Sale ends todayGet 30% off any course (excluding packages)

Ends in --- --- ---

AC Refrigerant Pressure Switch M1 wiring

EFI Wiring Fundamentals

Forum Posts



Tech Articles

Discussion and questions related to the course Motorsport Wiring Fundamentals

= Resolved threads


Hi All,

I'm probably the only one running air conditioning with Motec in my car which may explain why i can't find much on this topic, however, I would like to use my PDM to provide power to the Sanden AC compressor and the M150 to bump idle, turn on fan(s), and turn off compressor due to high pressures when AC is switched on.

I have a Restomod Air (similar to Vintage Air) AC system and am planning to replace the Trinary (pressure/fan) switch which is plumbed in the drier with a RaceGrade or FuelTech pressure sensor (500 or 1500psi). I will be using the Restomod Air HVAC control panel to turn on the AC. Below is the standard wiring schematic for this setup.

On the M150 side, how do I safely enable the AC Request Resource once the AC is switched on by the HVAC control panel? Next, since I am planning to provide power to the compressor via the PDM, would the AC 'on' and 'off' conditions be communicated to the PDM via CAN?

Thank you,


I would probably try the Blu (04) wire as it looks like it controls the relay to the compressor. If it's voltage changes, then splice that signal to an AV input on your MoTeC M150 and configure as the A/C Request Switch (no pullup).

Hi Neil,

This can be done without too much trouble.

Essentially you replace the compressor control relay with the M1 at the input or request side and the PDM at the compressor control side.

You would need to check and confirm but I believe the blue wire from the thermostat will supply 12v+ when the control unit activates the pump. This will become your request resource wired to the M1. The parameters under Air Conditioner Request should be set appropriately. You can see the measured voltage at the pin under Air Conditioner Request Pin Voltage.

Because you replaced the trinary switch with a pressure sensor this will be wired to an AV input of the M1 and assigned to the Refrigerant Pressure Sensor Resource. This will be referenced under Air Conditioner Enable Refrigerant Pressure.

To use the PDM to power the compressor clutch The Air conditioner Clutch Resource should be set to a PDM Byte X Mask X resource. I have attached the M1 to PDM CAN Messaging PDF which covers how to configure this correctly.

In your PDM you will need to configure a CAN Input based on the PDM resource used for the Air conditioner Clutch Resource.

The PDM output will be wired directly to the AC compressor.

Attached Files

Which Firmware are you using in the M1?

This is very helpful! Thank you Scott and David!

Stephen, I have G-Speed LS7 firmware on the M150. I've been told by them that everything I am planning to do, including running the AC compressor should work just fine.

Regarding the Trinary switch, the 'high pressure' setting is 406 psi and low is 30 psi, so would you recommend a 0-500psia pressure sensor?

Thank you,


That should be an okay range, most pressure sensors will have a burst pressure much higher than the measured range.

Some of the Partner Packages for the M1 do not use the same functionality and CAN messaging as the GP Packages, so it is good to know which Package is being used. What Scott said before will work with the MoTeC M1 GP Packages, but at least half of the M1 Partner Packages will not have the required support for it.

Stephen, thank you for this info. I can tell you that G-Speed is aware of how I plan to use my car/M150 and they have seen my PDM/ECU pinouts and components I plan to power up/control with the units.

Let me ask you, in the event the firmware loaded in my M150 will not allow certain functionality/CAN messaging, would I need to send my ECU back to Motec to have a GP/GPR firmware package installed?

M1 licencing and firmware package changes can be done in field. The ECU does not need to be returned if you wanted to switch to a different firmware.

With your Firmware, if it doesn't support the functionality that you want, the first port of call would be to G Speed to see if they can update the firmware to support your plans. If they are not willing/able to do so, then you could order the M1 License for one of the GP Packages from MoTeC through any Authorised MoTeC Dealer, Once you have this M1 License, you can then load the GP Package into the M1 (some of the tuning and calibration data may migrate, its been a while since I last looked at the G Speed Firmware to see where it matches the GP Packages) and configure that Firmware to suit your needs.

Thank you both and good to know this can be done in the field. Hopefully, the current firmware will work! I'll keep you posted. Harness builds ongoing...

We usually reply within 12hrs (often sooner)

Need Help?

Need help choosing a course?

Experiencing website difficulties?

Or need to contact us for any other reason?