2 different vac/boost readings - SRT Forums

Novak94 · 10-01-2008, 11:03 AM

ok well last night i was playing around with my duel setup and found something very weird. With vac/boost being displayed on both gauges i got 2 different readings. One said 19 inhg other said 14 inhg

now i know the 19 inhg is right cause i have a AEM boost gauge and my vac was normal about 19. Now has anyone ever had this happen where one gauge seemes to be scured? Anyway to fix it?

Also i have the AEM vac/boost gauge hooked up to the 5v input on the scan gauge, but i can figure a right slope and intecpt to get it to read right? anyone have this gauge setup and working correctly?

moreause · 10-13-2008, 11:18 PM

are they both in inch hg

i have an old setup the main gage is showing vac in inch hg
and the other gage is in psi (in vaccum that equal around 1/2 the value)

for the slope and intercept thing it's not that hard to do
first you need to know your gage output like 0 to 5 volt =??

after that the rest is easy

i would say try slope 9.95 and intercept -14.7 and this will give you psi when under vaccum nothing's perfect

but i will have to read my gage info to be sure

blab10 · 10-13-2008, 11:26 PM

Try this as a test: Turn the key to the "on" position but leave car off and see what the boost readings are on both guages. I only have one Aeroforce guage and for some reason it reads about 4 psi lower then the stock guage. At first I thought it was because my stock guage was bad or something but by doing the above test I found that the scan guage was showing at -4psi. I have been told that there is no fix though, oh well.

moreause · 10-13-2008, 11:33 PM

my gage read almost zero

but if not you can't do much since it's displaying what the pcm is seeing
so if your's is off by 4 psi

then your tip or map is off by 4 psi at around 0 psi since the pressure reading use the barometric pressure to place the 0 at the right spot.

blab10 · 10-13-2008, 11:40 PM

Quote: Originally Posted by moreause

my gage read almost zero

but if not you can't do much since it's displaying what the pcm is seeing
so if your's is off by 4 psi

then your tip or map is off by 4 psi at around 0 psi since the pressure reading use the barometric pressure to place the 0 at the right spot.

I don't think so because my buddy has the Aeroforce scan guage for his Cobalt SS and his reads the same -4 when engine is not running. It is more than likely our altitude (5300+ ft above sea level) even though from what I understand the scan guage should automatically adjust since there is no longer an "altitude adjustment" option in the setup.

moreause · 10-13-2008, 11:48 PM

Quote: Originally Posted by blab10

I don't think so because my buddy has the Aeroforce scan guage for his Cobalt SS and his reads the same -4 when engine is not running. It is more than likely our altitude (5300+ ft above sea level) even though from what I understand the scan guage should automatically adjust since there is no longer an "altitude adjustment" option in the setup.

stange in deed .... just for fun next time check the barometric pressure

and the offset compare to 14.7 psi

maybe that one of the reason they don't show the same pressure maybe one is corrected and not the other or something like that

Novak94 · 10-14-2008, 10:36 AM

Well i think i kinda figured out the problem, I have a duel setup and for what ever reason the 2nd gauge is the one that is reading wrong. The main gauge is right on with my AEM boost gauge, but the 2nd one was about 5 psi/vac off. The even weird part is that it sometimes fixes itself, it will be off and then the next day or same day will then show the correct reading. And now that i have left it alone and been watching it everyday it now seems to have fixed itself perminatly. As fot the slope and intercept ill have to play around with that.

N0sferatU · 10-14-2008, 04:23 PM

just throwing a thought out there

Where the MAP reads boost and where the mechanical gauge reads boost are at two different locations in the engine...perhaps this could account for slight flucuations in what you read?

Just a thought I'm throwing out there.

Novak94 · 10-15-2008, 01:01 AM

^^^ yea i understand that, and thats what i though at first, but 5 psi difference seems like alot. Its reading close to normal now so idk what the prob was.

forcefedjunkie · 10-22-2008, 09:00 PM

I think this was covered pretty well, but jsut to clarify aa few things...

Both the primary and secondary gauges look at barometer when the car is started and saves this to calculate vacuum/boost based on the MAP. The primary seems to be the most accurate, for some reason the secondary doesn't always get a good barometer reading. It most likely has something to do with the way this is obtained. The primary uses teh Chrylser enhanced method of acquiring the data, the secondary uses teh generic OBD2 protocol.

For this reason we are bringing back the old altitude adjust for the secondary gauge. You'll be able to make this as accurate as you wish. But like I said, the primary has always been right on.

About the Cobalt SS, the boost reading is as accurate as the car's barometer reading. If you see it off by 4 or 5 psi, the barometer must be way off. I'd like to know what it is reading. This would be much better discussed on our tech forum, aeroforcetech.com/forum, than here.

Todd

Novak94 · 10-23-2008, 12:22 AM

^^^ ill check out the barometer and make a thread over there, thanks much