TEST 4: Calculating The Amperage Draw Of The Fan Motor

It's not enough to apply 12 Volts to the fan on your Jeep Grand Cherokee to see if the fan comes on or not (to conclude that it's good or bad).

Why? Because the fan could come on and lead you to believe that the fan motor is OK, but in reality, the fan motor is drawing too much amperage!

If the fan motor is using way too much amperage, the end result is that the Pulse Width Modulated (PWM) fan relay you just bought (or are gonna' buy) is gonna' fry (again).

And you'll be back to square one and thinking you misdiagnosed the overheating problem. Now, you would think that this over-amp condition would blow the fan motor fuse but this rarely happens.

We can easily check the amperage draw of the fan motor using Ohms Law. It simply involves measuring the fan motor's resistance and then, using this value, calculate how much amperage the fan motor is pulling (Amps=Volts ÷ Ohms).

Now, don't worry, if you're starting to think this is too complicated. I'm gonna' show you how easy this is and how to do it in the following test steps:

  1. 1

    Disconnect the fan motor from its electrical connector.

  2. 2

    Set your multimeter to Ohms mode.

  3. 3

    Measure the fan motor's resistance by probing its two terminal with the multimeter test leads.

    NOTE: It's important that the fan blades do not turn while you're resistance testing the fan motor.

  4. 4

    Write the Ohms value down on a piece of paper once the Ohms reading on your multimeter stabilizes.

  5. 5

    Divide 12.5 by your resistance value.

    The result of this calculation is the amount of amperage the fan motor is using.

    To be a little more specific: Let's say that the resistance reading was 0.4 Ohms. This is what the math would look like: 12.5 ÷ 0.4 = 31.25 and this would translate to 31.25 amps. If you would like a little more detailed explanation about this, go to: Real Life Case Study in the next section.

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

CASE 1: Your test result resulted in a calculation of under 40 amps. This tells you that the fan motor is OK and will not fry the new PWM fan relay when you replace it (if applicable).

The next step is to check the Pulse Width Modulated (PWM) fan relay (if you haven't done so yet), go to: FAN RELAY TEST 1.

CASE 2: Your test result resulted in a calculation of an amperage over 40 amps. This indicates that the fan motor is bad and will fry the new PWM fan relay.

The next step is to check the Pulse Width Modulated (PWM) fan relay (if you haven't done so yet), go to: FAN RELAY TEST 1.

Real Life Case Study

To help you better understand how to test for the amperage draw of the fan motor, let me tell you what happened to a friend's Jeep Grand Cherokee.

He had installed a brand new fan motor and a brand new PWM fan relay (at the same time). About an hour later, the fan motor stopped working and the Jeep started to overheat once again. This is when he called me and I came over to check the Jeep.

I instinctively knew that the fan probably was using too much juice (even tho' it was a brand new one) and this is what I did, when I went over, to help him troubleshoot the problem:

  1. I tested the resistance of the fan motor, with the multimeter in Ohms mode and I got 0.2 Ohms for the result.
  2. I then divided 12.5 Volts (which is the battery voltage) by the fan motor's resistance value. This is what my math looked like:
    1. 12.5 ÷ 0.2 = 62.5 Amps (Ohms law: Amperage = Voltage ÷ Resistance).

This calculation told me that the fan motor (that my buddy had just replaced) on this 02 Grand Cherokee was using 62.5 amps and this is way over the 40 amp limit of the fuse (that protects this PWM fan relay and fan motor circuits).

To make the long story short, the new fan motor fried the PWM fan relay because it was defective and started to use way too much amperage. Yet the 40 amp mega fuse did not blow.

He replaced both again (this time with name brand aftermarket parts) and the problem was solved.

You DO NOT need to test the amperage draw while the fan motor is in action (for example: using an ammeter). Using Ohms Law (Ohms law: Amperage = Voltage ÷ Resistance) to figure out the amperage draw is very, very accurate.

You can avoid frying the new PWM fan relay (in case the fan is over the hill and about to croak) simply by figuring out the amperage usage of the fan motor before replacing the PWM fan relay with a new one.

If the amperage draw is greater than 40 amps, the fan motor needs to be replaced (even if fan motor comes on when you apply 12 Volts directly to it).

Hopefully this information helped! Return to FAN RELAY TEST 4

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Jeep Vehicles:

  • Grand Cherokee 4.0L
    • 1999,