Temp gauge re-code

[80stech]

I'm going to try and do something with an Arduino in the E-box over the holidays. All the necessary wires are there including access to the dash warning light and looks like won't be hard. Would anyone have access to the voltage to temp conversion numbers or logarithm ?

[Overboost]

I'm not sure what pull-up the factory DME uses but here is one of my calibration tables using a 1K ohm pull-up to 5V. It is a Bosch 023 temp sensor and should be roughly the same as what BMW uses.

[andrewwynn]

NTCs are typically linear over the useful range. Take a couple Ω readings when it's not connected with some known temps easy math from there. I would love to have the warning light come on at 115C

[80stech]

Yes, I was thinking of just tracking the temp with inpa and taking some voltage points and going from there. The beauty with the arduino is that it makes possible all kinds of flexibility including linearization, turning on the light at what ever temp, even variable brightness to temp, flashing etc. adding audible warning, even engine shut down if parked and self diagnostic features!

[Overboost]

Originally Posted by 80stech

Yes, I was thinking of just tracking the temp with inpa and taking some voltage points and going from there. The beauty with the arduino is that it makes possible all kinds of flexibility including linearization, turning on the light at what ever temp, even variable brightness to temp, flashing etc. adding audible warning, even engine shut down if parked and self diagnostic features!

Frustrating to know BMW thought to sound a gong when the outside ambient temp hits 37F but didn't bother to sound the gong on something so critical like a melting engine...

[80stech]

Thanks OB! that could prove to be very useful!

[Overboost]

Originally Posted by 80stech

Thanks OB! that could prove to be very useful!

If it is indeed a 1K pullup to 5V, I calculated the voltage low and high at .370V to 4.765V. Keep in mind the sensor is non-linear and I can get you all the voltage conversions if needed. I just used an online voltage divider calculator.

V_{out} = V_{in}*\frac{R_{2}}{R_{1}+R_{2}}

[andrewwynn]

I was thinking the same thing. Once you know the Ω value at a few values you can tweak the apparent resistance with either series or parallel to offset the temp to have the light come on at any temp you want.

[80stech]

Originally Posted by andrewwynn

I was thinking the same thing. Once you know the Ω value at a few values you can tweak the apparent resistance with either series or parallel to offset the temp to have the light come on at any temp you want.

No, first of all the 1/2 of the temp sender that works the light appears to be an old skewel normally closed switch, so can't really do much to change that other than maybe use a different sender. The wiring diagram for the temp sender shows both sides the same and NTC resistors internally but I think that is misleading and from what I can tell. I am going to double check that though. So what I am trying to do is read the temp from the DME side and turn the light on with that.

[80stech]

In this case there is enough eeprom space to keep a duplicate line of the code so there is no checksum needed.