Jeep PWM Fan Relay Test Troubleshooting an Overheating Condition

This article will help you to diagnose the Pulse Width Modulated (PWM) fan relay on the 1999-2004 Jeep 4.0L Grand Cherokee SUVs and/or the codes P1491 (Radiator Fan Control Relay Circuit). The dealer, at the time of this writing, will charge you $95 (US) to diagnose it... but if you have more time than money, you can easily diagnose it yourself, and I'll show you how.

Testing the fan relay is easy, the hardest part is getting to it... since Jeep, in all its wisdom, located the fan relay behind the right side (passenger side) of the front bumper... and you'll probably have to remove it to access it. You can pay Jeep an arm and a leg to do it, or you can, with my help, diagnose and repair the issue yourself, save that money and spend it on something like tools.

Here are the contents of this article at a quick glance:

  1. Important Tips.
  2. Symptoms of a BAD PWM Fan Relay.
  3. FAN RELAY TEST 1: Checking for Power.
  4. FAN RELAY TEST 2: Checking for Ground.
  5. FAN RELAY TEST 3: Checking for the Relay Control Signal.
  6. FAN RELAY TEST 4: Calculating the Amperage Draw of the Fan Motor.
  7. Real Life Case Study.

En Español You can find this tutorial in Spanish here: Cómo Probar el Relé PWM del Ventilador (4.0L Jeep) (at: autotecnico-online.com).

Important Tips

TIP 1: The way I've designed the Jeep PWM fan relay test, you'll need a scan tool that can read live data (not a code reader), since you'll need to keep an eye on the engine's coolant temperature with it.

You do not need the Jeep factory scan tool, just a generic scan tool that you can buy at your local Auto Parts Store or online and a multimeter. Now depending on your level of ‘wrenching experience’, you can modify this test so that you can do it without a generic scan tool (and I leave this up to you).

TIP 2: It's very important that you start out with a cold engine. If you have a scan tool, verify that the engine coolant temperature (ECT) value is under 190° F. If the engine has been running for an extended period of time, let it cool down for about 1 hour or more.

TIP 3: Before you jump into the tests, take a look at and read the entire article... so that you'll know what the test is all about and what you'll need to do.

TIP 4: The very first thing that you need to test/verify, is the fan. You need to physically see it come on by directly applying 12 Volts to it and if the fan motor does come on, calculating its amperage draw (and I'll show you how).

TIP 5: Also, you're not gonna' find any ‘remove and replace’ type instructions... since the article is all about the diagnosing the relay, not how to remove and replace it... so you may have to buy a repair manual or do further research online.

Symptoms of a BAD PWM Fan Relay

The most obvious symptom of a BAD PWM fan relay is that your Jeep is gonna' overheat.

You'll also see one or all of the following symptoms as the engine overheats:

  1. The electric radiator fan motor does not come on.
  2. Coolant bubbling out of the coolant reservoir.
  3. Temperature gauge on the instrument cluster is maxing out.
  4. Check engine light (CEL) is on and:
    1. DTC P1491 stored in the computer's memory

If your Jeep overheats beyond a certain point, you risk blowing the head gasket and/or cracking the cylinder head.

Thankfully, testing the PWM fan relay can be easily done right at home, without the need of taking it to the Jeep house.