For those of you with an electrical background...

[Justin B]

So there's this idea I have. It sounds simple, but I know it wont be. I don't even know if it's doable. What do you think?

I have an 02, and would really like to get a set of Autometer gauges in it. I do NOT want to hack up my gas tank either to put in an after market sender, but the BMW sender and after market fuel gauges don't like each other. As I recall, BMW senders are backwards (Low resistance when empty and high when full), and have a goofy range from E to F as well. I remember at one point I found what the range was for the sender I have (from an e30 318i), but don't remember now. That is besides the point. The gauge I'd like to use has a range of 240Ω when empty to 33Ω when full.

Where the question lies, is if it is even possible to have an input resistance, get it turned upside down and to a different ratio, and bleed out a different resistance on the output, linear with the input? What kind of cluster of a schematic am I looking at if it is? I think I'm off my rocker.

:crazy::hitler:

Go ahead, tell me it's a lost cause. I just want to know if its doable at all...

[euroshark]

I'm doing the same thing with my E24 but using VDO gauges... I am curious as well if it's possible to use the stock sender.

A VDO sender might fit with little modification though, right? I mean, VDO and BMW are like peanut butter and jelly...

[Justin B]

lol at the PB&J comment. I dont know, what does the after market VDO sender look like? What are the resistances on the gauges you want to use?

Either way, if I continued on my autometer path (I like a particular gauge they have thats got 4 in one, designer black kit 1408), I'd need some real help making up a circuit to confuse the resistances, or chop up my awesome gas tank that I just got finished restoring to put an extra hole in for an autometer fuel sender. I just remembered though that I used 3M undercoat spray on the whole outside of it for a fine texture besides where the original hole is for the sender/fuel pump. A new hole wouldn't seal too well without a generous amount of gas-resistant RTV, and the POR 15 sealant inside may lift around where the new hole is cut.

Meh. I started thinking to myself a bit there, but all the more reason to know whether this is even a valid possibility...

[TwoJ's]

I'm sure that there is some elaborate circuit that would make it work. Might even need a PIC. It seems to me the only way to do this easily is if the ratio of resistence to gauge position is linear with both the sender and gauge. It would also be easy if they were not linear, but had inverse proportions. Do you have access to a variable power supply? Have you tested the resistence of the sender in multiple tank capacities and plotted them to see if it's linear or close to it?

[TwoJ's]

I'll ask my EE teacher tomorrow if I make it to class.

[Justin B]

I have not tested it yet, but I could today. I was thinking it may require a PIC too. Gotta go to work :D

[Jscotty]

I don't know if this would be of any use to the situation or if this could give you an idea, but if you could get the schematic of an audio noise gate, that would probably get you close to where you want to be.

A noise gate is a device used in live sound and audio recording that prevents the audio signal from going through it unless the signal is of a certain strength. You can adjust the strength of the signal coming in and then you can adjust the strength of the gate. Essentially the "gate" keeps the resistance high on the back end of the circuit until the resistance is low on the front end and then it works vice-versa.

This does not exactly solve your problem but I think that the circuitry can help point you in the right direction.