Continuity)	The relay will not activate at all; no click sound.
Out-of-Spec Coil	< 50Ω or > 120Ω	Unreliable operation; may stick or fail to engage contacts properly.

Environmental factors like heat under the hood can cause resistance to drift slightly over years, but a significant deviation points to failure. Always compare your reading to the manufacturer's specification if available, as some specialized relays (e.g., for 24V trucks or specific ECU controls) may have different values. The 50-120 ohm rule, however, covers over 95% of standard 12V automotive applications, making it a reliable first-pass diagnostic benchmark trusted by technicians.

[DeDominic]

As someone who fixes my own cars, here’s my quick check: grab a multimeter. A good relay coil will almost always read between 70 and 90 ohms. That’s your green light. If I see anything below 50, I know the coil’s probably shorted and drawing too much power. If it’s way over 120 or the meter shows nothing at all (that’s an open circuit), the coil’s broken. In both bad cases, the relay is dead. I don’t overthink it—if it’s not in that 50-120 zone, I just swap in a new one. It’s the fastest way to rule out a relay problem.

[MacLila]

In my shop for the last twenty years, we’ve tested thousands of relays. The spec is clear: you want that coil resistance between 50 and 120 ohms. I’ve seen relays that still click but have a resistance of 140 ohms; they get weak, the contacts don’t make good connection, and you get an intermittent fault that drives owners crazy. The meter doesn’t lie. A reading of zero? That relay took out the fuse for the circuit. No reading at all? The fine wire inside the coil snapped from vibration or heat. We keep a known-good relay on the bench just to compare readings. Sticking to the numbers saves hours of diagnostic time.

[McMatteo]

If you’re not a mechanic, just remember this simple rule: a healthy car relay should measure between 50 and 120 ohms when you test the two smaller pins. This isn’t a random number—it’s engineered so the relay switches reliably without overloading your car’s electrical system. If your test shows a value way outside this range, or shows no number at all, the relay is defective. This is a very common failure. Telling your repair shop that you’ve checked the relay’s coil resistance and found it out of spec shows you understand the issue and can lead to a faster, more accurate repair.

[EmilianoLee]

The 50-120 ohm standard is a direct result of electrical design for 12-volt systems. The coil needs to be strong enough to pull the contacts shut reliably but must limit current to a safe level. Let’s break that down.

A lower resistance (like 50 ohms) means the coil draws more current—about 0.24 amps. This creates a stronger magnetic field, useful for heavy-duty relays. A higher resistance (like 120 ohms) draws less current, around 0.1 amps, which is easier on control modules.

The actual value depends on the wire gauge and length of the coil winding. Manufacturers choose a point within this broad range to balance performance, heat generation, and compatibility. Temperature affects it too; resistance increases slightly when the relay is hot under the hood.

So while 80 ohms might be “typical,” the entire range is normal. The key is consistency—any drastic deviation from this band indicates a physical problem with the coil winding that compromises its function.