I Rewired My Nissan's A/C... Is this a bad idea?

[85 Cutlass Brougham]

It's summer in Florida which means 95 degree heat with 90% humidity and I deliver pizza. The job is pretty much unbearable without A/C, and that was the unfortunate situation I found myself in. The problem was that my truck's A/C compressor would only turn on once the ignition was turned off, but then would immediately shut off when the key was turned on. I traced the issue to the computer which ran the ground side of the relay. Since it ran everything else fine, I decided to bypass it and ground the relay to the chassis, with the binary pressure switch wired in series with the ground wire in case of over or under pressurization. This eliminates the computer and the thermostat, but turns the compressor on whenever the ignition is on. The idle speed solenoid operates normally too as it is wired in parallel with the compressor clutch from the factory (it is a separate part that is not connected to the ISC stepper motor). Now, I noticed today that the lines completely iced up when it was running, but the compressor was not locking up and seemed to be operating normally. Is this something to be concerned about?

Oh, I do plan on putting an on off switch in the system too. This is just the proof of concept/Beta testing version. Right now, the positive is switched by the ignition key which also provides a fuse for the system. I also did not cut any wires and just extended the relay wires and cut a pigtail off a Maxima for the binary switch wiring. That way if I ever need to go back to stock , I can just plug it all back in as Nissan intended and not have to deal with cut and splice wiring.

 

[megaladon6]

your system, like most modern systems, is not meant to run continuously. when it reaches a certain pressure or temp it cycles off. then cycles on as needed to maintain system performance. this increases the longevity of the system and reduces fuel consumption and emissions
what i'd do is test the output pin at the ECU and make sure it's not just a bad wire or connector. then try and find someone with a good scan tool (not just a code scanner!!! something with factory level software) and make sure that the ECU is commanding the system on. if you have a bad sensor somewhere else it would keep the ecu from turning on the AC and look like a bad computer.

 

[85 Cutlass Brougham]

Well, the problem is not that it never turns the compressor on. It does. However, the oddity is that when you turn the key off, the relay activates the compressor. When you turn it on, the compressor is immediately disengaged. Now, the relay is triggered by two events. The first is that the key is on. This is the positive side of the relay's switching function. The negative side is triggered by the computer, and that is what is not working right. The computer uses both the thermostat under the dash and the binary switch to tell it to activate the compressor clutch. They are wired in series with each other so that both must activate in order to tell the computer to turn on. I have already verified the binary switch as functioning normally, but it may be that the thermostat is bad. The binary switch is the second switch in the series, and it always had 4.7 volts to it-whether the compressor switch on the dash is on or not. (The circuit goes: ignition switch to A/C switch on control panel to thermostat to binary to computer, and the computer then acts as a ground.) What I do not get though is that the relay activates when the key is off. I checked the positive relay terminal with my DVOM, and it is operating correctly-no current when switched off, current present when switched on.

As for having a proper Nissan Consult II diagnosis, I am broker than broke right now. I simply can't afford it as there are more pressing bills. I do, however, have the factory Nissan service manual for the truck. What I will probably do though is check the thermostat under the dash and see if it is working right. If so, and the dash switch is working right too, I will just use the output from the binary switch to trigger a relay which will trigger the negative side of the compressor relay. That way it will still have the thermostat in the system, but work without the computer until I can afford to fix it properly in the fall. Summer is a very hard time to deliver pizza, and with between 20 and 30 hours scheduled each week, my finances are not what they once were.