This reference provides the MAP conversion formulas and P72/P75 calibration values used in OBD1 ROM code. These values are intended for understanding legacy ROM structures and should be verified against specific ECU code, fuel-cut offsets, and sensor hardware before implementation.

Note

Most OBD1 ECUs follow similar logic, but verify the actual ROM code, fuel-cut offset, and sensor calibration before applying these formulas to non-P72/P75 applications.

Basic MAP-Byte Formulas#

The following formulas assume a stock MAP sensor maximum reading of 1764 mbar.

Stock Configuration#

Stock code utilizes a halved MAP byte value: pressure_mbar = (1764 / 255) * (map_byte / 2)

Uncorked Configuration#

When the ROM's MAP value is uncorked, the division by two is removed: pressure_mbar = (1764 / 255) * map_byte

Voltage Formulas#

The following formulas define the relationship between raw values and voltage, where the fuel-cut offset is 24 (located at address 0x4DCC in the referenced code):

• Stock MAP: volts = (5 / 255) * ((raw / 2) + fuel_cut)
• Boost MAP: volts = (5 / 255) * (raw + fuel_cut)

Voltage Conversion#

The following linear conversions are derived from standard vacuum and absolute pressure points:

• Inches of Mercury (inHg): inHg = 20 * volts / 1.85 - 30.81
• Millibars (mbar): mbar = 365.9 * volts - 29.9

Calibration Values#

Caution

The source documentation labels the final full-scale row as 256. As an unsigned 8-bit value tops out at 255, this value is preserved as an endpoint reference from the original technical notes.

Full Raw-Value Calibration#

Halved Stock-Map Calibration#

Sensor Interface#

Archived schematic for the three-wire MAP sensor interface.