6.0L Map reading backward?

[y5mgisi]

A buddy and i just recently put a 6.0l chevy motor in his jeep. Running a computer and harness from street and performance. The problem is that his map sensor appears to be reading opposite of what it should. He bought a scan tool and while watching the readings for the map while driving they are opposite of what they should be. Take off from a light and the vac(inhg.) shoots up to 25ish. Let your foot off the gas while cruising and vac reading drops to about 8. He has driven around with a vac gauge and the scanner hooked up and watched as they gave out opposing readings. the vac gauge did exactly what you would expect. 20" at idle, drop hard on accelerating, and sky rocket while coasting. Could a map sensor be wired backward and produce this effect? or could faulty computer programing possibly do it? He has called S&P and goten no where with them. Any ideas?


EDIT:
So can anyone tell me what readings i should have on the scanner? Im finding conflicting information. At idle, i know how much vacuum the engine has is about 19-20 or so per the vac gauge. But per the scanner, it only has something like 8-10. is the scanner supposed to be giving those readings? With the key in run but the engine not actually running the map reading is like 29.5in.Hg. i would think it that it should have 0 in that situation?

 

[Blue85]

Get the pinout for the MAP sensor connector. Maybe the 2 signal wires are swapped. Are these harnesses hand-built? Also, double-check the units to see if one is in in-Hg and the other psi. It also seems possible that there is a software/cal issue in the computer that inverts the signal for some reason.

You are sure it's not a MAF, right?

How does it run?

 

[Fordum]

MAP stands for Manifold Absolute Pressure

The absolute part means that it references a vacuum. So, when the engine is off, it reads barometric pressure.
So, when you put a vacuum on it, it will read less since it is getting closer to the vacuum reference.

What the tool shows depends on how it is setup. If it is set to read actual manifold pressure, then that is what you would see.
That is the absolute pressure in the maifold.

If it is set to read the amount of vacuum below atmospheric than it would get bigger numbers as the vacuum increases.

If the engine is running OK, I would check the settings in the scan tool.