TEST 1: Checking The Radiator Fan Relay Power Circuits
Before jumping into bench testing the radiator fan motor relay, we're going to check that the relay is getting power on socket terminals #2 and #3.
Relay terminal socket #2 gets power from fuse #4 (20 Amp) in the under-hood fuse box. Fuse #4 is the radiator fan motor's power source, once the fan relay is activated of course (see photo 3 of 5).
Relay terminal socket #3 gets power from fuse #14 (10 Amp) of the under-dash fuse box. Fuse #14 provides power for the relay's coil and is one part of the relay's control circuit (the other part is the radiator fan switch) (see photo 4 of 5).
IMPORTANT: It'll help the accuracy of this test to remove the A/C condenser fan motor relay before you starting probing the radiator fan terminals sockets for battery power. The location of the condenser fan relay is pin-pointed in image 2 of 2 (see photos above).
OK, this is what you need to do:
Remove the radiator fan relay from its place on the under-hood fuse box.
NOTE: You'll be making your multimeter test connections on the female terminals of the now empty relay socket of the radiator fan relay AND NOT on the relay itself.
Check female terminal labeled #2 for 12 Volts. Battery power should be present at all times (in other words: whether the key is in the on position or not).
Place your multimeter in Volts mode. Touch the red multimeter test lead to terminal socket #2. Black test lead to Ground (see the photo 1 of 2 above).
Check terminal socket labeled #3 for 12 Volts with the key in the On position (position II).
Place your multimeter in Volts mode. Touch the red multimeter test lead to circuit #3 female terminal. Black test lead to Ground.
Your multimeter should register 12 Volts for both parts of this test.
Remember: Female terminal #2, of the radiator fan relay socket, is hot all of the time. Female terminal #3 has power only with the key in the On position (position II).
Let's take a look at what your test results mean:
CASE 1: The multimeter registered 12 Volts for both circuits. So far so good, since this is the correct and expected test result.
This test result tells you 2 specific things:
- That fuse #4 (20 Amp) in the under-hood fuse box is good and supplying power.
- That fuse #14 (10 Amp) in the driver side dash fuse box is good and supplying power.
The next step is to jumper terminals #1 and #2 of the radiator fan relay socket. For this test, go to: TEST 2: Bypassing The Radiator Fan Relay.
CASE 2: Multimeter DID NOT register 12 Volts for circuit #2. Recheck your multimeter settings and connections and retest.
If you still don't get 12 Volts, then this usually means that fuse #4, in the under-hood fuse box, is blown.
Check the fuse and replace if necessary. If the fuse is blown, this is usually due to the fact that the radiator fan motor is bad. To test the radiator fan motor itself, see this tutorial: Radiator Fan Motor Test (2001-2005 1.7L Honda Civic).
CASE 3: The multimeter DID NOT register 12 Volts for circuit #3. Recheck your multimeter settings and connections and retest.
If you still don't get 12 Volts, then this usually means that fuse #14, of the under-dash fuse box, is blown. Check the fuse and replace if necessary and retest.
TEST 2: Bypassing The Radiator Fan Relay
After confirming that radiator fan relay terminal sockets #2 and #3 have 12 Volts DC, the next step is to use a jumper wire and jumper together terminal sockets #1 and #2.
This test bypasses your Civic's radiator fan relay and gets the radiator fan motor running.
This test will tell us two things: One: that the wiring between the relay socket and the radiator fan motor is OK. Two: indirectly tell us that the relay is not working.
IMPORTANT: Be careful not to damage the female terminals with your jumper wire. The jumper wire needs to be a wire gauge that will not open up/damage the female terminals of the sockets. If you open them up permanently, with the jumper wire, you'll need to replace the entire fuse box.
Alright, this is what you need to do:
Jumper female terminals #1 and #2 of the radiator fan relay socket (of the under-hood fuse box).
The radiator fan motor should come on, as soon as the two terminals are jumpered together.
Remove the jumper wire once you've confirmed that the radiator fan has or has not come on.
Let's take a look at what your test results mean:
CASE 1: The radiator fan came on. This is the correct and expected test result. This test result confirms the following important things:
- The circuits between the radiator fan motor and the radiator fan relay socket (#1 and #2) are OK (otherwise the radiator fan motor would not have come on).
- The radiator fan motor is good.
The next step is test the radiator fan switch. Go to: TEST 4: Bypassing The Radiator Fan Switch.
CASE 2: The radiator fan DID NOT come on. Double check all of your test connections and make sure that the radiator fan motor is connected to its connector and repeat the test.
If the radiator fan motor still doesn't activate and in TEST 1 terminal #1 of the relay socket has power, then the radiator is bad and needs to be replaced.
Now, if I where in your shoes, and if I hadn't already, I would test the radiator fan motor directly by applying power and Ground to it manually. I've written a tutorial for this particular test and you can find it here: How To Test The Radiator Fan Motor (Honda 2.2L, 2.3L).