TEST 1: Checking For Power
IMPORTANT: Before you start this test, on your 99-04 Jeep Grand Cherokee, you need to have calculated the amperage the fan motor is consuming. This is very important.
So if you haven't, you can find the out how to do this here: FAN RELAY TEST 4 and Real Life Case Study. Once you've calculated the amperage draw (and it's within specification), come back to this test step.
OK, to get this show on the road, the first thing you'll do is to make sure the PWM fan relay is getting power.
These are the test steps:
Grab your multimeter and select Volts DC mode on it and probe the Gray Wire labeled by the number 1 (in the photo in the image viewer) with the red multimeter test lead.
What I recommend you do is to use a wire piercing probe, or another suitable tool, to pierce the wire and then attach the red multimeter test lead to this tool. Do not probe the front of the connector, or you run the risk of damaging the female terminal.
To access the PWM fan relay, you'll need to remove the front bumper.
Now, Ground the black multimeter test lead on the battery negative terminal.
Since the Gray wire is always hot (always with 12 Volts present), your multimeter should register 12 Volts.
OK, read both test interpretations to see which one fits your test results:
CASE 1: Your multimeter registered 12 Volts. This confirms that the fuse that supplies the PWM fan relay with power is not blown. You can proceed to the next test, go to: FAN RELAY TEST 2
CASE 2: Your multimeter DID NOT register 12 Volts. This confirms that the fuse that supplies the PWM fan relay is blown. Replace the fuse and retest.
This fuse is found in the fuse box in the engine compartment.
FAN RELAY TEST 2: Checking For Ground
In FAN RELAY TEST 1, you verified that the Pulse Width Modulated (PWM) Relay is getting power in the form of 12 Volts DC. In this test step, you're gonna' verify that the relay has a good Ground.
This test is done with your multimeter in Ohms mode and I'll suggest that you disconnect the PWM fan relay from its electrical connector to do the test.
This is what you'll need to do:
Disconnect the PWM fan relay from its electrical connector and set your multimeter in Ohms mode.
With the red multimeter test lead (although it doesn't matter if it's the BLACK one or the RED one), probe the BLACK wire of the connector. This is the wire labeled with the number 4 in the image viewer.
Remember, don't probe the front of the connector... use a Wire Piercing Probe or another appropriate tool. Attach the multimeter test lead to this tool.
Ground the other multimeter test lead on the battery negative terminal.
You should see one of two results on your multimeter:
1.) You'll see continuity, in the form of the multimeter registering 0.5 Ohms or less or...
2.) Your multimeter will register no continuity, which is usually displayed with the letters OL on your multimeter.
OK, now that the testing part is done, let's take a look at what your results mean:
CASE 1: Your multimeter showed continuity in the Circuit: This means that the PWM fan relay has a good path to Ground.
So far you have verified that the PWM Relay is receiving power and Ground, the next step is to test that your Jeep's fuel injection computer (technically known as the Powertrain Control Module= PCM) is actually activating the Pulse Width Modulated (PWM) fan relay. This test will let you know if the PWM fan relay is OK or fried. Go to: FAN RELAY TEST 3.
CASE 2: Your multimeter DID NOT show continuity in the Circuit: This test result indicates that the PWM fan relay does not have a good path to Ground.
This is very unusual, what I suggest you do is to splice a wire into this circuit which you should connect directly on the battery negative terminal (without cutting it) and retest.