Megasquirt OEM Barometric sensor
02-28-10, 07:28 PM
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OEM Barometric sensor
After many Megasquirt installs for many different cars, I finally decided to install one on my personal car. I'm using DIYAutoTunes N52 DIYPNP kit. I decided I wanted the resolution of the supplied MAP sensor instead of the MAPDaddy dual map sensor...but still want barometric correction. So, this afternoon I spent a little bit of time with the stock barometric sensor, a multitester and a vacuum pump.
Incase anyone is wondering about my methods, this is what I did: Pulled out the ol' trusty Harbor Freight vacuum pump with the container for liquids (for a buffer, of sorts). Hooked one side to the vacuum pump, the other side to a vacuum hose. Installed a "T" fitting, and connected one side to the Megasquirt's MAP sensor, the other side to the stock barometric sensor (with the cap pulled off).
At atmospheric in my area, its right at 100kpa. Then I pulled vacuum by 10 kpa (per the Megasquirt) and measured the voltage from the stock sensor. This is what I came up with:
kPa Volt
100 3.87
90 3.5
80 3.15
70 2.75
60 2.35
50 2.00
40 1.60
30 1.20
20 0.80
When I graphed these numbers, I found it to be a nice nearly-straight line. Through extrapolation we should get:
5.0v ~130kpa
0.0v ~0kpa
Now, it'll be about 3 weeks before I get a chance to install the Megasquirt in my car...in the meantime, does this math look correct to everyone?
Incase anyone is wondering about my methods, this is what I did: Pulled out the ol' trusty Harbor Freight vacuum pump with the container for liquids (for a buffer, of sorts). Hooked one side to the vacuum pump, the other side to a vacuum hose. Installed a "T" fitting, and connected one side to the Megasquirt's MAP sensor, the other side to the stock barometric sensor (with the cap pulled off).
At atmospheric in my area, its right at 100kpa. Then I pulled vacuum by 10 kpa (per the Megasquirt) and measured the voltage from the stock sensor. This is what I came up with:
kPa Volt
100 3.87
90 3.5
80 3.15
70 2.75
60 2.35
50 2.00
40 1.60
30 1.20
20 0.80
When I graphed these numbers, I found it to be a nice nearly-straight line. Through extrapolation we should get:
5.0v ~130kpa
0.0v ~0kpa
Now, it'll be about 3 weeks before I get a chance to install the Megasquirt in my car...in the meantime, does this math look correct to everyone?
03-01-10, 11:12 AM
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I like your methodology and don't see anything wrong with your scheme. The only possible problem I can think of is if you can specify a separate calibration for the baro sensor. You'll want a 1K resistor to the MS input pin and need to be sure the OEM sensor ground is at the MS just like any other sensor. Keep us posted, this sounds like a great way to reuse perfectly good OEM parts.
03-01-10, 11:58 AM
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I like your methodology and don't see anything wrong with your scheme. The only possible problem I can think of is if you can specify a separate calibration for the baro sensor. You'll want a 1K resistor to the MS input pin and need to be sure the OEM sensor ground is at the MS just like any other sensor. Keep us posted, this sounds like a great way to reuse perfectly good OEM parts.
http://www.diyautotune.com/images/pr...y-ms2cfg_1.gif
I'll make sure to put a 1k resistor inline, thanks. I would hate to burn up one of the ADC's.
After studying it over the weekend, getting a signal out of the stock sensor was pretty simple. Apply +12v to one pin, vref (5v ref from MS) to another pin, ground, and the 4th pin is signal. Will not work with just 5v ref and ground, +12v is required.
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