Second MAP sensor, some hard found info and a question

[millsroger]

Here is what I found, all the turbo engines have an MAP sensor on the intake manifold to monitor manifold pressure. Some have a second MAP, referred to as the Baro, it is on the firewall drivers side and monitors atmospheric, or Barometric, pressure.

"In vehicles below EPA GVWR weight rating it would be used to determine when the EGR opens and closes based on the difference between atmospheric and manifold pressures. It would also be used to modify the boost limits for changes in altitude. In vehicles above the EPA rating, that are not equipped with an EGR system, it only needs to modify the boost limits for changes in altitude".

My source is GM archives and a partial description of the intended function of the second MAP. This does not mean that it ended up working that way. However, my conversations with four different chip programmers, including Heath, lead me to believe that the archive statement is accurate and that the ECM does function as stated.

In my case, a '94, the GVWR rating change was at 8600lbs.

My question is: Does anyone have the wiring for where the "Baro" sensor is connected to the ECM?

 

[Paveltolz]

I don't have access to mine as I'm out of town but I've found the Hanes manuals to be fairly accurate. Library may have a copy.

 

[millsroger]

Found it, checked it and it's working. Soon as I have gauges installed I can see if the ECM is really tracking it.

 

[buddy]

You can also see the value of the Baro sensor reported separately from the boost sensor on a scanner.

 

[millsroger]

Hi Buddy,
I would expect to but don't have a scanner. Was told by Joe at Heath that the ECM samples the Baro for atmospheric pressure every 104 seconds. In the later models that do not use the second MAP the atmospheric pressure is sampled while the glow plug heat is on before start up. Meaning that, if you then drove from Death Valley to the top of the Rockies without shutting the engine off it would not correct in between.