Haltech F9A User Manual download pdf (Page 55)

8.3 Barometric Correction

Fluctuations in barometric pressure varies the density of the intake air of the engine. At lower

barometric pressure, the engine can not breath in as much air, and therefore the amount of

fuel delivered to the engine must be reduced. In mapping in Throttle Position Mode, the Map

sensor provides a reading used for Barometric Correction. When in Manifold Pressure Mode,

there are two forms of Barometric Correction performed by the F9 and a third which is only

available on the F9A.

The Manifold Absolute Pressure Sensor (MAP Sensor) supplied with the F9 measures the

absolute pressure of the air in the inlet manifold, and therefore automatically correct for small

changes in barometric pressure. For a given geographical area, small fluctuations in the

barometric pressure can be adequately handled by the MAP Sensor. This is the first form of

correction performed.

Larger fluctuations in barometric pressure are usually caused by a change in altitude. When

the F9 is powered on it runs a small test to determine the barometric pressure. As you should

have read earlier, the ECU switches the fuel pump on at start up (fuel pump prime). If the

engine is not started, the fuel pump will be switched off. At this time, the ECU also reads the

MAP Sensor. If the engine is not running, the MAP sensor will indicate the prevailing

barometric pressure. The ECU remembers this pressure and uses it to perform a barometric

correction on the fuel delivery. Small changes in the barometric pressure from that point are

then handled by the MAP Sensor. This is the second form of barometric correction the E6A

performs.

If at start up the engine is cranked before the fuel pump prime has finished the ECU can not

read the barometric pressure from the MAP sensor as the engine will be applying a vacuum to

it. In this case, the F9 will use a pressure reading stored in its memory. This reading is set to

one atmosphere at sea level (1013 millibars) at the factory. This value can be reset to a

pressure that is close to what is expected in the geographic area the engine will be used in. To

reset this value, follow these steps :

1. Firstly make sure the throttle position sensor is properly calibrated. It must exceed

96% throttle for this to work. Also make sure that the MAP Sensor is set correctly

in the Identification.

2. Switch the ignition off.

3. Apply full throttle.

4. Switch the ignition on but DO NOT crank the engine.

5. Wait till the fuel pump prime finishes (about 5 seconds) then release the throttle.

The current barometric pressure as read by the MAP Sensor will be programmed

into the ECU’s memory.

It is not necessary or advisable to perform this reset regularly. It should only be done if

the vehicle’s regular place of garage is moved or if problems are suspected in the barometric

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Haltech F9A User Manual Page 55