Why the ECU watches A/C pressure
Refrigerant pressure tells you a lot about an A/C system. Very low pressure usually means a leak or low charge, which can starve the compressor of oil. Very high pressure can mean a blocked condenser or a cooling fan that isn't running. With the pressure sensor wired to the Nexus, the ECU can display it, warn you, log faults and feed that information into its air conditioner control strategy.
Most analog A/C pressure sensors have three wires: a 5 V supply, a signal back to the ECU and a signal ground. Some vehicles use a digital sensor that reports pressure as a duty cycle instead, and the Nexus supports both.
Wiring node
Start here, because the input type decides everything else.
| Input Type | Connect to | Pull-up |
|---|---|---|
| Analogue - Voltage | Any free AVI, with the sensor wired directly to the ECU | Disabled. The sensor is powered, so it drives its own signal |
| Digital - Duty Cycle | Any free SPI | Depends on the sensor's output type |
A common mistake is leaving the pull-up enabled on a three-wire analog sensor. That pulls the reading away from the real value and can make a working sensor look faulty.
Calibration node
This is where the voltage-to-pressure relationship lives. If the sensor came from Haltech, use Load from File to drop its calibration straight into the table. For a factory or third-party sensor, enter the manufacturer's data by hand, then use Save to File so you can reuse it on the next car. Sanity check: with the engine off and the system at rest, the reading should be steady and plausible for your system's static pressure.
Display node
This sets the lower end of the number and gauge displays, plus two warning thresholds:
- Warning Minimum: below this, any A/C pressure gauge or number flashes red for low pressure.
- Warning Maximum: above this, it flashes red for high pressure.
Our starting point is to base these on the vehicle's service information for normal operating pressures, then confirm in a log with the A/C running on a hot day.
Diagnostics node
Here you can turn on high and low voltage DTCs for the sensor. A voltage outside the sensor's normal range usually means a wiring fault, such as an open circuit or a short to ground or 5 V. Each DTC can be given a severity, which links it to an Engine Protection level. For most street cars, logging the fault is enough, and there's rarely a reason to cut engine power over an A/C sensor.
Quick troubleshooting
| Symptom | Usually |
|---|---|
| Reads maximum or minimum constantly | Open circuit, missing 5 V supply or a shorted signal wire |
| Reading is off but moves with pressure | Pull-up enabled on an analog sensor, or the wrong calibration |
| No reading from a digital sensor | Input Type set to analog, or the sensor wired to an AVI instead of an SPI |
Related setup: the ambient air temperature sensor.
