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The 3-Bar Map Sensor

The MPX4250AP was supplied with the “turbo” units from the first group buy, and ALL the units from the second and subsequent group buys, is in the Digi-Key catalog on page 1098. This is a 2.50 bar absolute pressure unit. That means it measure from a full vacuum to 2.50 bars (i.e., up to ~1.5 bar above normal atmospheric pressure). The stated pressure range is from 2.9 to 36.3 psi, equivalent to ~21.6 psi of boost.

Any sensor you replace the MPX4250 with must also be an absolute pressure type (so that it can measure engine vacuum as well as boost). Other sensors you might want to adapt for higher boost levels are shown on page 1096 of the Digi-Key catalog.

Note that instead of using the GM 3-bar MAP sensor, an alternative that's often cheaper and easier to source is the 4-bar sensor.

However, for the most reliable operation, you might also look at commercially available MAP sensors that have been used in automotive applications with high boost ranges. GM makes a 3.0 boost MAP sensor that can be made to work with MegaSquirt. This sensor can be used up to ~29 psi of boost. It was used on the 1989 turbo Trans Am with the 3.8L turbocharged V6 engine, and is GM part number 16040749, also available as Holley PN 538-23. The Echlin equivalent 3 bar sensor is PN CRB219481. They cost around $60 to $80. They have Pin A = Signal Ground, Pin B = Sensor Output, and Pin C = +5 volts (Vref), and the pins are marked A-B-C on the sensor.

To use any “non-standard” MAP sensor (anything but a MPX4250) you would have to:

• physically adapt the sensor to the MegaSquirt^® PCB by comparing pin-outs and fabricating wires, daughter boards, or whatever else is needed,
• electrically adapt the new sensor, making sure such things as supply voltage, etc., are correct,
• find the “characteristic curve” for the output of your new sensor (volts versus kPa),
• recompile the megasquirt.s19 file to recognize the new sensor's characteristics,
• load the new megasquirt.s19 to the MegaSquirt^® hardware.

For example, the 3-bar GM MAP sensors can be adapted to MegaSquirt. They use a 5 volt reference, just like MegaSquirt.

MPX4250/4115 Sensor Sensor output on Pin #1, Ground on Pin #2, and +5 Volts on Pin #3 Pins 4, 5, & 6 are not connected.	3-bar GM Sensor Ground on Pin A, Sensor output on Pin B, and +5 Vref on Pin C.

To wire a GM sensor 3 bar sensor, you need to:

• Run a single sensor output wire from Pin #1 (square) of the MAP holes on the PCB to a free pin on the DB37 (use #25, which can be connected by running the sensor output wire you just installed to X11 on the PCB),
• Then run a wire from this pin (#25) on the DB37 to Pin B on the sensor.
• Ground can be connected to Pin A the sensor from a common point on the engine where you ground the other sensors, etc.
• +5 Vref for Pin C can be taken from the supply for the TPS (DB37 Pin 22).

Then you need to make the sensor work with MegaTune.

For MegaSquirt-II, you can do this simply by changing the settings under 'Tools/Sensor Calibration/MAP Sensor'. For the 3 Bar GM sensor use the values: 1.1 and 315.5. Change your barometer sensor values as well.

For MegaSquirt-I, you have to change the code itself by completely eliminating the kpafactor and barofactor.inc files.

1. First hack up megasquirt.ini to set units on all incoming and outgoing MAP entries to ADC rather than kPa...

   [Constants] 
   
   mapBins = array,U08, 108, [ 8], "kPa", 1.23317, 0.918308, 20.0, 304.0, 1 
   
   [GaugeConfigurations] 
   
   mapGauge = map, "Engine MAP", "kPa", 0, 300, 0, 20, 250, 275, 1, 0 
   
   [RunTime] 
   
   mapBar = 0, 300 
   
   [OutputChannels] 
   
   barometer = { (baroADC + 0.918308) * 1.23317 } 
   map = { (mapADC + 0.918308) * 1.23317 } 
   

2. Next modify the embedded code to get rid of the .INC file use, this code replaces that big chunk of logic below the LOOPER label.

   clrh 
   
   lda baro 
   sta tmp11 ; Barometer Correction Gamma 
   
   lda map 
   sta kpa ; Manifold Air Pressure in Kilopascals 
   
   lda config13 ; Check if we use baro correction 
   mode (bit=1) or not (bit=0) 
   bit #$08 ; Use BIT instead of brset because 
   outside of zero-page 
   bne DoneBaroCorr ; Branch if the bit is set - we use the 
   MAP-obtained barometer 
   
   lda #$64 
   sta barocor ; Store 100% barometer correction 
   (correction not used) 
   bra CltCalc 
   
   DoneBaroCorr: 
   lda tmp11 ; Use the saved MAP-derived barometer value 
   sta barocor 
   

3. Assemble and download this, you should be clean ADC instead of kPa throughout the embedded code and MegaTune should show all MAP values in true kPa.

There are complete files here:

Be sure to thoroughly test any modifications you make (including the above files) on the stim to verify proper operation before use!