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Comment by LanceH | Hacker News Reader

My wife's car right now is low on windshield wiper fluid. It warns us at a particular spot while we are driving. Not a particular distance from home, a specific geolocation. First thoughts were that it was the amount of time as she went to something and came back. But it does the on screen and audible warning at that location every time, no matter what was driven before that, whether it was 5 minutes, 5 miles, an hour, or 60 miles. The only additional clue is that exact same spot is a dead zone for most phones/carriers.

We got the wiper fluid filled, so the mystery is in remission, but I'm wondering if all warnings will pop up right then. I'm guessing it has something to do with the telemetry of the car being nudged in that spot, waking up and saying something.

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Could be an incline is making the fluid level low at the sensor point at that location. Does it have an unusual incline in any axis?

Absolutely flat. There is a mild bend there, but nothing compared to corners and the traffic circle. For that matter, we can get out and drive all afternoon and it said nothing until that spot.

A lot of those sensors average the reading over quite some time to avoid false positives. It's possible that the last few miles of road have just enough average slope or contain just enough curves that the sensor averages "low" right at that spot with the speed of traffic.

I have a similar situation but in my case it's very obviously the gentle bend in that location combined with the relatively high speed that makes the fluid hug one side of the reservoir. I'm pretty sure that the sensor is mounted to the opposite side.

My Subaru warns me about the wiper fluid on highway exits exclusively because of the speed and inclination changes

Sounds like an area with strong EM interference. Maybe a stop-light with those underground sensors? or under a high-power line? or near a power transfer station? or high-power radio antennae?

Could be giving just enough weird EM interference to bump the sensor from "enough fluid" to "low"

(I know I sound like Scully from x-files but could just be?)

Experiencing this same issue acutually these days on a 2019 VW Passat, it triggers when you accelerate or break or you are doing a turn for a longer period. It's just the fluid being moved around in the container. I think today I cleaned my windshield for a bit just to make the car keep the low signal light always on.

Is there anything notable about the terrain at that point? A sharp turn, bumpy road, etc.

Long slow bend on completely flat road, 40mph. No braking or acceleration for a quarter mile each direction.

The road is like that for miles with some lights, traffic circles, etc... Only warnings in one spot.

I really didn't believe my wife, or thought maybe it was happening a few times when she made a regular trip like getting drive thru coffee and coming right back. But then we ran a bunch of errands all afternoon and got one warning there on the way out, and one warning on the way back.

> Long slow bend

That’s probably it.

If I were designing a wiper fluid warning, I would use some sort of fluid level sensor and I would denounce it aggressively: the indicator would only light up if the sensor detected a low level for more than a couple seconds. That way traffic circles, bumps, etc would not cause many false positives. I might even couple it to some kind of acceleration sensor so a warning would not turn on during or shortly after any heavy vibration or acceleration.

A long slow bend would cause a prolonged, steady centrifugal force and/or sideways acceleration due to a banked road, which would defeat these mechanisms.

The question then is does it only happen in one direction? If the answer is yes, than I think you've solved it. If the answer is no, it might still be the problem but the sensor might be top or bottom mounted and not side mounted from the center I suppose.

edit that’s too late to edit: debounced, not denounced.

Maybe it's a combination of the "bend" and the "long slow" part.

If you are making a turn the centrifugal force will push the fluid away from the axis of rotation. There is likely a level sensor only on one side of the tank, so the turn might push the fluid away from the sensor enough to trigger a warning.

The sensor likely has a time component to avoid triggering every time you make a turn, but if this bend is long enough, maybe the fluid is displaced long enough that it overcomes that minimum time.

Could be related to terrain a certain distance/time before that spot in the road. The car may only trigger the warning after the fluid level has stabilized which a flat road would contribute to.

Perhaps it's long and slow enough to make the fluid move to a certain point and stay there long enough for the light/alarm to come on. If it's electronic vs solely mechanical, I'm sure they've got some smarts so that certain changes are ignored for a period of time (incline/decline, sharp turns, short stops).

Maybe you found the sweet spot at a certain point in that long, slow bend?

Is it a Subaru? Mine is currently doing that when I go through a long turn in a highway interchange. I'm sure I need to add fluid but it's on the edge where it only shows up when the force of gravity is pulling the remaining fluid a certain way.

My thoughts too. I would often induce a quick g-force to get the last few drops out of the same fluid tank. I could see the same triggering a sensor under similar circumstances.

Same exact thing with my wifes 2023 BMW. Exactly the same spot.

Hilly area? Is the car at a particular angle in that spot? :)

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