<VV> RE: MAP sensor - utlimately for a Corvair

[burkhard at rochester.rr.com]

I agree with Craig except the part about the BARO estimation being off 
if you drive up a mountain after starting at sea level. I don't have 
knowledge at how every company does it, but at least SOME of them (and 
I'm betting most if not all) do have some pretty trick algorithms that 
can relearn the BARO estimate without shutting down the engine if it's 
suspected that BARO has changed a lot since the original determination. 
At one time, it's possible that this could have been a (minor) problem, 
but not really anymore. 

Also, not all the new ECMs have on-board BARO sensors. Some still 
figure BARO from the MAP sensor at keyup and then update when 
the "sense-checking" suggests a relearn is required.  It all depends on 
what the OEM is willing to pay for.  :-)

Jim Burkhard


----- Original Message -----
From: Craig Nicol <nicolcs at aol.com>
Date: Thursday, October 25, 2007 2:16 pm
Subject: <VV> RE: MAP sensor - utlimately for a Corvair
To: virtualvairs at corvair.org, chsadek at comcast.net

> Chuck asked:
> <snip>Manifold Air Pressure sensors used on more modern cars sense 
> vacuum in
> the manifold as I understand it. Some are mounted on a firewall 
> with a
> vacuum line to the manifold. 
> Questions. What voltage is supplied and what voltage range vs. 
> vacuum range
> do they sense? What is the reference? Absolute or ambient? If ambient
> pressure, how does the sensor get it? Is there a port on the sensor?
> Thanks Chuck S. BBRT <unsnip>
> 
> Craig Replies:
> The common GM sensor senses ambient (baro) at Key-on and this value
> (voltage) is stored in the ECM as "BARO".  There is no separate 
> port for
> sensing barometric pressure. Sensor variations after engine start 
> (caused by
> the engine operating) become the MAP (Manifold Air Pressure) 
> value.  Note
> that this value is always a pressure - the concept of manifold 
> vacuum is not
> typically used or relevant anymore.  The weakness of this sensor 
> setup is
> that BARO remains fixed in the ECM until the next key cycle, so if 
> you start
> in LA and drive to the mountains, the LA value is still the 
> supposed BARO.  
> 
> Newer ECMs have on-board (literally) baro sensors that track the 
> actual BARO
> value.  GM MAP sensors are available in 1-bar (for NA engines) and 
> 3-bar
> (for turbocharged engines).  They both run off of a regulated 5v 
> signalthat's sourced and monitored by the ECM.
> 
> Craig (Now you are talin' my bread and butter) Nicol