Mechanics

Relay Test

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Your car is constructed with many electrical relays. These control relays are used like a switch that utilizes an electrical trigger signal to activate. Once a relay is activated it will connect an electrical supply to a particular accessory. These accessories can range from the main computer PCM (powertrian control module) to a fuel pump. Most control relays work on the same principle even though their terminal configuration can vary slightly. There are two tests that need to be considered when dealing with a relay problem. Is the problem with the actual relay or the power, ground or trigger circuits that activate the relay. In this car repair guide we will test the relay contacts, operation and activation circuits. A relay is more prone to failure when it is warm. A rely can be considered into two separate halves.

The first or primary half of the relay utilizes an electromagnet to close the secondary electrical circuit inside the relay. This electromagnet is activated by a simple power (+) and ground (-) much like a light bulb circuit. The second half of the relay is the "switch" that controls the power to a particular accessory like a fuel pump or ignition system. So, to re-cap, if you activate the primary side of the relay (electromagnet) it closes the contacts (switch) to supply power to operate an accessory. The relay primary side activation can be done manually, example: headlight switch on the dash activates the headlight control relay to supply power to the headlights.

Or the relay primary side activation can be done automatically, example: When the PCM (computer) commands the fuel pump to turn "on" it will send an electrical signal to activate the fuel pump relay. The secondary side of the relay is the "switch" and does exactly as it is described; turn "off "and "on" power to an accessory. The guide below covers the most popular wiring configuration. Most relays are located in your car's fuse or power distribution center, to find a particular system relay for your car consult your owner's manual or a car repair manual.

Car System Relay
Typical Car System Relay

Relay Terminal Identification

  • Terminals 86 and 85 are the primary half of the relay that utilizes an electromagnet to close (connect) the secondary electrical circuit inside the relay. This electromagnet is activated by a simple power (+) and ground (-) much like a light bulb circuit.
  • Terminals 87 and 30 are the secondary half of the relay that is the "switch" that transfer's electrical current from one terminal to the other.
  • Terminal 87a is not widely used and does not need to be connected for the relay to operate. 87a can be used for many different things such as relay activation monitoring or connecting a separate circuit that uses power when the relay is not in use.
Testing and Checking a Control Relay Guide:

Tools needed to perform these tests: Test light or a multi meter, a small piece of wire and a small automotive bulb and socket. (A car side marker bulb and socket work great because it has small wires attached)

Step 1 - Anytime you have a problem with electronically controlled components such as a control relay for an engine, transmission, ABS brake, or SRS (supplemental restraint system, air bag) inspect all fuses using a test light and check the under hood power distribution center and under dash fuse panels. A fuse supplies power to operate the relay and to the controlled component. If all fuses test ok continue to the next step.

Step 2 - To check the relay operation have a helper turn the ignition key to the "on" position and then to the "crank" position while your fingers are on the relay in question. Either when the key is moved to the "on" position or when the starter engages your fingers should feel a click beneath your fingers. If not, remove the relay and inspect the connections, if corroded or overheated repair and reassemble with a new relay to recheck the operation, if the system is still not functioning properly proceed to next step.

Step 3 - Connect a test light or multi meter to ground (black lead). Turn the ignition key to the ''on' position (engine off) and remove the relay. Using the probe test all terminal sockets in the relay connector, two out of the four should have power. If power only exits at one terminal or no power exists re-check related system fuses. If all related fuses are ok and you still have no power a wiring diagram from a car repair manual is needed to trace the wiring back to the power source to be repaired. If ok proceed to next step.

Step 4 - With the ignition key in the "off" position take a small piece of wire (14 to 20 gauge) and strip it on either end. Insert each end of the wire into the 87 and 30 relay block terminals and turn the ignition key to the "on" position. The relay is now bypassed so the accessory that it controls should now be on. Example: if you are testing the radiator cooling fan relay, the cooling fan should be operating. If the accessory the relay controls are not operating, in this case the radiator cooling fan, use a test light and check for power at the cooling fan motor harness. If no power exists there is a short in the wire or connection between the relay power supply and the cooling fan motor. If power is present at the cooling fan motor, test the ground wire at the motor harness with the test light still grounded. If the test light doesn't illuminate the accessory or fan motor has failed and replacement is required. If power is present at the fan motor ground wire, the "ground" has failed and repair is needed. If all tests "ok" proceeded to next step.

Step 5 - Connect the test light lead to the positive battery terminal, this is done to test the relay "trigger" circuit. With the key in the "on" position (engine off) use the test probe and insert it into the either the 85 or 86 relay block terminal (whichever one did not have power). Next, command the relay to operate, this operation will vary depending on the relay being tested. Example: If you are testing a relay that can be manually controlled like a headlight relay, simply turn the headlight switch to the on position, the test light should illuminate. If you are testing a relay that is automatically controlled like fuel pump relay, crank the engine over and observe the test light, it should illuminate, if not consult a wiring diagram from a car repair manual to trace the wiring to the source. (note: control relay power and ground configurations may vary depending on application, if you are unsure of the configuration of a relay consult a wiring diagram from a service manual)

Step 6 - This test will help diagnosis intermitted failures which are common for relays and relay control circuits. Remove the relay in question, take a small wire strand about two inches long and insert it into the relay connector for the power or ground being tested. Next reinstall the relay while keeping the wire strand inserted and clear of any other terminals or grounds. With the wire strand secured in the relay terminal attach one wire of a small automotive bulb and socket. (A car side marker bulb and socket work great because it has small wires attached). The remaining wire of the bulb and socket attach to power or ground depending on your test. Example: if you are testing the relay ground trigger circuit, attach the remaining bulb wire to ground and vice versa. The bulb will now illuminate when the relay is use, and go out if a circuit fails. (Note: install the small automotive bulb in an area you can see while driving, taped to the hood or dash works well)

Common Problems:

  • As the relay heats up in normal operation, the electrical contacts inside the relay can short circuit causing the electrical flow to stop. When the relay contacts cool it will resume the flow of electricity. To test for this problem attach two test lights to terminal 30 and 87. Attach the test lead to the both terminals, a small piece of wire works well for this. Reinstall the relay and monitor the test lights while in operation. If one of the test light bulb goes out while in operation replace the relay. If both test lights go out trace the main power supply to that relay, consult a wiring diagram from a car repair manual and repair as the short circuit as needed.
  • If too much amperage is being drawn through a relay circuit it can cause the relay contacts to "stick" not allowing the power to be shut off to the accessory. Example: When an ABS system motor ages it will draw excessive amperage cause the control relay to "stick". This condition will run down the battery in a short time until corrected.
  • Moister can get inside a relay hindering the relay operation.
  • While testing for power, ground is contacted causing the fuse for that circuit to fail.
If further assistance is needed, our certified car repair technicians are ready to answer your car questions. Also, gain manufacturer specific instructions and information by clicking - Auto Repair Manual

Related Car Repair Information

Have a helper crank over the engine while you place your fingers over the relay, does the relay click under your fingers? If so the relay could be working, there is a chance the relay has burned contacts inside causing the problem.

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AUTHOR


Written by
Co-Founder and CEO of 2CarPros.com
35 years in the automotive repair field, ASE Master Technician, Advanced Electrical and Mechanical Theory.


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Article first published (Updated 2014-07-30)