TEST 3: Checking For The Fan Relay Control Signal

Jeep PWM Fan Relay Test Troubleshooting An Overheating Condition

Confirming that the Pulse Width Modulated (PWM) fan relay is receiving the activation signal (to activate the fan) from the PCM (Powertrain Control Module=Fuel Injection Computer) or not, will help you to confirm that the fan relay is good or bad.

You'll need a multimeter, a generic scan tool, a portable house fan, and an automotive 12 Volt test light for this test.

OK, here are the test steps:

  1. 1

    Set your multimeter in Volts DC mode.

  2. 2

    Connect the black multimeter test lead to the wire labeled with the number 3 in the photo above.

    IMPORTANT: The PWM fan relay must remain connected to its connector, use an appropriate tool (like a back-probe or a wire piercing probe) to probe the light green (LT GRN) wire.

  3. 3

    Connect the red multimeter test lead to the battery positive (+) post.

  4. 4

    Disconnect the fan motor from its electrical connector.

  5. 5

    Now connect the metal probe of 12 Volt test light to green (GRN) wire labeled with the number 2 (see photo).

  6. 6

    Connect the alligator clip of the 12 Volt test light to the battery negative (-) terminal.

If the AC Works:

  1. 7

    Make sure that the AC is turned off and then start the Grand Cherokee.

  2. 8

    Your multimeter should read 0 Volts and the test light should be off with the AC off and the engine running.

  3. 9

    Now, turn on the AC and look/listen for the AC compressor to come on.

  4. 10

    Your multimeter should read 10 to 12 Volts and the test light should come on when the AC compressor runs.

  5. 11

    Turn off the engine and go down to the test interpretations at the end of these test steps (no further testing is required).

If the AC DOES NOT Work:

  1. 7

    Connect your scan tool to the OBD connector inside your Jeep Grand Cherokee and select the Live Data function on it, so that you can see the engine coolant temperature reading in degrees Fahrenheit (or degrees Celsius).

  2. 8

    Start the engine and let it warm up

    While the engine is warming up, you need to keep your eyes glued on the scan tool and the multimeter. On the scan tool, you need to see what the engine coolant temperature is.

  3. 9

    Your multimeter should register 0 Volts DC and the test light should be off at any temperature around 190° F or less.

  4. 10

    At any temperature around 200 to 215° F, your multimeter should read 5 Volts DC.

    There's a good chance that your multimeter may read 5 Volts for a few seconds and then register 0 Volts and then register 5 Volts again, this is normal.

  5. 11

    Once the temperature of the engine coolant temperature sensor (on the scan tool) reaches 217° F, your multimeter should register 12 to 13 Volts DC and the test light should be fully lit.

    IMPORTANT: Do not let your Jeep Grand Cherokee's temperature go beyond 220° F.

  6. 12

    Turn off the engine and place the portable house fan on top of the engine. Turn on the fan and let it cool the engine down.

Alright, let's find out what your test results mean:

CASE 1: The multimeter registered 12 Volts and the 12 Volt test light DID NOT come on. This confirms that the PWM is fried and that it will need to be replaced.

Here's why: The 12 Volts your multimeter registered indicate that the PCM is activating the PWM fan relay. The fact that the test light did not light up means that the PWM fan relay is unable to process the PCM's activation signal because it's fried.

CASE 2: The multimeter did not register the indicated voltage. Recheck all of your multimeter connections and retest.

If the multimeter doesn't register the indicated voltages, then you have eliminated the PWM fan relay as the cause of the issue, since without the activation signal from the PCM, the PWM fan relay will not activate the fan motor.

It's beyond the scope of this article to find out why the PCM is not activating the PWM fan relay, but you have at least eliminated it as the cause of the P1491 code or overheating problem.

CASE 3: The multimeter registered 12 Volts and the 12 Volt test light did come on. This is the correct test result and it confirms two things:

  1. The PCM is activating the PWM fan relay.
  2. The PWM fan relay is activating the fan motor.

Jeep Vehicles:

  • Grand Cherokee 4.0L
    • 1999,