How to troubleshoot a control relay and a blower motor

Jan. 1, 2020
This article, part two of a two-part series on electrical bug hunting, explains troubleshooting a control relay and a blower motor.

Editor's note: This article was originally published March 1, 2012. Some of the information may no longer be relevant, so please use it at your discretion.

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:

  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.
  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.
  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.
  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.
  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.)
  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:

  1. 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.
  2. 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.
  3. Moisture can get inside a relay, hindering the relay operation.
  4. While testing for power, the ground is contacted causing the fuse for that circuit to fail.

Troubleshooting a blower motor

An electrical motor is used to push air through the heater and air conditioner system. If the motor fails, it won't force air through the system. Ground a test light lead to a good ground source, such as a metal brace under the dash, seat mount bolt or under-hood brace. Turn the key to the on position, and check the fuse in the heater or blower motor fuse panel with a test light. Then lightly touch the service port at the top of the fuse on both sides. If the test light illuminates on both sides of the fuse, the fuse is working properly. If the fuse lights the test light on one side, it's blown and needs to be replaced.

Starting problems, especially in cold weather on a system that worked well last summer, often can be traced to a weak battery. Subzero temperatures can cut a battery's amperage output by more than half. Check out the battery condition. If the battery is OK, look for a locked or dragging starter, which is one with high internal resistance, worn brushes or open circuits in the windings or armature.

You can use a multimeter to check for starter problems. A starter current draw test should be performed to see if you have to replace the starter. Measure the amperage on the battery-to-starter cable while cranking the engine. As a general rule, under normal load conditions, the starter should draw about 1A per cubic inch of engine displacement, plus or minus about 25 percent. Check the service manual for the specifications for your vehicle.

If the current draw isn't too high, check for resistance in the starter circuit. Starter current can exceed 200A, even 300A on large displacement engines, so even low resistance can cause a significant voltage drop. Rather than measure resistance, which is usually off scale for most DMMs, measure the current drop instead. A drop of only 0.2 to 0.3V may be enough to reduce performance in automotive circuits. Set the DMM to the millivolt scale and connect the positive lead to the side of the component nearest the battery and the negative lead to the opposite side of the component. Systematically, check the drop between the battery post and connecting cable, the solenoid posts and the wires that attach to them, and across the solenoid. Also check connections to the starter, alternator, and ground strap link to the engine block and body.

The moment you disconnect either lead from your battery, it's possible to cause thousands of dollars in damage because the battery does more than just yield electricity. It also shorts AC, spikes and transients to ground. Removing the battery from the circuit allows those spikes and transients to travel around, endangering every semiconductor circuit in the car – the ECU (the computer that controls fuel mixture, timing, and much more), the speed-sensitive steering, the memory seat adjustments, the cruise control, and even the stereo.

Even if the computers and stereo remain intact, in a great many cases, removing the battery burns out the diodes in the alternator, necessitating a new alternator. If disconnecting the battery interferes with the voltage regulator's control voltage input, it's possible for the alternator to put out hundreds of volts, frying everything.

If you disconnect the battery and the car conks out, you don't know if it conked out because of insufficient alternator current, or whether the resulting transients caused the ECU to spit out bad data, shutting down the car.

Problematic instrument cluster

Several problems in a Ford Focus, for example, appear to be the same or similar but have different sources. The driver complaint will be an erratic dashboard display. The information display may flicker, go out entirely, partly fail or create odd displays. While electronic information displays have been known to do all of these things, avoid the temptation to start tearing out the instrument cluster for closer inspection because there are a few other things to check out first.

The old Tempo/Topaz and several other models shared a similar malady: the alternator connections. Older Fords have a three-wire electrical connector at the rear of the alternator that included the battery positive lead. Because these consisted of all flat connectors, they'd work loose and/or start to wobble over time. This created a poor electrical connection because of a poor mechanical connection.

As the connection deteriorated, it wobbled to the point where the connections would start to arc. From there, the connection would deteriorate quickly. The result was a completely fried connector that usually resulted in an alternator failure and plug pigtail replacement. This was common enough that several aftermarket suppliers included a replacement pigtail along with a rebuilt alternator. You also could find them hanging in the parts store electrical section.

While the Focus connector is different, the condition is similar but with an added twist. The Focus connector doesn't usually cause an alternator failure but can cause a charging system problem.

With newer electronics, electrical arcing and poor connections create strange behavior in many electrical circuits. In the Focus, for example, this commonly creates erratic dash instrument cluster operation. An under-hood connection is creating a dash display problem. The display will come and go, operate oddly, and sometimes lie to you. Keep in mind, with the charging system warning on the dash, there's a direct tie between the two components. Combine that with frequency and rough electrical pulses, and you have the cause of the odd behavior.

Wiggle test

While a helper watches the dash, reach behind the engine on the passenger side, and wiggle and twist the alternator connector. Ford calls this a wiggle test, and it's still as good now as it was when it was named. If the dash behaves properly or becomes worse while wiggling, you've found the culprit.

However, in many cases, you don't need to replace the plug and connectors. You may be able to remove corrosion or tighten the contacts to repair the problem. Be sure to check for pin fit in the connector. Of course, replacement is always a possibility. Don't make the mistake many techs make. Don't just fix it and let it go out the door. Apply the dielectric grease to the connector to prevent any repeat performances of the problem. Dielectric is critical for the longevity of almost any electrical repair on a vehicle. Skipping the dielectric grease will cause the car to return.

The wiring harness in the hinge/hatch opening area experiences quite a bit of flexing. The obvious problems involve the rear lighting systems, lift-gate release, rear wiper and defroster, etc. But again, turn signals and warning lights tie into the dash. And once again, arcing creates frequencies and poor voltages that confuse the instrument cluster. In many cars, the first symptoms would be noticed by those following the car, but in this case the driver may see the dash act oddly before the rear lighting problems are detected.

Over time, the hatch harness wires get soft and weak. Soon, you have the hard-to-find open circuit. But the wires can rub through their insulation and create shorts. The opens and shorts are intermittent in many cases. Once again, the wiggle test is helpful.

Caution: When you find one of these problems, don't attempt to perform a repair in the hatch area. The repair splice will be stiffer than the original harness and will resist flexing when the hatch is opened, thus shortening the life of the repair a great deal and potentially causing more problems with associated wiring. Instead, remove the damaged wire from the hatch hinge area, and replace it with a new wire. Make the splices outside of the area of movement. While that involves removing several interior panels to gain access, the result will be a longer lasting and better repair.

About the Author

Tim Sramcik

Tim Sramcik began writing for ABRN over 20 years ago. He has produced numerous news, technical and feature articles covering virtually every aspect of the collision repair market. In 2004, the American Society of Business Publication Editors recognized his work with two awards. Srmcik also has written extensively for Motor Ageand Aftermarket Business. Connect with Sramcik on LinkedIn and see more of his work on Muck Rack. 

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