G2: DIY: Diagnosis: Testing the Radiator and Condenser Fans

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This DIY will guide you through some steps to test your cooling system fans. The cooling fans should turn on when the sensor in the bottom passenger side of the radiator detects that the coolant temperature has exceeded 172°F, or when the air conditioner is turned on. If your fans are not running when they should (or are running when they shouldn't), these steps will help locate the problem.




First you'll want to make sure that all of the required fuses are good. You can simply replace the fuses with known-good ones, or test them. For this guide, you will want to check fuses 3, 15, and 20 in the under-dash fusebox, and 47 and 50 in the under-hood fusebox. Refer to this page for locations of these fuses: G2: FAQ: General: Fuse And Relay Locations.

There are two methods for testing a fuse, you will need a multimeter for both. A basic/cheap multimeter is show here, to the right. For both of these tests, you will install the black and red leads as labeled in the image. You can also visually inspect the fuse but a blown fuse is not always obvious so it's good to double-check with a multimeter.

Method 1: Voltage


To test fuses using this method, you should either start the engine or at least turn the ignition key to IG2. This is to make sure that all fuses are powered and should be receiving voltage. Set your multimeter to test DC Voltage. If you have one that is auto-ranging you probably only have one setting for DCV, use that. If you have one like shown above, you will need to set it to the correct range. We are testing 12 volt circuits, so you will set the meter to the point labeled "20" within the DCV section.

You will notice that every fuse has two test contacts on them. These are what you will be touching with the test probes. Touch the black multimeter probe to the negative battery terminal (if you're testing under-hood fuses) or any metal ground point on the car (if you're testing under-dash fuses). Locate the fuse you want to test. Touch the red probe to either of the two test contacts on the fuse and note the value on the multimeter. It should be roughly 12 volts. The exact voltage is not important for now, as it will vary with the level of charge in the battery, the output of the alternator, etc. Now touch the probe to the second test contact on that same fuse, and again note the value on the multimeter. If you got a voltage reading of 0v on one contact but 12v on the other, that fuse is blown and needs to be replaced. If you got 0v on both contacts, make sure your black lead is properly grounded to the vehicle body/frame or negative battery terminal. If you still get 0v on both contacts, that fuse is not receiving power and further testing will be required. If you get ~12v on both contacts, the fuse is good and you can move on to the next fuse you want to test.

Method 2: Resistance


To test fuses with this method, you will want the engine off and the key removed from the ignition. Remove the fuse(s) you want to test. Set your multimeter to test Resistance or Ohms (Ω). Again, if your meter is auto-ranging you will only have one dial setting for resistance. If your multimeter has multiple options, for our purposes you can set it to any of them. Some multimeters may also have a setting for Continuity, if yours has this setting you'll want to use that instead. Hold the test probes apart from each other and note the reading on the multimeter display. This reading means "infinite resistance", or an open circuit. Each multimeter is different in how they signify and open circuit, so remember what yours shows. Now touch the two multimeter probes together and note the reading on the meter. This is a closed circuit (ideally 0 resistance or 0 ohms, but a little bit of resistance is OK). If your multimeter is set to test continuity, you may hear it beep when you touch the probes together.

Now touch the black probe to either of the test contacts on the fuse you removed, and touch the red probe to the opposite contact. You should get the same reading you got earlier when you touched the two probes together (closed circuit). This means the fuse is good. If your multimeter shows the same result as when you held the probes apart (open circuit), the fuse is blown and should be replaced.

Fan Motors, Relays, Wiring


There are three relays used by the fans: Radiator Fan Main Relay, Radiator Fan Relay, and Condenser Fan Relay. Be sure you don't confuse the Radiator Fan Relay and Radiator Fan Main Relay while doing the tests below. They are all in Relay Box A which is attached to the upper drivers side corner of the radiator, next to the battery and are pictured here. The following tests will confirm that your fan motors are working properly and that these relays are all good. If you find a problem with any of these tests, it is probably unnecessary to continue with the rest of them. Repair whatever problem you find, and then see if your fans are working again.

With the engine off and key removed from the ignition, remove all three fan relays and set them aside. With your multimeter set for testing voltage (see Method 1 of testing the Fuses above if you don't know what setting to use), touch the black probe to the battery negative terminal. You will keep the black probe here for all tests unless otherwise specified.

Touch the red probe to the White wire of the Condenser Fan Relay socket. If you do not have voltage on this wire, double check fuse #50.

Touch the red probe to the White wire of the Radiator Fan Relay socket. If you do not have voltage on this wire, double check fuse #47.

Put the key in the ignition and turn it to ON (IG2), do not start the engine.

Touch the red probe to the Yellow/Black wire of the Radiator Fan Relay socket, then the Yellow/Black wire of the Radiator Fan Main Relay socket. If you do not have voltage on either of these wires, double check fuse #3. If you have voltage on one and none on the other, remove the key from the ignition and test for continuity between these two Yellow/Black wires.

Sound your horn just to confirm that it does work. Remove the Horn relay and set it aside. Take the Condenser Fan Relay and put it in place of the Horn relay. Make sure the horn still sounds. Do the same with the Radiator Fan Relay. If the horn does not sound for either of these (or the horn sounds constantly as soon as the relay is installed), replace that relay.

Disconnect the negative battery terminal. Reinstall the Radiator Fan Main Relay. Take a paper clip and unbend it (or preferably make a short test jumper with male spade terminals on each end: like this), and insert one end into the White terminal of the Radiator Fan Relay socket, insert the other end into the White/Blue terminal of the same socket. Touch the negative battery terminal to the negative battery post for no more than a few seconds. The passenger side fan should begin to run. Remove the negative battery terminal again, do not leave the fan running like this, it could melt the paper clip or your jumper wire. Repeat this test using the White and Pink wires of the Condenser Fan Relay. This time both fans should run. If your fans ran as they should, this confirms that each fan motor is working and the wiring from the relays to the fans is good and the Radiator Fan Main Relay is at least partially working.


Coolant Sensors

There are three "coolant temperature sensors" in the Legend. Only one of them relates to the operation of the fans, but I will explain all three here anyway to eliminate confusion over which does what.

Radiator Fan Control Sensor

This sensor is located in the bottom passenger side corner of the radiator. It detects the temperature of the coolant and signals the Fan Control Unit (FCU) to turn the fans on at low speed when the coolant is above 172°F, and high speed at 194°F. They will remain at high speed until the temperature drops below 183°F. If this sensor detects temperatures above 230°F, the FCU will disable the AC compressor clutch.
G2 RadiatorFanControlSensor.jpg

Engine Coolant Temperature Sensor (ECT or CTS)

This sensor is on the passenger side of the water passage between the intake manifold and throttle body. It sends the detected coolant temperature to the ECU. The ECU uses this information to run its calculations when there is need to compensate for the engine temperature. It is also known as the TW Sensor in the EDM service manuals.
G2 CoolantTempSensor.jpg

Engine Coolant Temperature Sending Unit

This sensor is above the upper radiator hose, just behind the bleeder bolt. It is used by the coolant temperature gauge in the instrument cluster.
G2 CoolantTempSender.jpg

Oil Temperature Sensor

This sensor is used by the Fan Timer Unit (FTU). When the oil temperature is above 214°F, the FTU will turn both of the fans on at low speed after the engine has been turned off.
G2 OilTempSensor.jpg

Control Modules

There are two control modules that are required for proper operation of the cooling fans. The Radiator Fan Control Module (RFCM) and the Fan Control Unit (FCU). Another frequently referenced part is the Fan Timer Unit, although this is not actually a separate control module, it is only a part of the RFCM. Both control modules are located under the dashboard on the passenger side: the RFCM is behind the right kick-panel, the FCU is mounted to the underside of the ECU cover underneath the carpet. They are not easily tested, but like the Legends PGM-FI Main Relay, they are known to go bad with cracked solder joints. If you are familiar with soldering, you can disassemble them and resolder any bad joints. You can also replace them with modules from the junkyard, they should be very cheap but you won't know for sure that they are good.
G2 RadiatorFanControlModule.jpg G2 FanControlUnit.jpg

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