[wwmiller3]

Hi,

I wanted to post my experiences with chasing down a false windshield washer fluid low warning on my 2014 335i in case it helps someone avoid the rabbit hole I ventured down. A couple of months ago, I started to get a check control warning message for low washer fluid after each ignition on and engine off cycle. These persisted even after topping off the reservoir to the max.

As many probably know, BMW advises against using washer fluid other than their own product because other formulations can contain ingredients that may interfere with sensor operation, leading to the sort of symptoms I experienced (I believe there is a TSB on this issue as it is fairly common).

However, not in my case as my car has never had anything other than BMW washer fluid in the tank (mixed from concentrate in appropriate ratios for winter/summer), as I do my own maintenance and repairs. Nevertheless, on the off chance that something had fouled the sensor over time (which basically is no more than two metal strips built into the tank that allow for a capacitance measurement) I swapped out the reservoir with a new one (the sensor is built in).

The problem persisted even after the swap, so I checked for continuity between the two sensor wires: one to chassis ground, the other to the FEM (with no intervening connectors to worry about) and all was well. I went so far as to scope the signal from the FEM, which is a square wave from 0 to -3.3 volts with a period around 90 ms that, I assume, the FEM uses to measure the decay time once voltage is shut off to determine overall capacitance of the circuit.

Everything looked just fine as far as I could tell (I do not know how BMW performs the capacitance calculation so I cannot confirm the signal behavior correctness). I then stumbled across an F80 forum post where in one case using tap water--and not distilled or deionized water--to mix the washer concentrate fixed a similar problem.

I always have used distilled or deionized water to prep the washer fluid final concentration with no issue for 10 years, so I was skeptical of such a fix. But being at my wits end, I did top off the reservoir with a small batch of washer concentrate using tap water and a small amount of Himalayan sea salt to re-mineralize the fluid already in the reservoir (it was about 90% full) and sure enough the warning message went out!

So, it appears BMW assumes some mineral/ion content in the washer fluid in setting its thresholds for triggering the low washer fluid warning. This in spite of having no issues with using distilled water in the mix for so long. I'd have to assume that my car at 10 years has developed enough resistance in the circuit in spite of a new tank/sensor to skew the readings in combination with low-to-no mineral/ion content. It does not take much resistance to change what has to be very low capacitance values (pico Farads).

Here I thought I was being a conscientious owner by keeping hard water out of the washer system, but no good deed goes unpunished.