i just noticed last night when i put the engine under a load
the scanner showed a higher vacuum reading and when i
remove the load it showed a lower vacuum reading .
i just tee'd a vacuum gauge in with the map sensor and
confirmed it,
light acceleration vacuum gauge showed 6 in hg & map showed 18 in hg

decelerate and vacuum gauge showed 20 in hg & map showed 8 in hg

i swapped map sensors , no change .
i checked voltage , 5.04 vdc reference to chassis ground , 5.00 vdc on outside pins on connector , .05 vdc signal to ground .
all are what my manual says is correct

this causes me to say WTF ???

any ideas ? this is needless to say making tuning impossible

 

Could be that the scanner is lying to you (wrong conversion for vacuum variable), or that the ECM is doing a pseudo baro read between tests. And getting the read incorrect.

RBob.

 

Just to verify...

Pin A (black) is supposed to be sensor ground

Pin B (green) is supposed to be MAP signal to ECM

Pin C (Gray) is supposed to be 5V reference from ECM

You're saying you're getting 5V on both outside pins?

 

FWIW, maybe he's just reading inches of vac on one meter (negative number), and
inches of pressure on the other (positive number) - without knowing how meters work.

Just a thought.

 

key on engine off
black - gray wires = 5.00 vdc
green - gray wires = 4.98 vdc
black - green wires = .005 vdc

analog vacuum gauge scale = 30 in hg - 10 psi
scanner scale = in hg

 

I checked the ALDL stream of the $8D code and there is no vacuum variable. I assumed you are using the stock '7730 ECM.

Which means that the reading you are getting in inches of mercury is pressure. The MAP reports absolute pressure within the intake manifold. To convert that value to vacuum the barometric pressure neeeds to be accounted for.

Taking the readings from your first post:

light acceleration vacuum gauge showed 6 in hg & map showed 18 in hg

decelerate and vacuum gauge showed 20 in hg & map showed 8 in hg

Assume that you are at sea level on a good day and the barometric pressure is 31" hg. Subtract the MAP pressure from that and the result is vacuum.

31 - 18 = 13 " hg VAC
31 - 8 = 23" hg VAC

To make the values match what you saw on the vacuum gage, I would say that there is some elevation involved. Working backwards:

6 vac + 18 map = 24" hg baro

20 vac + 8 map = 28" hg baro

There is a 4" hg difference in baro between the two. Which may be caused by inaccuracies in the gage or MAP, or just by trying to read the two at the same time.

RBob.

 

the problem is not so much an accuracy issue as
it is a direction of scale.
when i apply a load ( accelerate ) the map is displaying a
greater vacuum condition , not possible , and when i remove
the load ( let off throttle ) the map is displaying a less vacuum
condition , not possible .

hopefully i can make some changes tommorow to the VE tables and
watch the wideband and see if it is just a dislpay issue or if when
i make a change to the cell with the rpm from my tach and vacuum
from my analog gauge i see the appropriate chane on the wide band

 

It is because the scan tool is displaying pressure, not vacuum. If there is a metric/english (sae) selection for the display put it into metric mode. You should then get the MAP in KPa (of pressure).

Only downside to this is the vehicle speed will be displayed in KPH.

RBob.