Steve Preston <steveophonic@yahoo.com> wrote:
> This helps me a little bit. But it creates a new
> question in my mind: should a person have the same
> reading from their MAP sensor via scanner that they
> would have from a vacuum gauge? Because I,in fact,did
> not get the same readings. The scanner said 10 in
> HG,and my gauge said about 19. But your subtraction
> that you came up with is making me wonder whether
> there should be a disparity?
Its been a while since I've used a scanner on a vehicle so I don't
recall all of the readings, and I've never used the DRB series of
tools, so I'm wondering - is the scanner actually giving you a vacuum
reading? Bernd mentions "MAP vacuum" (which seems like an oxymoron to
me) as distinct from "MAP pressure", but its possible... Is the
scanner actually giving you a vacuum reading or just manifold
pressure?
The reason I ask is because the computer doesn't really know what
the vacuum is to be able to give such a reading. The MAP sensor
(Manifold Absolute Pressure) is simply a barometer, which measures
pressure, not vacuum, and there is no vacuum sensor. It is possible
for the computer to give you a vacuum reading of sorts because when
the key goes to the on position just before (and/or while) the engine
is cranked, the computer can use the MAP sensor to determine the
current ambient air pressure (barometric pressure), and then of course
once the engine is running, it knows the manifold pressure, so to
determine vacuum, it can just subtract the manifold pressure from the
barometric pressure. (Of course, if the baseline barometric pressure
reading is being taken during cranking, there is going to be some
vacuum in the manifold because the pistons are moving, and the
baseline reading will be at a lower pressure than actual barometric
pressure, which will cause any subsequent vacuum calculation to be
lower than actual.) Another problem with such a vacuum reading is that
it is accurate only immediately after startup, because there is no
ambient barometric pressure sensor, the computer has to assume that the
barometric pressure is still the same as it was just before the engine
is started. If you drive through a weather system or up and over the
Rockies, or through Death Valley, any guess by the computer at the
current manifold vacuum is going to be wildly inaccurate.
(Fortunately this isn't an engine management problem because the
computer doesn't need to know the vacuum to run the engine, it only
cares about the air density, which is adequately conveyed by the MAP
sensor.)
--
-Jon-
.- Jon Steiger -- jon@dakota-truck.net or jon@jonsteiger.com -.
| '96 Kolb Firefly, '96 Suzuki Intruder, Miscellaneous Mopars |
`-------------------------------- http://www.jonsteiger.com --'
This archive was generated by hypermail 2b29 : Tue Oct 02 2007 - 15:23:18 EDT