To check if a relay is bad, use a multimeter to measure the coil resistance and test the switch for continuity. Then, use a 12V battery to power the coil and check if the internal switch properly opens and closes.
Common indicators that a relay is malfunctioning include unusual clicking noises, failure to activate, and intermittent operation. These symptoms can signal underlying electrical relay problems that require attention.
For a standard 12V automotive relay, the coil (pins 85 and 86) should typically measure between 50 and 200 ohms. Many common relays hover right around 75 to 90 ohms.
Yes, a bad relay can still click.
To tell if an electrical relay is bad, you can quickly swap it with a known good relay of the same part number, perform a click test with a battery, or use a multimeter to check coil resistance and switch continuity.
Contact wear and corrosion are two of the most common. Relays can develop wear and arcing damage from the repeated operation, and it can also result in circuit failure. Contact wear and arcing damage can lead to high contact resistance and short-circuiting.
In the datasheet, the pull-in voltage is listed as 80% of the nominal voltage. The nominal voltage of a 12V relay is 12V, so 80% of this is 12 x 0.8 = 9.6V. So in theory, this relay should activate at 9.6V.
Numbers of a Relay
Note that each pin is numbered. 85 and 86 are the coil pins while 30, 87, and 87a are the contact pins. 87 and 87a are the two contacts to which 30 will connect. If the coil is not activated, 30 will always be connected to 87a. You can think of this as the switch in OFF.
When using an ohmmeter (or the resistance setting on a multimeter), you are primarily checking the internal coil and the continuity of the switch contacts to ensure the relay operates properly.
Yes, you can easily test a standard 12-volt automotive relay with a 9V battery. The battery supplies enough voltage to activate the internal electromagnet (coil).
Set Multimeter to Continuity or Ohms Mode (Ω): This checks for electrical connection (resistance or continuity) between the terminals. Test the Coil: Connect one probe to pin 85 and the other to pin 86. A good relay coil should show resistance (typically between 50–200 ohms).
Listening for a Clicking Sound
As the circuit is completed, the relay should make an audible click that indicates that the electromagnet is working and is completing the circuit. If the relay doesn't make an audible click, it might mean that the relay has stopped working and isn't completing the circuit.
The main symptom you may experience if a relay is failing is a lack of power to one or more systems on your vehicle, but more specifically this could cause issues like: Failure to turn the engine over (starter relay) No fuel pressure and a no-start condition (fuel pump relay)
Understanding relay lifespan
On average, mechanical relays typically last between 1 to 5 years due to their moving parts, which are prone to wear and tear. In contrast, solid-state relays offer a significantly extended lifespan, often exceeding 15 years.
Yes, you can often fix a relay, especially mechanical ones used in cars or appliances. However, because they are inexpensive, replacement is frequently the most practical route.
To tell if an electrical relay is bad, you can quickly swap it with a known good relay of the same part number, perform a click test with a battery, or use a multimeter to check coil resistance and switch continuity.
Changeover relay
Two circuits (terminals 87 and 87a) have a common terminal (30). When the relay is at rest 87a is connected to 30, and when the relay is energised 87 becomes connected to 30 (but never both at the same time).
A 50/51 relay is an electrical protective device that monitors current to prevent damage from overloads or short circuits. It combines two ANSI/IEEE standard functions:
It takes less than 1 amp (typically between 0.15 A to 0.2 A or 150 to 200 mA) to trigger a standard 12V automotive or mechanical relay. Small logic relays or those with built-in driver modules need even less, requiring as little as 0.003 A (3 mA).
No, a 24V relay will almost certainly not work on 12V.
The 5V relay module requires a 5V signal delivered from a microcontroller or sensor to trigger the switch. Its working is also very simple. When the input pin is HIGH, the relay turns on, and when the input is LOW it turns off.
Excessive Heat
Just as cold weather and moisture can damage relays, excessive heat can also shorten their lifespan. In vehicles, for example, high engine temperatures or a poorly ventilated fuse box can cause relays to overheat, triggering electrical malfunctions.
Relays can include four types of terminals - coil, common, normally open (NO), and normally closed (NC) The coil (also known as control input) terminal is generally connected to a low-power source that controls the relay's switching mechanism.
Yes, AutoZone carries specialized relay testing tools, such as the OEMTOOLS Relay Circuit Tester, which allows you to test both the relay and the circuit without a standard multimeter.