How To Test IAC With Multimeter (5 Step Guide)

Before we get into the exact details, please read the table of contents below.

What you need to know about how to test iac valve with multimeter

Carbureted controlled engine vs electronic controlled engine

Firstly, we need to take a bit of a history lesson. It will be very short so you do not need to worry.

However you will have a greater appreciation on how to test the iac valve with multimeter after we are done.

Before the advent of electronic controlled engines, motor vehicles used to run off a carbureted engine, now when your car was on idle, you needed to warm it up by applying revs to the acceleration.

By increasing the revs you practically slightly open the throttle plate. This would allow air to flow to the engine.  

The old style carbureted engines had two screws. They were referred to as warm screws and hot screws. Once the car gets started and it went on idle the flap would open up in the cold screw position.

It would idle until it became hot enough, and then it would move to the hot screw position, and the car could hence idle without needing to be rev up.

As you can imagine this creates a problem because it meant that you couldn’t idle a car until it was sufficiently warm using a carbureted engine.

Then came in the electronic controlled engine and the IAC. To solve this problem, when the car was idling air would pass through the IAC to keep the engine running, as the butterfly valve in the throttle plate would be shut.  

Also very interestingly the IAC controls air flow when the car is decelerating.  

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Symptoms that your Idle Air Control Valve is failing

Smooth Idle

The key thing to think about here, is that your car idle’s once the car heats up and stabilizes should be very smooth.

The key thing to consider in this regard is that when you turn on your engine and the car idles it should stabilize after a few seconds.

Typically when you think about it a smooth idle is typically within the range of 500 – 1000 RPM. This value is heavily dependent on the vehicle that you are operating at the time of reading this article.

To get the precise idle RPM value, be sure to check the cars manual for the exact figure.

Rough Idle

As we had already stated, when the iac is functioning correctly, what would typically happen is that the idle would operate relatively smoothly.

When the car begins to rough idle what you would typically experience is that the car would experience a vibrational force, resulting in an oscillation of the RPM readings.

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Clogged

When the iac is operating correctly, the air flowing in and out of the idle control valve can often leave some form of debris as contaminants that stick to the housing of the opening chamber.

This blockage can cause malfunctioning of the idle air control valve.

Failing IAC

The iac has a solenoid, if there is a discontinuity then the valve will no longer receive an electrical signal.

Upon this happening the car may not even be able to start. The condition that this creates also leads to a condition of troubled starting.   

Equipment You Will Need To Learn How To Test Iac Valve With Multimeter

To ensure that you can perform the tests effectively, please see the items that you would require below.

• Digital multimeter - My Top Pick:⭐ Fluke 101 Multimeter ⭐
• Service manual from your vehicle;
• Digital infrared thermometer - My Top Pick:⭐Etekcity Infrared Thermometer⭐
• Terminal connector.

5 Steps on how to test iac valve with multimeter

Let’s look at the various steps and tests that you will need to undertake to ensure that you test your iac valve correctly with a multimeter.

Word of caution, if you are not comfortable performing work underneath the hood of your car, please ensure that you outsource this work to a mechanic instead.

Step 1: Locate the iac valve from service manual

Open the car’s service manual, this will assist you in locating the iac valve. This iac is typically attached at the top of the throttle body.

Step 2: Inspect the IAC Valve operation

Use the terminal connector on your engine diagnostic, then start the engine and let it run until it reaches it’s optimal temperature. The engine should have a RPM. Slowly rev it up to between 1500 RPM keep it there for about 4 to 6 seconds.

Then release, if your iac valve is operating correctly, then it will return back to the idling RPM’s.

Remove terminal connector from the diagnostic tool.

Step 3: Test the resistance of the coils

Here you will need a pointed infrared thermometer, because you will need to get the temperature of the actual IAC valve and not the temperature of the engine.

Typically in your service manual they will provide a temperature range for hot and cold. Record your temperature and compare it to what is in the service manual. The typical cold and hot temperature ranges are shown below.

Cold: -13 degrees C to 50 degrees C (17 to 25 ohms)

Hot: 50 degrees C to 100 degrees C (21.5 and 29.5 ohms)

The IAC sits at the bottom of the throttle body, depending on your vehicle, you might need to disconnect the air tube from it’s connectors to ensure that you get access into to the IAC itself, and access to terminal points to measure resistance.

Hence forth use the multimeter to check the resistance. Choose the ohms resistance reading between the 3 terminals. Black connector to ground and the red connector to ground.

If the values are within range as shown above, then it passes the ohms resistance test.

Step 4: Check supply voltage from the ECU

The iac valve although being attached to throttle body is actually electronically controlled by the engine’s control unit.

This therefore goes without saying that you should check the ECU’s voltage to ensure that it is within range and is able to fire correctly and within the right sequence.

Step 5: Check the coil for short circuit and for short circuit to earth.

Another common reason why the iac might not function correctly is that the solenoid could be damaged. If this is the case you would check it's continuity. 

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Final Thought on how to test iac valve with multimeter

Testing your iac with a multimeter is not difficult, if you know how to work under the hood of your vehicle. Contact a mechanic if you are not comfortable testing your iac valve with a multimeter.

Hopefully this guide has been helpful. All the best.