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BMW E36 Duel Fuel Level Sensor Calibration

PostPosted: Fri Jun 18, 2021 2:52 am
by 01-0041
Hello all,

I’m preparing to do some work to the car and could use some assistance/clarification before I do so.

I’ve been looking at the previous fuel level discussions and still had some questions that lead me to this ‘pull up resistor calculation’ document from AiM: https://www.aimsportline.com/download/f ... 00_eng.pdf

My example - using the one from AiM and the factory service manual values: the floats indicate 500 Ohm (250 Ohm ±5) with the fuel tank full and 20 Ohm (10 Ohm ±2) with the fuel tank empty, take 500 Ohm, multiply it by 9 and you get 4500 Ohm - not factoring in the tolerance values.

Looking at readily available resistors I could use 4.53 kOhms (±1% (1/4W)), but 4.64 kOhms (±1% (1/4W)) and 4.7 kOhms (±1% (1/4W)) are available as well.

Given the provided information - is there something I’m missing for use with a C12X?

Thank you!!

Re: BMW E36 Duel Fuel Level Sensor Calibration

PostPosted: Sat Jun 19, 2021 8:07 pm
by MikeD
Hi,
to answer your question at the beginning, you could use any of those values mentioned and it will work...

Anyway I'm not sure where the term "multiply the highest value by 9" comes from, maybe they think about to use something like a 12V pullup voltage or a very low amperage 5V supply...? This kind of dimensioning isn't something I've seen before as you loose a fair bit of resolution with such a high pullup.

With a C1xx dash and the values provided I would use an AT input which has a 1k pullup to 5V integrated. This means straightforward wiring from AT input to one of your fuel level senders (I assume you have two of them connected in series) and the other end to ground (if the senders itself don't have a connection to ground then use the sensor 0V GND from your dash as well for best accuracy).

Cheers,
Mike

Re: BMW E36 Duel Fuel Level Sensor Calibration

PostPosted: Tue Jun 22, 2021 12:12 am
by 01-0041
Hi Mike,

Thank you for the reply!

Over thinking things based on too much searching and not enough working…

You are correct - they will be in series.

While I was pluming in the new hardware this weekend I thought I'd setup a quick table and experiment. For the test I used AT1 and 0V and did a sweep of a single fuel level sensor and it worked great!

Thank you for the push!