3 bar Linear/Offset what should it REALLY be?

[BAD LS1]

Working on a '02 F body converted over the a 3 bar OS a long time ago.

Problem is that the KPA being reported to the PCM and being seen thru the scanner PID is nowhere close to the actual "gauge pressure"

The scanner is reporting up to 281 KPA by 6k RPM and the pillar gauge says about 14 psi, they both get their signal out of the brake booster line. How can there be that much of a discrepency? i mean that technically means like 26 psi if its real!

This car has a F1C on it, its a 347 and has a 4.25" pulley on the blower and 7.6 crank pulley with aster bracket etc.

The linear was at 305 and offset was at 10.33 for the longest time. Found a chart this morning for the GM 3 bar map thats in it (so i think anyway) and changed it to linear - 315 and offset - 6.00. It made zero change at idle in vacuum from before and now it showed up to 292 kpa this last test drive.

So can these settings being really off slew the map readings by THAT much under boost? OR is really a 2 bar map in a 3 bar system using the 3 bar pid to log it? Would it still read over say 215kpa on the scanner if it really wasnt a 3 bar in it?

I just want to make sure its not really getting whacked with that much boost. I have NOT verified it with another boost/pressure gauge yet to confirm if its the gauge or not.

Idea please? Was just trying to get the street manners close for this new build and head to the dyno tomorrow, but now im trying to sort this problem out.

[DSteck]

Find a part number off the MAP sensor.

If you want a MAP sensor that just plugs right into the factory location and wiring, PM me. It'll go to 22psi, and it will be extremely accurate because I have the right data for it.

[BAD LS1]

Well there is no decal or part # on this one that i can see. it was in 2009 this map sensor went into it. So i guess maybe thats the first step is finding a known good 3 bar to try in it.

Maybe im all wrong here, but the theory of this would be, since its getting so close to 15 psi, if it is actually a 2 bar map, would that make the scanner think its getting close to the end of the rope of the 5V scale hence why its getting close to 300 kpa? I mean if it were a 2 bar, the numbers on the gauge would fall right in line and would be getting near the end of 5v scale it would work from if it were to be a 2 bar. Or am i just straight up wrong? I guess putting in 2 bar data would answer this too if gauge boost tracked the KPA almost paralell?

[DSteck]

Yep. Add barometric pressure to your scanner and see what that is. It should be 95-100.

[BAD LS1]

So guess what, i just went out the car and looked, and it is a genuine GM map sensor in it... problem is that it has a "886" stamped into it, meaning it is Indeed a 2 bar! FML

Thanks for the help anyway!

[blownbluez06]

As I was reading through this thread, I was thinking this was the problem. Had you not had a sticker on there (and for future readers) I was going to advise that you place a volt meter across the green and orange wires, turn the key on and retrieve the voltage. A 1 bar will read ~4.9 volts, a 2 bar 2.4, a 3 bar 1.6. and a true 2.5 bar sensor, 2.0.



If your factory wiring scheme doesn't agree with those colors, simply find your 5v, your reference low and then your sensor output and measure these voltages between the sensor output and the reference low.

[DSteck]

As I was reading through this thread, I was thinking this was the problem. Had you not had a sticker on there (and for future readers) I was going to advise that you place a volt meter across the green and orange wires, turn the key on and retrieve the voltage. A 1 bar will read ~4.9 volts, a 2 bar 2.4, a 3 bar 1.6. and a true 2.5 bar sensor, 2.0.



If your factory wiring scheme doesn't agree with those colors, simply find your 5v, your reference low and then your sensor output and measure these voltages between the sensor output and the reference low.

2.5 = 1.78v at atmospheric. Ask me how I know.

If looking from the top down, with pins pointed down, the middle pin is the 5v signal, left is the 5v source, and right is ground (assuming you're still using the factory plug for the AC Delco sensors).

[blownbluez06]

2.5 = 1.78v at atmospheric. Ask me how I know.

If looking from the top down, with pins pointed down, the middle pin is the 5v signal, left is the 5v source, and right is ground (assuming you're still using the factory plug for the AC Delco sensors).

Your meter must me totally fried because mine reads 1.77.