How To Test Your Car’s Coolant With a Multimeter

Introduction

If you think the only job of antifreeze is to prevent freeze-up during the winter, you're wrong. Antifreeze also plays an important role in preventing corrosion caused by electrolysis, which occurs when two dissimilar metals start swapping electrons. Over time, this causes internal corrosion.

To protect the aluminum, copper, cast iron, steel and magnesium alloys found in most engines, antifreeze has additives to prevent all of that electron swapping. But, as it ages, those additives become depleted and can't do the job anymore. The good news is that it's pretty easy to check the conductivity of your coolant with a digital multimeter. If the conductivity is high, it's time for a coolant flush and fill.

What Is Coolant and What Does It Do?

Coolant refers to the liquid that circulates throughout the engine that removes the heat caused by internal combustion, and dissipates that heat through the radiator.

Is Coolant and Antifreeze the Same Thing?

These terms are often used interchangeably, which is fine for modern cars, but they don't exactly mean the same thing.

If you fill your car's radiator with straight water, that would be considered "coolant." Water is very good at removing heat, however, in order to keep it from freezing, an "antifreeze" is added in the form of propylene glycol or ethylene glycol. This lowers the freezing temperature of water and, as a bonus, increases the boiling point. This mixture can be referred to as coolant or antifreeze, but in fact, it's a blend of both.

Why You Should Test the Coolant in Your Car

If you let your coolant age beyond its years, it will no longer protect your engine and will harm it. In fact, worn coolant becomes a pretty darn good electrical conductor, accelerating internal electrolysis. Doing this particular test regularly (and keeping good notes) may serve as an early warning if you notice a sudden change in conductivity.

How a Multimeter Works for Coolant Testing

As the anti-corrosion properties of the antifreeze/coolant mixture start to fail, it no longer prevents electrolysis within the engine. Over time, this leads to a build-up of metal molecules in the coolant, which leaves a tell-tale signature in the form of conductivity. Set to the appropriate range, a multimeter detects this conductivity as a voltage.

What Are the Multimeter Measuring Ranges?

A multimeter will have multiple ranges to choose from when taking measurements. Whether measuring DC or AC voltage, it's important to set the appropriate range for the magnitude of the measurement being made. This allows the multimeter to read in the best resolution, providing you with the most meaningful reading.

For our purpose, the multimeter needs to be set to DC voltage in the 20 volts and under range. Most multimeters have a dial allowing you to choose the correct setting for the particular voltage range you're testing for, but some adjust to the appropriate range automatically.