Mastering Cooling Fan Motor Relay for Superior Performance

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Purpose

The Cooling Fan Motor Relay is an electrically operated switch that turns the coolant fan motor on and off, as commanded by the Engine Control Module or Powertrain Control Module (ECM/PCM) or, in older vehicles, the coolant temperature switch.

Location

The coolant fan motor relay in many vehicles is located on a relay block within a plastic case in the engine compartment. This relay block includes other relays and fuses. A diagram on the underside of the case cover maps the location of each fuse and relay.

On other vehicles, the coolant fan motor relay is mounted separately, often on the driver’s side inner fender, underneath the battery shelf or air intake box, or attached to the frame near the radiator. When the relay is separately mounted, it can be located by tracing the wiring from the radiator fan motor back to the fan motor relay.

Operation

A relay is an electrical switch that opens or closes another switch magnetically. Within the relay, electrical current flows through a coil of wire wound around an iron core, creating an electromagnet. 

This creates a magnetic field in the “control circuit” that pulls closed a nearby switch in the relay’s “power circuit.” This switch is spring loaded. When the current flow in the control circuit is turned off, the magnetic field collapses and the switch springs open, interrupting the flow of current through the power circuit.

Relays allow a low voltage electrical circuit to control a higher voltage circuit. In most modern vehicles, the cooling fan motor relay control circuit receives a 5 volt signal from the ECM/PCM and the power circuit operates at battery voltage of 12 volts. 

In older vehicles, a thermostatically controlled coolant temperature switch sends a current signal to the relay. When the ECM/PCM or the coolant temperature switch grounds the control circuit, allowing current to flow through it, the relay closes the power circuit and the fan turns on. When the control circuit is opened, the power circuit also opens, and the fan turns off.

Vehicles may be equipped with one or two cooling fans. If equipped with two, there are usually three cooling fan relays. Typically, one relay’s control circuit will power both fans by connecting the fans in series – current flows through the first fan to the second and then to ground. This divides the voltage equally between the fans, operating both at low speed. 

For high speed, current also is sent through the second and third relay’s control circuits. This adds a second power circuit for the second fan and removes that fan from the first fan’s circuit. The fans are now connected in parallel, each receiving full circuit voltage and, consequently, operating at full speed.

Drivability Symptoms

There are three common symptoms of cooling fan motor relay failure: a cooling fan does not run when it should, does not run at the speed it should, or does not turn off when it should. In the first two situations, the engine temperature rises when the vehicle is idling or moving slowly, but falls when higher speed increases airflow through the radiator. 

A fan operating continuously may continue operating after the vehicle is shut off. (In most modern automobiles and light trucks, however, fan operation for a brief period of time after engine shut off is normal.)

Failure of a cooling fan motor relay in a vehicle equipped with Second Generation On-Board Diagnostics (OBD2) – 1996 and later automobiles and light trucks - lights the instrument panel Malfunction Indicator Light (MIL) on the second consecutive occurrence of the malfunction.

Inspection, Test, and Diagnosis

WARNING: An electric cooling fan can turn on without warning, even when the engine is not running. Keep hands away from the cooling fan blades during all diagnostic procedures.

When a defective fan motor relay is suspected, retrieve any stored Diagnostic Trouble Codes (DTCs) using a Code Reader or Scan Tool. OBD2 “generic” DTCs are shared by all vehicle manufacturers. 

Generic DTCs P0480 through P0482 indicate a fault in cooling fan motor relay control circuits one through three, respectively. DTC P0485 indicates a fault in a relay power supply circuit.

These codes are set by a defective relay or an open or short in the circuit wiring. In addition, many vehicle manufacturers have “enhanced” DTCs specific to the manufacturer that further refine the diagnostic process.

If any of these codes have been set, remove the relay corresponding to the circuit. Verify the operation of the relay by identifying the positive and ground terminals on the relay for both the control and power circuits and the corresponding slots in the relay block into which the relay installs. (The relay may have a diagram printed on the side of the relay case and/or numbers cast into the bottom of the relays case identifying the circuits.) 

After checking for corrosion on the relay and relay block, turn the ignition switch on, engine off, and use a 12 volt test light connected to a good ground to probe the slot in the relay block corresponding to the relay’s positive power circuit terminal. If the light does not illuminate, there is no electrical current reaching the relay and the problem likely is in the wiring leading to the relay.

If the 12 volt test light illuminates, current is reaching the relay. Turn the ignition switch to the “off” position and connect a fused jumper wire between the power circuit’s positive and negative slots in the relay block, then turn the ignition key to the “on” position. If the fan runs, either the relay is defective or there is a fault in the control circuit. 

With the relay still removed, connect its positive control circuit terminal to the battery positive terminal with a jumper wire. Then, using a second jumper wire, connect the relay’s negative control circuit terminal to a good ground, such as the exhaust manifold. 

There should be an audible click as the current flow closes the power circuit switch inside the relay. If not, the relay is defective and should be replaced. If the relay functions, the fault is most likely a short or open in the control circuit wiring.

If a cooling fan runs constantly, the relay may be stuck, keeping the power circuit closed. Tap the relay. If the fan shuts off, the relay is defective. If it doesn’t, the relay is still the most likely cause. 

When the vehicle is not operating, it may be necessary to remove the fan circuit fuse to prevent discharging the vehicle battery.

Reprogramming

Replacement of a cooling fan motor relay does not require reprogramming.

Repair/Replacement

If the cooling fan relay plugs into a relay block or fuse center, replacement is accomplished by removing the old relay and inserting a new one of the same specification. 

On vehicles where the cooling fan relay is mounted separately, replace it by disconnecting the wiring pigtail, removing any attaching screws or bolts, and removing the relay. Installation is the reverse.

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