How To Test Coolant And Coolant Sensors

Engine coolant keeps the waste heat of the combustion process at bay. There’s no denying the importance coolant plays in keeping an engine running, but how do you know when it needs to be changed? But also to make sure your coolant is doing its job it needs to be monitored by a coolant sensor. Let’s look at how to test engine coolant, how to pressure test a cooling system, and equally important how to test coolant temp sensor operation. 

Why Test Engine Coolant?

Your engine coolant is part of an enclosed system, but that system consists of many components of varying materials. Over time under the stress of extreme heat exposure the coolant loses some of its ability to control and conduct those temperatures. There are also parts of the cooling system that can corrode internally leaving tiny rust flakes that act like silty mud. There are all reasons why your engine coolant needs to be tested and periodically replaced when it has reached the end of its service life. But you need to test not only for the right coolant mixture ratio, but also the chemical composition of the coolant.

Testing Engine Coolant Ratio

The easiest way to test coolant mixing ratio with an antifreeze tester. This neat little device tests the specific gravity of the coolant using either little colored floating balls or a swing arm. The balls and swing arm are calibrated to float at different levels based on the specific gravity of the coolant. Simply draw coolant into the antifreeze tester and compare the results to the included chart. Typically on a floating ball type tester the higher the concentration of ethylene glycol, the more balls that float. You can then estimate the freezing point of the coolant and how well you are protected against the cold. Just be aware that there are different testers for propylene glycol and ethylene glycol, so choose a tester that matches what is used in your cooling system.

For a more accurate measurement of your coolant’s freezing point you can use a refractometer. There are analog and digital refractometers but they both work on the same idea. Simply place a few sample drops of coolant in the tool. For the analog refractometer you then look through the eyepiece and read the inside gauge. For the digital refractometer you just have to push a button and the reading will be displayed on the screen. You will need to read the instructions and be familiar with the tool to understand what the results of each one means to the specific gravity of your coolant.

Testing Engine Coolant Condition

As mentioned earlier your coolant can actually degrade over time. Luckily a simple engine coolant test kit can give you a glimpse of what is in your coolant. When the engine is cool and depressurized (never work on a hot engine’s cooling system) just remove the radiator cap and dip in a testing strip. Make sure to read the directions included with the testing strip to make sure you get a good reading. Most test strips can tell you the pH level, nitrate concentration level, and liquid freeze point. If any of these readings are out of specification, it is time for a coolant system flush and refill. 

How To Test A Coolant Temp Sensor

Knowing how to test coolant temp sensor output is a bit more technical. You will need a multimeter to read the resistance of the coolant temp sensor during the test. You will also need to remove the coolant temp sensor from your vehicle, so refer to a repair manual for the specific procedure. For sensor range testing you will need a container of ice water and a container of boiling water. Finally you need the factory sensor range specifications (usually found in the repair manual) along with a pen and paper to take notes.

Once you have the sensor out of the vehicle attach it to the connections on the multimeter. Most sensors have two connections and since you are testing resistance, it does not matter which order is used. If your sensor has more than two connections refer to a vehicle wiring diagram to find the ground connection and the voltage input connection. 

You will be testing engine coolant temperature sensor resistance output in cold water and hot water, then comparing the two readings to the factory specification found in your repair manual. Check the temperature of the ice water to make sure it is as close to freezing as possible (32 degrees F or 0 degrees C). Set the voltmeter to the 20,000 ohm range. Dip the tip of the sensor in the cold water and observe the reading on the multimeter. When the reading stops changing, write it down on the paper as the cold reading. Repeat the same process with the boiling water, being careful to hold the sensor with tongs or similar tool to reduce the chance of touching the boiling water. Write down the hot temperature reading from the multimeter. 

Now you can compare the two voltage readings to the factory sensor specifications. If the readings are not within specifications the sensor is bad and should be replaced. Now that you know the steps for how to test an engine coolant temperature sensor, you can decide if it is worth your time or if the sensor is cheap enough to just replace it and move on.

How To Pressure Test A Coolant System

Luckily learning how to pressure test coolant system components is pretty easy. You will need an engine coolant pressure tester which looks like a bicycle tire pump attached to a universal radiator cap. Start with a cool engine (never work on a hot engine cooling system under pressure). Remove the radiator cap or coolant reservoir cap if so equipped. Attach the pressure tester to the same place where you just removed the radiator cap or reservoir cap. The pressure tester may have a universal rubber fitting or come with an array of adapters to connect with your particular cooling system. Now use the pump to add pressurized air to the cooling system. Watch the pressure gauge on the pressure tester and add roughly 15 psi of pressure (but no more than that). The pressure gauge should hold steady indicating no leaks. If the pressure gauge goes down or does not register any pressure, double check your pressure tester connection just in case. If the system will not hold pressure, you will need to repair the leak. You can use UV dye to help locate the leak if it is not easily apparent.

Check out all the cooling system components available on NAPAonline or trust one of our 17,000 NAPA Auto Care locations for routine maintenance and repairs. For more information on how to test engine coolant sensor output and other cooling system parts, chat with a knowledgeable expert at your local NAPA Auto Parts store.