MAP Sensor Output

While both the ST165 and ST185 models use a vane Air Flow Meter (AFM) as the primary airflow input to the ECU,  all models (excluding Japanese specification ST165's) also have a Manifold Air Pressure sensor (Turbo Pressure Sensor Assy in Toyota-speak).  This sensor simply measures the pressure inside the plenum chamber and sends it to the ECU as a 0-5 volt analog signal.

While the signal is used by the ECU for fuelling calculations,  the most noticeable functions are operating the factory boost gauge and fuel cut.

I have used a calibrated digital manometer and a calibrated Fluke digital multimeter to perform this testing.  5V DC was applied to the MAP sensor using a PC power supply.  Pressures were then applied using a Mityvac vacuum/pressure pump and the signal voltage measured using the multimeter.

Two MAP sensors were tested:

P/N 89420-17030         This sensor is fitted to the 89-Oct93 MR2 Turbo and also (using a different P/N) the ST185.

P/N 89420-17050         This sensor is fitted to the Oct93-99 MR2 Turbo and also (using a different P/N) the ST205.

Results are in the graph below.

MAPsensor.jpg (35243 bytes)

As can be seen the relationship is perfectly linear.  Excuse lack of vacuum units,  I couldn't be bothered working them out :-).  As can be seen the ST185 sensor flatlines at about 18psi,  above which it reads a constant 5.05 volts. 

In contrast the ST205 MAP sensor reached 5 volts at around 26 psi.  I couldn't manage higher pressures unfortunately to test higher response.

The relationships are :

ST185:    V = 0.1319*psi + 2.7000  up to a maximum of 5V

ST205:    V = 0.1025*psi + 2.3293  up to a maximum of 5V

Using this relationship and a simple voltage comparator circuit an adjustable boost switch relay can easily be made for operating fans,   pumps etc.  Better than unreliable pressure sensors.

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