Wiring My Car Air Conditioner for MAXIMUM Cooling!




The air conditioning system in my 1999 civic is in great shape EXCEPT the OEM controller cycles the compressor a bit too often (not rapidly in the cases of low or high refrigerant pressures). I live in a hot climate and AC compressors should not be cycling often. They should be ON more often than OFF. Time to fix this crap.

In the page of the honda civic manual I highlighted the two wires in red ovals. These I will need to connect to 0 volts to engage the AC Compressor Clutch and the AC Condenser Fan. The honda OEM temperature controller will be removed in this mod. I just want simple on off control by the flick of a switch.

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Step 1: Time Delay Relay Plus Control Relays.

I needed an ON delay timer to prevent the compressor clutch and condenser fan from engaging when the engine is starting.

This timer from Amazon.com is what I used to give me a ten second delay for my purpose.

I used two relays from an 8 relay card to provide that ground connection to the clutch and fan separately. To activate each relay, the respective inputs on the relay card has to be tied to 0Volt.

Step 2: Providing a Replacement for the OEM Controls.

Since I had previously removed the recirculate and mode control motors from my car ventilation system (since I always use recirculate and don't care for cold air on my feet), I wanted the OEM controller out since it was doing a poor job of controlling the air conditioner.

I used a spare toggle switch and a length of cat5e wire pairs to control my two relays. The other wires in the harness were used for the potentiometer with my pwm controller for the AC blower motor. I used Evo stick to secure the switch to my dashboard console below the deck where the oem AC button used to be.

Step 3: Completion and Testing.

I am now using my glove box as the staging area for my car automation project. I wired the delay ON relay, control relays, power terminal block plus wiring harness in that area.

To fill the voids left by the removed OEM controls, I cut and glued foam pieces into the console. At least the wife wont complain about the "pretty" factor" for this intermediate car mod. Final phase of this project is the car automation pc.

Now the AC works amazingly well. In the second pic you can see the Temperature of the vents (6 Celsius). The air from the dash vents are freezing cold even on the hottest days and I'm extremely pleased. No more 1 minute ON, 15 seconds OFF cycling and hence extra wear/tear on the AC compressor clutch. Once the car cabin is cold enough, I simply flick the AC switch off. In fact this is how I have always used the AC even with the OEM controller for the past 12 years.

If you live in a hot climate and wonder why your car does not cool well, this mod may work for you. Please make sure your refrigerant pressures, compressor, condenser fan etc are all working perfectly before doing this mod.

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    6 Discussions


    Question 2 months ago on Introduction

    My proton preve air-con control is spoilt. Proton service said better if I can get someone to hot-wire/do separate switch. Anyone can do the job/recommend someone?


    Question 6 months ago on Introduction

    I would like to know solution to my Honda civic 2004 car ac problem. The ac switch on the dashboard show light but does not supply to the compressor.
    I took it to my rewire here and he tap from the fan wire to power the ac compress from the fuse box under hood. Lo and behold the compressor work and the ac cool to satisfaction. The major concern is that this connection always drain my car battery that I won't be able to start my car the following morning except I jump start. When we remove this connection I don't have problem with my battery again but I need my ac working.
    Please help me out.


    4 years ago

    I'd like to have an embedded pc in my car with an 11" monitor in the center console/dash for real time fuel management monitoring. I also would like to set up a fly by wire control system with a Boing 777 joystick for steering, acceleration and breaks. having problems with the stepper motors for the steering. my test car is a 2004 mini (cooper/bmw) and the steering is still mushy for lack of power and speed. anyway I would like to see your finished project along with the specs, stability enclosure type... passively cooled or active (fans, thermoelectric or liquid cooling).
    thanks and great job

    1 reply

    Reply 3 years ago

    My system wont have that wish list you described. Definitely wont apply for your project.


    4 years ago

    Gotta be careful on this one. Most vehicles have a high pressure switch that cuts the compressor out if the pressure goes over a certain value. This helps prevent a/c system damage in the event of a plugged orifice tube or other malfunction. And then there are the systems that are designed to cycle on and off fairly often. Overriding the cooling controls to force the compressor into operation is risky business. Chances are there is something wrong somewhere and this is patching a symptom, not diagnosing and fixing the problem.

    A good idea using a car pc for climate control, but not sure sure bypassing all the safety aspects of the system is the right way to go about it. Good write-up, though!

    1 reply

    Reply 3 years ago

    thanks! The safety aspect is good for now with my particular car. All pressures are good. Should that change and the compressor be damaged then I will simply replace it and do another instructable.