For people installing ECUs on street cars, where this is legal to do so, often people will want to run the factory air conditioning system through the aftermarket ECU. This has several advantages over being wired directly.

Firstly, the biggest advantage you can gain is in the idle control. The air conditioner compressor puts a large torque load on the engine, so the ECU can compensate for this with idle control. Secondly, the ECU can disable the compressor if the engine is about to stall, and lastly the ECU can disable the compressor at wide open throttle.

Let’s first discuss how to wire in an air conditioner system to the ECU. Firstly the ECU requires a digital input to be triggered when the air conditioner request is active. On most cars, this is a wire that pulls to ground when the air conditioner is required to turn on. Normally this comes from the user switch or a thermostat, connected in series with a triple pressure switch on the refrigerant line. Connect this to a spare digital input pin, and select that input in the Digital Inputs setting as being air conditioner request. If your pin connects to power when active rather than ground, then you will need to select that input as being active high.


Wiring an air conditioner system

The ECU then must be able to drive an external relay, which provides 12V power to the compressor magnetic clutch. On almost every car, this output to the relay coil must pull to ground to activate the relay (and turn on the air conditioner). In this case you can connect the output to any of the unused injector, ignition or auxiliary outputs on the ECU, and configure the output to be air conditioner.





In terms of settings, let’s discuss idle control first of all. There are several parameters to get right here, but when you do, it works really nicely. The first setting is the new, higher idle speed when the air conditioner is on. We used to have an additional RPM that gets added to the target, but that meant that the target RPM became very high at low engine temperatures, so instead we have a new “minimum idle RPM”, so whichever is greater out of the base target idle table and the aircon RPM value will be the actual target RPM. So wherever you want to set this is up to you, but often this is set to about 1000 – 1100 RPM where the engine has a bit more torque reserve than at the low speeds and has some room to recover if a stall is imminent.

The next setting is the additional idle effort when the air conditioner is on. The easiest way to set this is practically; if you already have closed loop idle working, then you can watch what the idle effort is before you turn on the air conditioner, and then watch what it changes to, after turning on the air conditioner and the RPM has stabilized to the new target. Remember that this will also include the extra effort for the thermofans which will also come on at the same time, so their extra efforts have to already be configured when you do this.

This difference, minus the difference for the thermofans, is what you need to put into the additional idle effort for aircon setting.

The next setting is the delay for turning on the compressor. When the input is first enabled, the new idle target and idle effort is applied straight away to bring up the idle speed. After a short delay, the compressor is enabled. This gives the engine some time to prepare for the additional load of the compressor, which helps stabilize the idle when cycling the aircon compressor. Here’s a graph of how I got it to behave on a pretty much stock GTR. Notice how smooth the RPM trace is; there’s a small amount of overshoot but this is a well set up system.



The next setting is the minimum RPM. Below this value, the ECU won’t enable the aircon compressor at all. So you can set this to a low value eg 600 RPM, so that if the engine is about to stall, this will help save it. It has the other benefit of disabling the aircon compressor during cranking.

The ECU will always disable the aircon during a wide open throttle condition.

Sample of logs from ECU are presented below.

Thank you and happy learning!