Washer fluid level sensor in my truck has failed again. What’s going on?

It’s really annoying, not just because I won’t get a warning when the fluid is actually low, but because, about 30 seconds after every start, my truck dings and a big “Washer Fluid Low” message appears on my dash.

I’ve seen on the internet that the sensor is pretty easy to replace, but I’d like to know why it keeps going bad, and what I can do to prevent it.

Washed Up

There are two ways that automakers typically use to determine the level of fluid in the washer system’s reservoir.

The most common method is to use a float. Just as the name implies, there will be some type of buoyant, well, float that’s captive in a tube or has a stem going through its centre to allow it to rise up or sink down in a specific location inside the tank. In all of the ones I’ve encountered, a magnet is attached to the float, and it interacts with a magnetic reed switch or Hall-effect sensor (an electronic switch that reacts to magnetic fields) when it reaches the bottom of its range.

In the simplest designs, the reed switch just grounds one side of an indicator bulb in the cluster, while the other side of the bulb has ignition-switched power. Primitive and dead simple, and, unless the float sinks or sticks (more common in coolant tanks or brake master cylinders), or the bulb burns out, it’s going to work.

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• Dec 31, 1969
• Comments

• Dec 31, 1969
• Comments

Of course, this type of level sensor does require several different parts, including a float, a magnet, and a switch. And the sensor would have to be assembled from several pieces to contain the float and provide it with a range of motion. The cost to manufacture a part like that wouldn’t be excessive, however tallied up over 1500 pickups Ram sells on average every year (well over half a million), even a small savings becomes significant.

So Ram parent company Stellantis uses a different, far simpler type of sensor; conductive probes.

Let me clarify the “simpler” part: while the sensor, itself, is incredibly basic, implementing it to do the job actually requires electronics and software. Fortunately, the body control module (BCM), or main computer bus for the vehicle, that all 1500s already have is more than capable of handling this task.

The 1500’s washer level sensor is, effectively, a plastic bung with two metal conductors moulded into it; on the inside, they protrude into the fluid, and on the outside, they serve as the pins the truck’s wiring connector plugs onto. No moving parts.

The two wires from this sensor go to the BCM, which sends five volts down one and measures what comes back on the other. If the probes are submerged, the water in the washer fluid conducts some of the voltage across the pins, and the BCM determines that there’s still fluid in the tank. If the BCM doesn’t see enough voltage coming back, it knows the level is below the sensor and thus “low.” Logic is employed to reduce false results from splashing.

“Fine,” you say, “but why does mine keep failing?”

I’d be willing to bet that you use a premium-grade washer fluid, one that has hydrophobic chemicals in it that creates a water-repellent coating on your windshield to improve wet-weather visibility. “Rain-X” is the most well-known brand, however competitive products, such as Canadian Tire’s Reflex line, likely contain the same type of polymers.

My theory is that the same properties that work on glass to cause beading also coat and repel water away from the sensor’s probes, reducing how well the weak electric current can conduct between them, ultimately fooling the BCM into thinking the level is low.

I’ve actually come across this failure several times in Ram pickups, and have heard from fellow techs that it affects other makes, such as Volkswagen and Mercedes products, as well. While it’s possible that there’s a way to clean the old sensor, in the Ram 1500 a new one from the dealer is under $20 and relatively simple to replace, so I’ve always just fitted a new sensor.

My recommendation for you is to remove the remaining fluid from the tank, replace the contaminated sensor, and switch to a fluid that doesn’t contain additives for water-repellency or bug removal.

While less convenient, applying Rain-X or a similar product to your windshield directly will provide the benefits you desire without affecting the function of your level sensor.

Ask a Mechanic is written by Brian Early, a Red Seal-certified automotive technician. You can send your questions to wheels@thestar.ca. These answers are for informational purposes only. Please consult a certified mechanic before having any work done to your vehicle