1995 OBDI MAP sensor limits?

[buddy]

Your 94 should have a BARO sensor on the firewall, driver side, with its port open to atmosphere. The emissions trucks with EGR also used it for EGR control, verifying vacuum levels.

The 2-bar sensor doesnt seem to have any trouble reporting all the way to 30psi (just past 2-bar), and tracks right along with boost gauge. We werent having an accuracy issue, and the voltage levels and ranges are understood. The PCM code does have a table for MAP pressure vs RPM to determine how much fuel to cut, for altitude compensation. GM decided not to populate the table on the non-emission trucks, but it is on emissions trucks. So it can be populated for any truck program.

The PCM also has a bit to define whether your engine was NA with one 1-bar sensor, or turbo with the 1-bar sensor and the 2-bar sensor.

The PCM code uses a conversion formula, and as mentioned, we modified the PCM program to perform the 3-bar voltage conversion.

 

[millsroger]

Wiring diagram does not show the BARO, will go look today. My point re altitude is that the MAP may read 2.4 at sea level and start from there for vac/boost, but at 6000 ft elevation it will read alot lower to start with. that means 20" of boost will be less in "absolute" terms. Ergo, less fuel at a given RPM at altitude than at sea level.

You said that GM did have a "correction table" but did not use it on non EGR?? did I read that right?? Did GM use the BARO with a separate table???, kind of redundant.

Atmospheric standard is 29.92 at 59 degrees at sea level--two bar is twice that or 59.84" so a two bar sensor should read to 30psi accurately, I am sure it will read some past that to account for a high pressure cold day.

 

[WarWagon]

GM had their 5.7 Olds diesel banned in CA for 1985. So emission vehicles would want to keep the smoke down. Going from -200' to around 8000' in altitude is possible (Death Valley, CA) without shutting the engine off. So a separate baro sensor is needed. Using MAP for baro readings with the engine running is ugly. Gas engines can take a reading at WOT. Diesel NA anytime, turbo I doubt. Air filter restriction making it ugly.

MAF is different system with regard to baro needs.

 

[millsroger]

OK Buddy, I just checked, ur rite! Xtra unit is there where you said it would be, has a different part number than the one on the dog house. Later on I will check the voltages on each but I suspect the dog house one is a two bar and the other is a one bar, logic says so, but----. So, from your earlier posts, my 94 is using the extra one for barometric pressure as an altitude sensor and so it also prolly has the chart for it in the PCM.

Might be going off the "MAP limits thread" but is there a version or rev code list to tell which PCM is correct for what??

 

[buddy]

My point re altitude is that the MAP may read 2.4 at sea level and start from there for vac/boost, but at 6000 ft elevation it will read alot lower to start with. that means 20" of boost will be less in "absolute" terms. Ergo, less fuel at a given RPM at altitude than at sea level.

Less fueling at a given RPM would be the PCM performing some compensation for altitude, and the fuel derate table in the program is purely intake absolute pressure based, not boost based. My point is that what you have stated was never in question, it is understood.

Atmospheric standard is 29.92 at 59 degrees at sea level--two bar is twice that or 59.84" so a two bar sensor should read to 30psi accurately, I am sure it will read some past that to account for a high pressure cold day.

I was pointing it out since you said it would be inaccurate near its limits.

 

[buddy]

Xtra unit is there where you said it would be, has a different part number than the one on the dog house. Later on I will check the voltages on each but I suspect the dog house one is a two bar and the other is a one bar, logic says so, but----. So, from your earlier posts, my 94 is using the extra one for barometric pressure as an altitude sensor and so it also prolly has the chart for it in the PCM.

I posted before that it has a 1-bar BARO sensor and a 2-bar Boost Sensor, thats what you have. NA versions would only have the 1-bar sensor.

ALL 94-95 6.5 trucks have the BARO sensor, its used to calculate boost in the PCM. The non-EGR ones did not utilize the fuel derate table, even though its there. OBDII 96+ trucks use the single MAP to measure the Baro and boost, and only the EGR equipped trucks had the separate 1-Bar BARO sensor.