Who knows how BMWs fluid level senors work? (not as simple as you think)

[Estoril-E46]

I was thinking this forum or the tuning forum since there's no electrical forum, but since this is for a forced induction car I figured more people in this section would be interested and or want to help.

So BMW uses the same fluid level sensor for pretty much everything needing it being measured (with a few exceptions) You can see the piece I speak of and the numerous places here: http://www.realoem.com/bmw/partxref.do?part=17137524812

Now this sensor does NOT come in contact with the fluid, it is held in a cavity of the tanks that gets surrounded by fluid but is completely separate from the fluid itself. I know by looking at it that it gives some sort of variable resistance, but what I want to figure out is what force acts on the sensor to have it change. I cant think of really any measurable environmental variables that change when the fluid gets low where the sensors are placed, again they are not in contact with the fluid, but in a cavity built into the tank.

I'm sure I'll be able to figure it out, just wanted to see if anyone knew and could save me the time.

BTW when I do figure it out, I'll be trying to integrate the washer level switch into the HPF tank. Low washer fluid will turn into low meth :)

[paraklas]

I don't get it. Fuel level sensor is different in design from the coolant level sensor and the coolant sensor is different from the oil level sensor. AFAIK all sensors come in contact with the fluid, perhaps not all parts of the sensor.

[Mr Paddle.Shift]

Non contact principles boils down to ultrasonic or a heated wire element of some sort. I doubt the radiator level or washer level sensor is using ultrasonic (would be cool though).

So my bet is on the heated wire. Similar principle to the MAF. You heat up a wire by sending a constant current. You measure the Vd across. The height of the fluid level is proportional to the Vd across this wire. Geeky stuff but fun.

But if you research more, the oil level sensor is much more expensive. Still, I doubt it is ultrasonic.

update: forgot to mention, it's based on heat dissipation. Heat transfer rate is different when the wire sense fluid surrounding and lack of. Still no-contact required.

[Estoril-E46]

Non contact principles boils down to ultrasonic or a heated wire element of some sort. I doubt the radiator level or washer level sensor is using ultrasonic (would be cool though).

So my bet is on the heated wire. Similar principle to the MAF. You heat up a wire by sending a constant current. You measure the Vd across. The height of the fluid level is proportional to the Vd across this wire. Geeky stuff but fun.

But if you research more, the oil level sensor is much more expensive. Still, I doubt it is ultrasonic.

update: forgot to mention, it's based on heat dissipation. Heat transfer rate is different when the wire sense fluid surrounding and lack of. Still no-contact required.

Yea, I was saying the oil sensor was one of the few ones BMW uses that actually contacts the fluid. I'll take a pic of the sensor, but if it's like you just said I'm guessing the target reading is calibrated to a specific tank.

[TxZHP04]

Can't verify without removing it to see but it's likely a reed switch triggered by a magnetic float in the expansion tank.

[phelix]

Can't verify without removing it to see but it's likely a reed switch triggered by a magnetic float in the expansion tank.

Yup; on the other end of that float is the stick that tells you the level when you remove the cap.

[AMG ETR]

Please someone figure out how to adapt this to the HPF methanol tank, its VERY annoying running out all the time! Having to pull the ECU cover and risk loosening the wires just to purge the system!
Evan

[nuc1]

Please someone figure out how to adapt this to the HPF methanol tank, its VERY annoying running out all the time! Having to pull the ECU cover and risk loosening the wires just to purge the system!
Evan

Run out of meth = pull air in = have to purge? Correct or am I in left field?
Curious. Why do you have to pull the ECU cover?

Mike

[AMG ETR]

Run out of meth = pull air in = have to purge? Correct or am I in left field?
Curious. Why do you have to pull the ECU cover?

Mike

Correct, you have to pull the ECU cover to access the methanol connection and plug in the purge harness HPF supply's with the kit!
Regards,
Evan

[Estoril-E46]

Can't verify without removing it to see but it's likely a reed switch triggered by a magnetic float in the expansion tank.

Yup; on the other end of that float is the stick that tells you the level when you remove the cap.

No and no, that is just a float completely unrelated the the low level warning system, it's just there to tell you how much is in the tank so you can keep it in the right range.

I say this because the same float is not present in any of the other tanks the switch is used in (not floats at all) and it does use the same switch, it also has no other wires coming off besides that switch.



Have been very busy so no time to play with the switch. It might be very hard to adapt to the tank though as it is an immersible switch. I'm thinking about using a non OEM switch at this point connected to an LED.

[AMG ETR]

Decided to look further into this myself...

I called a company that sells floating switches and they explained how I could use one to make this work. Basically you mount this switch on the bottom of the tank inside at the height you want the light to eluminate. Then when the level drops below the half way point of the floater inside the unit it would no longer float and make contact with the bottom of the unit. The wires would have to be run to a relay then to an LED which would illuminate once the level dropped below the floater...

Here are a couple photos of the unit.
http://www.fluidswitch.com/images/fs11.jpghttp://www.fluidswitch.com/images/FS11-0001sm.gif
I somewhat understand how it would work, and might try it. I have a couple concerns though. The main one being you have to drill a hole in the methanol tank to mount this floater switch and with all the methanol leaking issues out of connectors and the pump, how would I seal this hole?
Evan

[Estoril-E46]

I somewhat understand how it would work, and might try it. I have a couple concerns though. The main one being you have to drill a hole in the methanol tank to mount this floater switch and with all the methanol leaking issues out of connectors and the pump, how would I seal this hole?
Evan

They have sensors you can literally stick onto the tank that will work, that is what I am currently looking into.

I wouldn't use that float unless its rated for corrosive environments. Plus it is entirely to big to be used.

[gasser]

they make specific switches for meth tanks. look at snow performance and other meth sellers, very easy install and cheap.

[TxZHP04]

No and no, that is just a float completely unrelated the the low level warning system, it's just there to tell you how much is in the tank so you can keep it in the right range.

I say this because the same float is not present in any of the other tanks the switch is used in (not floats at all) and it does use the same switch, it also has no other wires coming off besides that switch.

Have been very busy so no time to play with the switch. It might be very hard to adapt to the tank though as it is an immersible switch. I'm thinking about using a non OEM switch at this point connected to an LED.

BMW has been using reed switches in applications like this for years. You're going to have to prove me wrong on this one. :hi:

If you look inside your wiper fluid reservoir, you will notice a little housing just above the cavity where the sensor resides. This most likely contains the magnetic float which triggers the switch, same as with the expansion tank.

I believe you misspoke above when you labeled the switch immersible. You actually described it properly in your original post when you stated that the sensor never comes into contact with the fluid.

The fact that the switch never comes into contact with the fluid leaves very few options for triggering it. Reed switches are commonly used in this type of application.

Why don't you post up a pic of the switch itself so we can settle it once and for all?

[Flavourless]

I understand the desire to use the OE BMW sensor and that way the low washer fluid light on the dash lights up. We did this on lots of VW's and it was awesome. Could you not use another type of sensor and install a LED somewhere so when the sensor reads low it triggers the LED? Not the same as OEM but would work well and should be fairly easy.

Also what is the need to remove the Ecu cover? Can you not jump the meth pump directly at the pump to purge it?

[bimmer07]

only problem you might run into is the viscosity of the meth is going to be different than a mix of distilled water and washer concentrate

[AMG ETR]

Also what is the need to remove the Ecu cover? Can you not jump the meth pump directly at the pump to purge it?

5 Torx Screws... Not a big deal, but I have already had to replace 2 jumper boxes and 1 wiring harness due to loose wires from having to remove the ECU box so many times... The purge is inside the ECU box and must be removed...
Evan

[Estoril-E46]

BMW has been using reed switches in applications like this for years. You're going to have to prove me wrong on this one. :hi:

If you look inside your wiper fluid reservoir, you will notice a little housing just above the cavity where the sensor resides. This most likely contains the magnetic float which triggers the switch, same as with the expansion tank.

I believe you misspoke above when you labeled the switch immersible. You actually described it properly in your original post when you stated that the sensor never comes into contact with the fluid.

The fact that the switch never comes into contact with the fluid leaves very few options for triggering it. Reed switches are commonly used in this type of application.

Why don't you post up a pic of the switch itself so we can settle it once and for all?

I meant not immersible, just didn't type the non part. But gold star for you, on closer inspection there is in fact a Piece next to the cavity for the switch that is moving around. That pretty much ends any chance of it being worth the time to try and use it with the meth tank.


When you were talking about a float I thought you were refering to the little stick that pops up when you open the cap.

[AL330CI]

There are three types of switches for these kind of applications:
1) Reed Switch (cheapest): Using a magnet as the triger. When magnet comes close to the reed switch, the switch triggers. The magnet is usually embedded in the float.
2) Inductive Sensor Switch: Works same way as a reed switch but uses a metalic (with iron content) object as a trigger for the switch.
3) Capacitive Sensor Swtich (most expensive): Can be clamped on to the outside of the tank. When liquid comes close to the sensor on the inside of the tank, the capacitive field changes and triggers the sensor.

You can use multiple reed switches with multiple resistors encased in a rod form factor to create a variable resistance level sensor. As the magnetic floats up and down the rod, it would trigger a different switch. Each switch would close/open the switch to a different resistor to give the variable resistance sensor.

Also you'll also need some kind of time delay on the sensor signal to prevent sloshing of the liquid from triggering and giving you a false signal. It's not as simple as just adding a sensor.

[Flavourless]

5 Torx Screws... Not a big deal, but I have already had to replace 2 jumper boxes and 1 wiring harness due to loose wires from having to remove the ECU box so many times... The purge is inside the ECU box and must be removed...
Evan

I understand not a big deal but still should be a fail safe way integrated near the pump in order to purge the pump in case of running out, so things don't happen such as in your case where you have replaced 2 jumper boxes and a wiring harness....I think that having to take the ecu cover off to purge the system is just asking for trouble which I feel you agree as well.