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Old 09-02-2008, 10:16 PM   #1 (permalink)
EcoModding Lurker
Join Date: Aug 2008
Location: Massachusetts USA
Posts: 84

Ziggy - '95 Audi S6 Sedan

Manfred - '97 Audi A6 Quattro Sedan
90 day: 20.61 mpg (US)

Clarabell - '03 Audi A4 Quattro Avant

Sherman - '98 Audi A6 Quattro Avant

Cab - '96 Audi Cabriolet
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MPGuino for Audi

I've been lurking here for a while. Now I have an iDuino and I'm ready to start implementing in my 97 Audi A6 Quattro. Fortunately, Audi provides several connection points for VSS. It's a signal that's output to the other systems by the speedometer. Even better, Audi provides a fuel consumption signal that's intended for the optional trip computer. I'm hoping this signal will avoid the need to measure injector pulses directly.

Here's what the VSS signal looks like at 60 mph:

I've been told the signal from the sender is 8 pulses per tire revolution. Given the stock tire size, I expected about 11 pulses per 100 mS. The oscilloscope proved it.

Now the fuel consumption signal at idle (700 rpm):

And at 60 mph steady:

And accelerating hard 55 to 65 mph:

The only expanation of the fuel consumption signal that I've found is this:

"The fuel consumption signal is a data message which is conditioned by the engine control unit. The sum total of the high levels during a defined period of time corresponds to the injected fuel quantity."

So, it sounds like I just have to count the pulses that occur within, let's say, 100 mS. The problem is, what volume of fuel does that number relate to? I may have to just estimate a ballpark volume and then adjust with every fillup.

I know that I get about 22 mpg at 75 mph. If I use the scope to capture the number of pulses at 75 mph, I should be able to get close to the fuel volume those pulses represent.

Does anyone have a better way to do this?

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