The REM only reads what the ECU sends it and will then do math to get actual readings. The only reading that the REM directly reads is the Oxygen Sensor Heater Relay Voltage, everything else is straight from the ECU.
Now there is a bug in the latest update becuase it was really only a quick patch but if you set the Pressure Unit to Voltage, the MAP will read as expected but the Vacuum is also affected by this and may read in BAR or something odd so I need a good work around. Setting the pressure to any other unit will read properly for MAP and VAC. I may add the voltage reading to the diagnostic menu or figure out what the best way to show it would be.
As for the ECU, I'll give you a quick overview of how it works.
All of the sensor are fed into the Renix ECU. The ECU will convert those analog readings into a digital value and then do the necessary adjustments to the engine for it to run appropriately.
Those Sensor Readings and Adjustments Values are then all grouped together and sent to the ECU Transmit pin on the diagnostic connector to be read. My REM reads the TX pin looking for the beginning of the stream and will then read the entire string in when found. You only need the TX and Ground pin in order to read all the info from the ECU.
Each piece of information is sent as a byte which is only a value from 0 to 255. The REM will do the necessary calculations to convert that 0-255 number into something more useful such as Voltage, or a unit of measurement. They are then displayed on the REM with some screen formatting so that they stay in place.
Now for example, let's look at the MAP reading.
The MAP byte is the 4th byte in the stream. When the engine is off this number will read very high, let's use 247 for example.
That number can be converted into:
Volts: _____247 / 51 = 4.84V
BAR: _____ 247 / 255 = 0.969bar
In Mercury: 247 / 9.13 + 3.1 = 30.15"Hg
KPA: _____ 247 / 2.7 + 11 = 102.48kpa
PSI: ______247 / 18.6 + 1.5 = 14.78psi
Now that will give you the Absolute Atmospheric Pressure from the MAP sensor.
In order to get the Relative Engine Manifold Vacuum we will need the help of another byte, the Barometric Pressure before engine start (#17 in the stream)
We will take the MAP reading and subtract it from the Baro reading so that the reading is zeroed at current atmosphere and you get a negative pressure reading, also known as a vacuum. Then we can convert it into our desired pressure unit.
So now, let's use 91 as MAP and keep 247 for Baro to get out vacuum reading:
BAR: _____ (247 - 91) / 255 = 0.612bar VAC
In Mercury: (247 - 91) / 9.13 + 3.1 = 20.19"Hg VAC
KPA: _____ (247 - 91) / 2.7 + 11 = 68.78kpa VAC
PSI: ______(247 - 91) / 18.6 + 1.5 = 9.89psi VAC
Alright, so hopefully I didn't pop any brains yet :explosion
Hope that clears anything up, if not I'd be happy to answer any other questions.
