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.

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.

Back to Technical Notes Page.

Back to Home Page.