Air Conditioner Pressure Sensor

What the A/C pressure sensor is for

Most modern A/C systems have a pressure transducer on the high side of the refrigerant circuit. The factory ECU uses it to protect the compressor, decide when the condenser fan needs to run, and cut the compressor if pressure goes too high or too low. When you swap to an Elite ECU, the A/C Pressure Sensor function gives ESP that same reading, so it's available for Air Conditioner Control, logging and dash displays.

Setting it up comes down to four tabs: Wiring, Calibration, Display and Diagnostics.

Wiring tab

Pick the input type

First, check what kind of signal your sensor puts out. The calibration data from the sensor maker tells you.

• Analogue Voltage: the sensor's calibration is given as voltage versus pressure. This is by far the most common, and it wants an AVI (Analogue Voltage Input).
• Digital Duty Cycle: the sensor reports pressure as a changing duty cycle. This needs an SPI (Synchronised Pulsed Input) or DPI (Digital Pulsed Input).

Analog sensors

Use an AVI where you can. On an AVI, the Pull Up setting decides whether the ECU supplies a reference through a resistor:

• Leave it disabled for a powered three-wire sensor with its own 5V supply, which is what most A/C transducers are.
• Enable it for a two-wire resistive sensor that needs the ECU to provide the reference.
• If the sensor is also wired to a factory ECU (for example in a piggyback install), that ECU normally supplies the pull-up already, so disable it here.

Out of AVIs? An SPI can read an analog signal too. Set its Sensor Type to Hall Effect for any analog use. The SPI's internal pull-up is a weak 10k resistor. If a two-wire sensor needs a pull-up, the 10k can throw off the preloaded calibrations. The cleaner fix is to leave the internal pull-up off and fit an external 1k resistor between the signal wire and the +5V sensor supply. That makes the SPI behave like a true AVI, and the stock calibrations read correctly.

Duty cycle sensors

On an SPI or DPI you'll set:

• Edge Select: which edge of the signal the ECU measures from. Falling is the usual choice unless the sensor's documentation says otherwise.
• Sensor Type: digital Hall Effect or reluctor, to match the sensor.
• Pull Up: some Hall-type outputs need one if neither the sensor nor the harness provides it. Turn it on only when the signal needs it.

Calibration tab

ESP ships with a set of ready-made sensor calibrations. Click the yellow folder (Open) icon to load one. If your sensor isn't in the list, build a custom calibration from the manufacturer's voltage (or duty) versus pressure data, then use the blue disk (Save) icon to store it for the next car. Check the reading against a set of A/C manifold gauges if you're not sure the calibration matches your sensor.

Display tab

These only change how ESP displays the value. They don't switch the compressor on or off by themselves.

Diagnostics tab

These settings catch wiring faults by watching for a raw voltage that a healthy sensor should never produce.

Both are off by default. Once enabled, and if you're running the Engine Protection function, you can also give each code a DTC severity so the ECU reacts to it. For an A/C sensor we'd normally just log the code rather than have it limit the engine.

Troubleshooting

If you'd rather measure evaporator or cabin temperature as well, see the A/C temperature sensor setup. Running a Nexus? The NSP version is covered in Air Conditioner Pressure.