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How to troubleshoot a control relay and a blower motor

Thursday, March 1, 2012 - 01:00
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Part two of a two-part article (IMAGE / ANNE SORBES)

Troubleshooting a control relay and a blower motor are two important electrical skills for collision repairers. This article, part two of a two-part series on electrical bug hunting, explains those tasks.

To check a control relay, a test light multimeter, a small piece of wire and a small automotive bulb and socket are needed. A car side marker bulb and socket work well because it has small wires attached. Then follow these six steps to troubleshoot a control relay:

Step 1. Anytime there's a problem with electronically controlled components such as a control relay for an engine, transmission, ABS brake or SRS (supplemental restraint system – airbag), inspect all fuses with 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. When the key is moved to the on position or when the starter engages, you should feel a click beneath your fingers. If not, remove the relay, and inspect the connections. If it's corroded or overheated, repair and reassemble it with a new relay to recheck the operation. If the system still isn't functioning properly, proceed to next step.

Step 3. Connect a test light or multimeter to the 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 of the four should have power. If power exits only at one terminal or no power exists, recheck related system fuses. If all related fuses are OK and there's still no power, use a wiring diagram from a car repair manual to trace the wiring back to the power source to be repaired. If it's 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 it controls should be on.

Example: If you're testing the radiator cooling fan relay, the cooling fan should be operating. If the accessory the relay controls isn't 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's 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 a repair is needed. If all tests OK, proceeded to next step.

Step 5. Connect the test light lead to the positive battery terminal to test the relay trigger circuit. With the key in the on position (engine off), use the test probe, and insert it into the 85 or 86 relay block terminal, whichever one didn't have power. Next, command the relay to operate, which will vary depending on the relay being tested.

Example: If you're testing a relay that can be controlled manually such as a headlight relay, turn the headlight switch to the on position, and the test light should illuminate. If you're testing a relay that's automatically controlled like a 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 the application. If you're unsure about the configuration of a relay, consult a wiring diagram from a service manual.)

Step 6. This test will help diagnose intermitted failures that 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. Attach the remaining wire of the bulb and socket to power or ground, depending on your test.

Example: If you're testing the relay ground trigger circuit, attach the remaining bulb wire to the ground and vice versa. The bulb will illuminate when the relay is in 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

Keep these four common problems in mind while troubleshooting a relay operation:

  • 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, they will resume the flow of electricity. To test for this, attach two test lights to terminal 30 and 87. Attach the test lead to 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 bulbs 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, causing the control relay to stick. This condition will run down the battery in a short time until corrected.
  • Moisture can get inside a relay, hindering the relay operation.
  • While testing for power, the ground is contacted causing the fuse for that circuit to fail.
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