schematics and diagrams: How to test relays on car?

how to check relays and the switches that operate them

Quick tip: If you don't care what a relay is and just want to try a quick test, you can swap the questionable relay with another one. If the relay has the same number of terminals, it is generally interchangable. Note: when turning on the ignition switch to see if a relay is working, you may need to remove other relays that may also click with the key on to avoid confusion about which relay is clicking.

Also, the horn relay is a handy one. You don't need it for the engine to work, so you can always borrow it to fix a bad one and replace it later. Plus, if you suspect you have a relay that is shorted on, you can push it into the horn relay socket to see if it blows the horn without pushing the horn button. The horn relay and socket is a great troubleshooting tool!

What is a relay? A relay is used to turn something on that needs a substantial amount of power. It may be something that you use manually, like the horn, or something the car's computer turns on, like headlights that come on automatically. Here are some typical components that use relays: A/C compressors, brake lights, radiator fans, headlights, fuel pumps, and horns. Below is a typical relay out of the engine compartment relay/fuse box:

How does it work? A relay uses a smaller "command" signal to close a switch that can carry a larger current. The "command" is usually just a 12 volt signal from, say, the brake switch in above your brake pedal. By pressing the brakes, you flip a small switch that connects to the relay. The relay then closes and makes a much larger current available to the brake lights. To work, a relay needs 4 terminals as shown below:

The command terminal on the left is connected to the switch (say your brake switch) that is turning the relay on. The current this causes in the coil creates a magnetic field that closes the contacts. The contacts thereby connect the hot input to the load terminal, which is connected to your brake lights or whatever is being turned on. The brakes lights, having their own ground connection to the chassis, will then light until the command signal is removed from the relay.

Note that the command signal can be negative or positive. Buttons that commonly use a negative (ground) signal are the horn and door switches. If the command signal is negative the other coil terminal must be supplied with positive voltage and vice versa.

Now that you understand the relay's operation, you are ready to troubleshoot your circuit. There are two things to check for at the relay.
1. Is the relay receiving the command signal?
2. Is the relay working?

Note that if the command signal is positive, you don't have to know which terminal is which to troubleshoot. There is only one hot terminal when the command signal is absent (may be always hot or hot with the key on, depending on what the relay goes to--for example, the brake lights always have a hot input so they will work even when the key is off). When the command signal is present, there are 2 hot terminals. So by turning the switch on (by say holding the brake pedal down), you should have 2 hot terminals in the relay socket.
Using a test light (available for a few dollars at your parts store or local WALMART) with the clip connected to a ground point, test all terminals in the socket. The two photos below shop a typical test light connected to a chassis ground and then being used to check a relay socket for the presence of a command signal.

If you find 2 hot terminals, the command switch is working.
In the case of a negative command signal, you actually have to verify that the ground is present when the switch is on. For example, to troubleshoot the horn switch, you would need to check each pair of terminals in the socket with the horn button on and off to see if the command signal is coming through. Another approach would be to use an ohmmeter to check for the negative command signal by verifying that the command teminal has no resistance to a ground point.

Next, with the command switch still on, push the relay back into the socket and listen for the contacts to click as you do. If you hear a click, the relay is probably working.
Finally, you can check the wiring between the relay and the load by checking the connector terminals at the load. For example, remove the connector from the horn and have someone hold the horn button on while you use your test light to check the terminal(s) in the horn connector--there should be one hot terminal.

Failure modes: Now you know how to troubleshoot a relay circuit, but there are a few more things worth knowing, depending on what your car's issue is. The relay can fail open or closed. If any of the terminals becomes disconnected inside the relay, it will fail open--that is to say it will not supply voltage to the load. Other ways the relay can fail open are if the coil shorts itself and burns open or the contacts become damaged from arcing, thus no longer making a good contact.
If the return spring breaks and no longer pulls the contacts apart when the command signal is removed, the relay can fail closed, thus providing voltage to the load even in the absence of a command signal. Another way the relay can fail closed is if the contacts become fused due to extended arcing. A fail closed mode is very uncommon, and the condition of "uncommanded" voltage at the load is more often caused by a failed closed command switch (for example, your horn button being stuck on).
Trade Tricks: Note that the relay coil does not care about the polarity of the voltage applied--it will pull the contacts closed no matter which end of the coil has the positive voltage applied. The industry, therefore, has generally used this to design the sockets and relays such that the relays can be rotated 180 degrees without changing the function. See the figure below in which I have swapped the position of the Command and Load terminals to accomplish this purpose. Industry has also generally made all 4-pole relays that will fit the same sockets of the same power ratings, so that you can swap and rotate relays all you want without causing problems.
Diagram of a relay that can be rotated 180 degrees: