LCD Instrumentation for Cougar Controller

[harlequin2]

20 to 220 ohms - I'd be surprised if there is 12V across the sender, particularly as it is driving the computer. Can't find a circuit diagram of my car, they are all car electrician things that just show "green wire to plug A" type of thing. Maybe jackbauer you could measure it (if you haven't already) and confirm that voltage? Just seems strange to be pushing 600 mA through the the thing.
I've tossed out my old fuel tank and the sender went with it.

[jackbauer]

Yes its 12v but there are other resistances in play. Attached are 2 quick printscreens from the factory wiring manual.

[Freyguy]

Great, those screenshots are very helpful. Looks like both senders are in series, one end connected to ground (31) through unknown resistance and the other to +12V (15) through another unknown resistance. You could figure out the resistances by putting one 500ohm resistor in place of the series senders. Then measure voltages on low side of 500ohm (ref gnd) and high side of 500ohm. Then it's ohms law since you know current through 500ohm and difference in high/low side measurements to gnd &+12V.

So is each sender 20-220ohm meaning in series you'd need to create one 40-440 ohm resistance to interface to the cluster ?

[jackbauer]

I got it sorted. Today at lunch time i had a play. Rigged up a 500r pot to the brown/green wire from the instrument cluster and put the other end to ground. To my surprise it worked! I could adjust the gauge up and down over the range 20-220 ohms. As the resistance increased , the voltage on the wire increased. 2v = 1/2 tank , 3v = 3/4 etc. Again though i could "fill" much faster then "drain". There is some form of damping in the dash computer.

I still have to play about getting it calibrated but as this little video shows it works a charm! Well done Chris

http://www.youtube.com/watch?v=BO04UTbUPLM

[harlequin2]

Nice video, but I missed exactly how you are driving the fuel gauge from the LCD unit. You said you connected a 500 R pot, but I thought you meant a manual one. Can you clarify that please?

[jackbauer]

Sorry. I have the output from the lcd module directly connected to the brown/green wire from the instrument cluster. I experimented with a 500r pot to determine which wires effected the gauge reading.

[harlequin2]

Ah, I see. So it accepts the variable pulse width from the LCD board and just integrates it all to an average voltage.
I couldn't really read those BMW circuit diagrams, resolution to poor, but I wondered if you could scan a couple of those pages and PM them to me? It looks as if the wiring is very similar to my E30 model.

[jackbauer]

Can do. Sorry i had to scale them back to fit on here. I might be able to source the e30 diagrams.

[Freyguy]

Wow! Wasn't really counting on it being that simple. I was going to suggest as a next step to do the 500r pot thing, because ultimately, the computer justs wants to see a voltage from the sender/resistor divider network. If that worked and you figured the voltages that generate a particular tank level (which you did!), I was then going to suggest RC filtering the PWM and following it with a opamp buffer, but it appears the computer is happy enough with the PWM. Could be there is a passive or active low pass filter at the computer's ADC input. Good design practice since cars are a noisy environment.
I am a bit confused though as I thought you'd need an inverted PWM to work.
100%SOC=100% PWM (high from U6 pin 13) drives Q1 low all the time which should be empty tank for you. Maybe I'm missing something here.......well OK, I'm SURE I am

[mrbigh]

Originally Posted by jackbauer

I did a little video. Couldn't help myself

I have a question for you JackBauer,
What type of screw terminal connectors (phoenix ?) and pin spacing are you using on the PC board. Picture included....
Thank you