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Old 09-02-2008, 09:16 PM   #1 (permalink)
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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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Old 09-02-2008, 09:36 PM   #2 (permalink)
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looks like about 14040 vsspulses per mile?

The "fuel consumption signal" looks basically like an injector signal.
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Old 09-02-2008, 11:46 PM   #3 (permalink)
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Now I'm confused. The stock size tire turns 811 times per mile. The sensor is supposed to output 8 pulses per tire rev. 811*8=6488 pulses per mile.

Crosschecking that number, if the scope shows 11 pulses in 100 mS at 60mph, that's 110 pulses per second * 60 sec = 6600 pulses per minute (1 minute = 1 mile at 60 mph). So 1 mile = 6600 pulses. Pretty close.

What am I missing?
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Old 09-03-2008, 07:09 AM   #4 (permalink)
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I guessed 117 pulses per second (7 pulses in 60ms) per the graph, and the guino will multiply that by 2 (counts rising and falling edge). You can always calibrate it using the cars odometer so close enough for getting started.
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Old 09-03-2008, 11:20 AM   #5 (permalink)
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I'm currently working on getting mine to work on my '98 A4, and was wondering where you were able to tap into the fuel consumption signal...

I was looking through all of the wiring diagrams in the bentley manual, but I guess I missed that whole section.
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Old 09-03-2008, 10:13 PM   #6 (permalink)
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Quote:
Originally Posted by secmil View Post
I'm currently working on getting mine to work on my '98 A4, and was wondering where you were able to tap into the fuel consumption signal...

I was looking through all of the wiring diagrams in the bentley manual, but I guess I missed that whole section.
Ahh...an Audi owner to commiserate with!

Bentley wasn't much help for me. I found a thread on AudiWorld on retrofitting a trip computer. The fuel consumption signal came from a connector behind the passenger kick panel. I checked my car and it has the proper color wire in that spot. I hooked up the scope and it looks like the right signal.

It's a Blue/Black wire in the red connector here:



I removed the connector bracket, pulled the connectors and pushed a solid wire into the top of the connector to bring the signal out to where I could attach an alligator clip:



Remember, this is a 97 Audi A6. I don't know if the A4 will have this signal in exactly the same place. Audi is, however, pretty consistent. Since the A4 has a trip computer as an option, there must be a consumption signal somewhere. I'll bet that you will find a Blue/Black wire in a red connector in roughly the same place.

For the VSS, I found a schematic that showed the VSS going to the cruise control module. I pulled the module connector and found the signal there on a White/Blue wire. I found the same colored wire behind the kick panel and a continuity check showed them to be connected. So, I pushed a solid wire into the kick panel connector to bring that signal out.



Let me know if have any questions. Good luck.

Bob.
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Old 09-03-2008, 11:08 PM   #7 (permalink)
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good to see others trying to remedy the audi mpg curse ... '99 A4 2.8 here
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Old 09-04-2008, 06:23 PM   #8 (permalink)
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Can't cure the curse that Audi wrought. Best to know your enemy well though! Jump right in and join the fray.
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Old 09-05-2008, 02:25 PM   #9 (permalink)
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Quote:
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.
After logging more fuel consumption signals, I'm coming to the conclusion that the 97 A6 signal is one pulse for each injector firing and the pulse width is the length of time that the injector is open. So, the signal includes the injection time for all 6 injectors not just one. Summing the pulse width time is the total injector open time.

Now I need to monitor the pulse width at 75 mph where I get about 22 mpg. Then I can calculate the approximate injection time per gallon. Soon.
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Old 09-05-2008, 02:50 PM   #10 (permalink)
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If it actually conditions the signal, then you might need to adjust your "Injector DelayuS" from its default of 500. Or just tap an injector lead

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