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Old 02-28-2009, 01:53 PM   #24 (permalink)
EcoModding Lurker
Join Date: Feb 2009
Location: colorado
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Thanked 3 Times in 1 Post
Below is the MPGuino installed in my '96 Passat TDI:

This pic was taken in the early stages of calibration, so its reading a bit optimistic.
Many thanks to dcb and rmccomiskie for their helpful posts on this subject.
What I ended up doing is tapping into the ECU fuel consumption signal as a substitute for the lack of a 12V injector lead in a TDI. The ECU fuel consumption signal is a "negative pulse" which requires a minor change to the MPGuino code:
attachInterrupt(0, processInjOpen, FALLING);
attachInterrupt(1, processInjClosed, RISING);

attachInterrupt(0, processInjOpen, RISING);
attachInterrupt(1, processInjClosed, FALLING);

Again, thanks to rmccomiskie and dcb for figuring this out - it would have taken me a whole lot longer (if ever) without their help.

I tapped the VSS and fuel consumption signals at the instrument cluster plug. A Bentley manual is very handy for identifying the wire #s. On my Passat, it was wire 26 and 27 IIRC. Green and white wires spliced in below:

I procured an ashtray from a salvage yard Passat and cut a hole in it to make a somewhat stock appearing enclosure. Since the cigarette lighter was nearby, I tapped the constant 12V here.

With about 50 miles on the MPGuino, it looks like it's going to read fairly accurately. I had a 98 Jetta TDI with scangauge and the instant MPG readings on the MPGuino look fairly consistent with what I used to see with Scangauge - i.e. Idle shows ~0.17 GPH, coasting in gear 0 GPH, steady 60 MPH cruising shows 45-50 mpg on flat ground, 5-20 MPG while accelerating or climbing hills.

I'll post back with my calibration settings when I get a few more test miles racked up.
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