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Old 12-18-2008, 10:42 AM   #1 (permalink)
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Instead of vacuum gauge: multimeter on MAP sensor

Just had a thought - probably not original:

Instead of buying a vacuum gauge, another option is to wire a cheap multimeter to the MAP (manifold absolute pressure) sensor (if your car has one).



It wouldn't be too hard to drive a little bit and see what voltage corresponds to the lightest load, then use it as a relative indicator of fuel consumption. Very useful if using the DWL (driving with load) technique.

Maybe make a simple LED array that does the same thing, so you don't have to read numbers.

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Old 12-18-2008, 11:21 AM   #2 (permalink)
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wasn't there some talk recently of MAPs having a frequency output as opposed to V output?
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Old 12-18-2008, 11:26 AM   #3 (permalink)
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Most MAPs have voltage outputs. I'm sure some maker somewhere thought they'd be smart and put in a frequency output on something though.
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Old 12-18-2008, 12:09 PM   #4 (permalink)
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Quote:
Originally Posted by MazdaMatt View Post
wasn't there some talk recently of MAPs having a frequency output as opposed to V output?
Right - joelmaldonado used a Ford frequency output MAP sensor for his load-smart cruise control: Cruise Control (with MPG sensitive logic)

And I just remembered where else I've seen a MAP sensor + multimeter as instrumentation: Test: F-150 pickup "cap on" vs. "cap off" @ 60 mph

Ironically, they were using a Ford, so bought a junkyard GM MAP sensor to take relative readings of engine load during their tests.
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Old 12-18-2008, 12:42 PM   #5 (permalink)
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TDI MAPs are 0-5v. But then again we don't run under vac. It's useful to get the load though.
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Old 12-18-2008, 12:45 PM   #6 (permalink)
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I took some readings off the metro map sensor that coyotex sent. I used an atmega168 to turn 5 volts into a 0 to 1023 number, but a voltmeter on 5 volt scale would do fine too (for a geo):
Reading a MAP sensor

I really have no idea how many cars use pulses vs sensors or what all the pinouts are or the signals or voltages involved might be though.


Oh, and an analog meter would be nice for vacuum.
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Old 12-19-2008, 03:13 AM   #7 (permalink)
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I "think" my 95 sonoma has a map sensor. Anyone confirm? 4.3 CPI motor(92-95), not TBI or CFSI (96+ only).

I don't think the GP has one though, and that's my highway cruiser.
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Old 12-19-2008, 03:19 AM   #8 (permalink)
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I did this with my acclaim. I found that the dmm I was using, having 1/2 second measurement, wasn't responsive enough. Something a bit faster, and it would work great.
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Old 12-19-2008, 12:43 PM   #9 (permalink)
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I like this idea, one two wire cord coming into the cabin and my multimeter would make a good proof of concept and load indicator, very nice idea, I will likely try this sometime this weekend
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Old 12-19-2008, 12:53 PM   #10 (permalink)
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Cool - let us know how it goes.

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