What this function is for
Many factory A/C systems use a temperature sensor, commonly on or near the evaporator, so the control system can avoid icing and manage the compressor. If your Elite is running the engine and the A/C, wiring that sensor to the ECU gives it a proper A/C temperature channel that you can display, log and put limits on. It pairs naturally with the air conditioner control function and the A/C pressure sensor.
In ESP, add the A/C Temperature Sensor from the Functions list in Main Setup. It has four tabs: Wiring, Calibration, Display and Diagnostics.
Wiring tab
Pick the input type
| Input Type | Choose it when |
|---|---|
| Analogue Voltage | The sensor outputs a signal that varies between 0 and 5 V |
| Analogue Resistance | The sensor is a resistive (thermistor) type whose resistance changes with temperature |
| CAN Input | The temperature comes over the vehicle's CAN bus instead of a wire to the ECU |
Choose a connection
An AVI (Analogue Voltage Input) is the preferred connection. If you've run out of AVIs, an SPI (Synchronised Pulsed Input) will also work.
On an AVI, the only setting is the pull-up, which here is a 1k resistor to 5 V.
On an SPI, set Sensor Type to Hall Effect, as you do for any analog signal on an SPI. Its pull-up is a weaker 10k resistor to 5 V. That difference matters: the standard temperature calibrations in ESP are built around the AVI's pull-up, so if you use an SPI you'll need to adjust the calibration to suit. Our article on using an SPI for temperature sensors explains why.
Should the pull-up be on?
This is the setting people most often get wrong. Our rule of thumb:
- Two-wire temperature sensor, wired only to the Elite: pull-up enabled. A thermistor has no power supply of its own, and the pull-up is what creates a voltage for the ECU to read.
- Sensor with its own supply (usually three wires): pull-up disabled.
- Sensor shared with another ECU, such as a piggyback install alongside the factory ECU: pull-up disabled. The other ECU normally provides the pull-up already, and adding a second one skews both readings.
Calibration tab
ESP ships with a set of ready-made calibrations. Use the yellow folder icon at the top left to open one, and the blue disk icon beside it to save your own. If your sensor isn't listed, build a custom calibration from the manufacturer's data sheet: a temperature-versus-resistance or temperature-versus-voltage table. Once it reads correctly, save it so you can load it straight into the next map.
Quick sanity check after setup: with the car cold and the A/C off, the A/C temperature should read close to ambient. If it's pegged at an extreme, suspect wiring or the pull-up before the calibration.
Display tab
Warning Minimum is the one setting here. When the A/C temperature drops below it, any A/C temperature gauge or number in ESP flashes red. Set it just above the point where you'd worry about the evaporator freezing for your system.
Diagnostics tab
These settings let the ECU flag wiring problems by watching the raw sensor voltage. Both are disabled by default.
| Setting | What it does | Our advice |
|---|---|---|
| Raw Min | Logs code P1352 if the voltage falls below your limit, pointing to a short or open circuit | Set it below anything the sensor produces in normal use |
| Raw Max | Logs code P1353 if the voltage rises above your limit, pointing to a short or open circuit | Set it above anything the sensor produces in normal use |
If the Engine Protection function is active, you can also give each fault a DTC severity so the ECU takes action. For an A/C sensor, a logged warning is usually enough. Full code definitions are in the DTC index.
Troubleshooting
| Symptom | Usually |
|---|---|
| Reads maximum or minimum all the time | Open or shorted wiring, or pull-up set wrong for the sensor type |
| Reading is plausible but off by a lot | Wrong calibration, or a standard calibration used on an SPI |
| Both the Elite and the factory ECU read strangely | Two pull-ups on a shared sensor, so disable the Elite's |
| P1352 or P1353 logged | Check connectors and the sensor ground, then review your Raw Min and Raw Max limits |
For more on chasing bad temperature readings, see temperature sensor problem diagnosis.
FAQ
Do I need an A/C temperature sensor for the Elite to run my A/C?
Not always. It depends on how the A/C system is controlled on your vehicle. The sensor adds monitoring and protection. If you're unsure what your car needs, ask us.
Can I use a spare Haltech temperature sensor?
If it physically suits the location, yes. Load its calibration from the list and enable the pull-up if it's a two-wire sensor.
