How To Test The Blower Motor Resistor (Ford 4.0L)

If the blower motor only runs in HI or one of the blower speeds is not working... then there's a good chance that the blower motor resistor assembly is bad.

Thankfully, the blower motor resistor on your Ford Explorer (Ranger, Mercury Mountaineer) can be checked with a few simple continuity tests.

With the help of this tutorial, you'll be able to test it and say: ‘Yes, the blower resistor is bad’ or ‘No, the blower resistor is not fried’.

To aid you in further knowing if this tutorial applies to your Ford Explorer, Ranger, Mercury Mountaineer or Mazda B4000... if your vehicle uses one of the following blower motor resistor numbers, this tutorial applies to your vehicle:

  1. Airtex/Wells 3P1368.
  2. Dorman 973010 and 973411.
  3. Duralast JA1506.
  4. Four Seasons 20321.
  5. Motorcraft YH1700.
  6. Standard Motor Products RU404.

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

  1. Basics Of Troubleshooting The Blower Control Resistor.
  2. TEST 1: Checking Continuity Of The Blower Resistor.
  3. TEST 2: Using A Jumper Wire To Bypass The Blower Resistor.
  4. Where To Buy The Blower Resistor and Save.
  5. Related Test Articles.

En EspaƱol You can find this tutorial in Spanish here: Prueba: Resistencia del Motor del Soplador (4.0L Ford) (at: autotecnico-online.com).

If the blower motor is not working at all, you should start by testing the blower motor first. The following tutorial explains how to test the blower motor:

  1. How To Test The A/C Heater Blower Motor (Ford 4.0L).

Basics Of Troubleshooting The Blower Control Resistor

How To Test The Blower Motor Resistor (Ford 4.0L)

The component that makes it possible for the blower motor to run in the 3 speeds lower than HI is the blower motor resistor assembly.

If you have already removed the blower resistor assembly from your Ford Explorer (Ranger, Mercury Mountaineer), you know that it's made up of 3 resistors.

The purpose of these resistors is to reduce the amount of the blower motor's current that's passing through them by converting some of this current into heat energy (and boy does the resistor assembly get hot!).

It's this reduction in current (by converting some of it into heat energy) that makes the blower motor run slower than full speed (HI).

The one blower motor speed that doesn't depend on the blower motor resistor assembly is HI... since the blower motor switch accomplishes this speed by completely bypassing the blower motor resistor assembly.

Usually, when the blower motor resistor fails, the resistors are physically burned ‘open’... but not always. This is when knowing how to continuity test the resistor assembly comes in handy to find out if it's good or bad.

Blower Resistor Circuit Descriptions
Pin Wire Color Description
1 LT GRN/WHT M1 Speed Input From Blower Switch.
2 ORG/BLK Input From Blower Motor.
3 BLK Output to Chassis Ground.
4 YEL/RED M2 Speed Input From Blower Switch.

NOTE: There's a good chance that the color of the wires described above DOES NOT match what's on your particular vehicle. This is no cause for concern since the circuit descriptions are the same for all of the Ford vehicles covered by this repair tutorial.

TEST 1: Checking Continuity Of The Blower Resistor

How To Test The Blower Motor Resistor (Ford 4.0L)

OK, to get your blower motor resistor diagnostic on its way, the first thing we'll do is to remove the blower resistor from it's location (next to the blower motor) and check the continuity of the individual resistors (that make up the assembly).

These are very simple tests (the only hard part is getting to and removing the blower resistor assembly) and in the test steps below I'll show you which terminals you'll be testing with your multimeter in Ohms () mode.

Before we get started let me tell you that it's common for the blower resistor connector's lock tabs to break. This is due to the fact that the connector is made of plastic and is exposed physically to the heat that the blower resistor produces (when in use) and exposed to heat that the engine radiates (when running).

The tendency of the locking tabs is to become brittle (from the heat) and to break when you unplug the resistor assembly. The good thing is that you can buy the blower resistor connector and replace it. You can find the link to the connector in this section: Where To Buy The Blower Resistor and Save.

NOTE: All of the continuity tests, described in this test section, are done on your Ford Explorer's (Ranger, Mercury Mountaineer's) blower motor resistor and not on its connector.

OK, these are the test steps:

  1. 1

    Disconnect the blower motor resistor from its electrical connector and remove it from its location. Set your multimeter to Ohms mode ().

    Don't have a multimeter or need to upgrade yours? Check out my recommendation: Abe's Multimeter Recommendation (found at: easyautodiagnostics.com).

  2. 2

    Visually inspect the condition of the blower resistor's coils and it's electrical connector.

    Is the connector melted? Are the coils in one piece?

  3. 3

    Probe terminals 2 and 1 with your multimeter leads (see image above).

    Your multimeter should register a resistance of about .9 to 1.4 Ohms ( ).

    NOTE: See image 4 of 4 to identify the proper male spade terminals to probe).

  4. 4

    Probe terminals 4 and 1 with your multimeter leads (see image above).

    Your multimeter should register a resistance of about .6 to .8 Ohms ( ).

  5. 5

    Probe terminals 4 and 3 with your multimeter leads (see image above).

    Your multimeter should register a resistance of about .2 to .3 Ohms ( ).

    This test also test the continuity of the blower resistor thermal fuse.

IMPORTANT: There's a good chance that you won't see exactly the resistance values I've listed (on your multimeter). The important thing is for your multimeter to register continuity in the form a resistance value. If your multimeter registers OL (Open Loop), then that resistor is bad.

Let's take a look at what your test results mean:


CASE 1: The blower resistor did not show continuity in one of the indicated tests above. Double check that you're testing the correct blower resistor pins and repeat the test(s).

If your multimeter still does not show continuity for those particular blower resistor pins (you're testing) then this test result confirms that your Ford Explorer's (Ranger, Mercury Mountaineer's) blower motor resistor is bad and needs to be replaced.

CASE 2: The blower resistor passed all of the continuity tests indicated above. This is good and is the correct and expected test result that tells you that the blower motor resistor is OK and not the cause of the blower motor speed problem.

You have eliminated the blower resistor as bad on your Ford Explorer (Ranger, Mercury Mountaineer). If you're still having a blower motor speed issue, then I suggest that you perform the next test.

The next step indirectly tests the blower switch on the A/C Heater control panel by bypassing the blower resistor with a jumper wire: TEST 2: Using A Jumper Wire To Bypass The Blower Resistor.