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Mercedes-Benz C-Class and C-Class AMG: Why is the Washer Fluid Light On?

Does your "Check Washer Fluid" warning light stay on even if you've just topped off the washer fluid reservoir? Your problem might have something to do with chemistry.

By Tom Cavanagh - May 29, 2015

This article applies to the Mercedes-Benz C-Class and C-Class AMG (2007-2014).

If your "Check Washer Fluid" light won't go out, even though the reservoir is full, you might be experiencing one of two problems. Both of them deal with chemistry and the conductivity of fluids. The C-Class uses a dual probe sensor (MB Part # 221 820 91 10 M22) that, when working properly, will tell you when the washer fluid is running low.

The basic principle is fairly simple. We all know the dangers of using electrical equipment around water. Water is an excellent conductor of electricity. When your reservoir is full, a small charge passes from one of the sensor probes through the liquid to the other. The sensor tells your "Check Washer Fluid" light to stay off. But when the liquid level drops below the sensor probes, the charge cannot pass through the air and your warning light goes on.

Figure 1. Mercedes-Benz sensor.

That is how it is designed to work. However, if the warning light stays on, you may have a bad sensor, or the fluid you are using doesn't have the conductivity to pass the charge from probe to probe. Plain water conducts electricity the best, but it doesn't have the cleaning agents necessary to cut the dirt. This article will take a look at both problems, and how you can fix them.

Materials Needed

  • Washer fluid
  • Replacement sensor (optional)

Step 1 – Inspect the sensor

  • Make sure that the reservoir is empty.
  • Pop the sensor assembly out of the side of the reservoir. Inspect it to see if there is any corrosion or build-up of sludge on the sensor probes. If they are fouled, it might be symptomatic of a problem this article will discuss later. If they are, clean them off with a clean, damp cloth and put the sensor assembly back into the reservoir.
  • Fill the reservoir up with water. If the "Check Washer Fluid" remains on, you probably have a faulty sensor and it needs to be replaced
Figure 2. Washer fluid reservoir diagram.

If the sensor is not the issue, move on to Step 2.

Step 2 – Research and/or replace your washer fluid

  • If is not the sensor, the problem may lie with the type of fluid you are using.
  • The cleaning agents in the Mercedes-Benz SummerFit additive were designed to maintain conductivity when mixed with water. In cold weather, many owners simply use the SummerFit with a cold weather washer fluid instead of using the WinterFit concentrate.
  • Many owners rely on Rain X, but there are mixed reviews on it as a washer fluid. Some owners feel that it leaves an orange residue in the reservoir.
  • A number of owners simply use distilled or de-ionized water.
  • It is important to use a washer fluid that conducts the electrical charge for the sensors and doesn't cause a residue build-up on them.
Figure 3. Mercedes-Benz "SummerFit" washer fluid.

Pro Tip

Owners report many different results with many different washer fluids. The key thing to remember is that the washer fluid needs to conduct electricity well enough for the sensor to work properly.

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