Output for fuel economy (fuel rate) gauge?

[SamSpade]

The '86 BMW I'm working on has a fuel economy gauge:



Haven't tested it yet, but I suppose it works the same as the electronic tachometer which feeds on a symmetric square wave.  I understand that the P259 is configured to control the tach for this.

Of course computing for fuel economy requires a combination of a speed signal and probably injector duty cycle.  The Motronic ECU which the BMW uses has a vehicle speed signal input (VR sensor from rear wheel), so it's straightforward for it to compute for fuel economy.

Anyone have any idea how to implement such a scheme using VEMS?

[rob@vems.co.uk]

I think there's something to do with vacuum with that gauge.
In order to find a solution you really need to find what signals go into that gauge.

[gunni]

I´ll just tell you how it works

It is a square wave but it´s non linear so any output would have to be on a curve.

http://uk.youtube.com/watch?v=S5KwL2DUkro

I belive the boost output meter in current firmware would have to be further developed to work with the bmw gauge.
It´s also not body style comparable as there is a coding chip and other engine and diff related calculations in the dashboard going on,.

[Sprocket]

At less than 25mpg do you really need a consumption gauge anyway, cant you just use the fuel level gauge instead

 

[gunni]

There are templates out there now to turn that gauge into a
oil temp
oil pressure
boost
could be wideband if wanted

all kinds, but I think those will all be seperate purchasable thing so no standalone relations.

[SamSpade]

I´ll just tell you how it works

It is a square wave but it´s non linear so any output would have to be on a curve.

http://uk.youtube.com/watch?v=S5KwL2DUkro

I belive the boost output meter in current firmware would have to be further developed to work with the bmw gauge.
It´s also not body style comparable as there is a coding chip and other engine and diff related calculations in the dashboard going on,.

Gunni, as usual great info.  I think I can borrow a signal generator so I'll post here how it maps frequency to output.  My car's also in L/100km, just picked a photo off google images and got that one with the MPG counter.

There are templates out there now to turn that gauge into a
oil temp
oil pressure
boost
could be wideband if wanted

all kinds, but I think those will all be seperate purchasable thing so no standalone relations.

I like this idea.  I'll probably settle with oil temp, oil pressure or fuel pressure (when the turbo goes in there).

I guess there's no programmable frequency output in the current firmware right?  If not, maybe a simple voltage-to-frequency circuit would do well at the moment.  I think I have a spare 100psi, 0-5V output pressure sensor lying around.

Thanks everyone!