Miata AC in the Molvo. Questions. Mostly wiring.

I am trying to make clear in my head what wiring I need to make an AC system work in the Miata.

I will be using a ford Mustang compressor and the rest of the system will be stock Miata.

I am assuning that I will somehow be able to make hoses that attach to the Ford compresor on one side and the Miata parts on the other end.

I do not know or know how to figure out what the compressor is made for R12 or 134A.  No hints on the tag. I know my 1990 Miata was converted from R12 to 134A so expect the same can be done with this is need be. It was not as cold on 134 either as a loss of efficiency with the r12 system or because ?????

Same deal with the Miata AC system that I have. No idea if it was r12 or 134A. the high pressure switch might be a clue as I think they are different>

Anyways, back to wiring since it is the more imediate concern, if I can get the electrical infrastructure in place by this driving season, I shall be happy. I am hoping that all the control wires that are required from the climate control panel are actually already in place. I do have the AC button on the climate control panel.

I am guessing that this connector is where the AC relay harness plugs in? Someone at the factory has gone to great lenghts to cap this thing!  

I am working from this diagram in the 1991 Miata wiring manual

The goal seems simple enough. I need to get the 20 amp Blue and Yellow wire (LY) to deliver power throuh a relay and  a pressure sitch to the magnetic clutch.

What is confusing me is the signal from the switch TO the relay via the blue wire.The AC switch switches to ground, and that would be fine if the blue wire was not a powered wire. So I am guessing that the AC switch is not what actually toggles the relay, but rather a bit of ground sorcery from the PCM?

I'll spend some time digging through this tonight if I get a chance. Note that custom hard lines can be challenging - your best bet might be to bring in a set of the Mazda lines as a source of fittings, we found they were not easily available off the shelf. Our local NAPA can make the hard lines if they have the parts.

The PCM does exert some control over the AC, it disengages the clutch a WOT because the hamsters are overwhelmed at that point.

I have made adaptor lines by getting compressor-to-generic adaptors and push lock fittings to go from those to hose, and went to the old hoses.

It is not a very visually appealing way to do it, but that Fiero got a modern compressor 

In reply to Keith Tanner :

Thanks Keith. 

The good news is that all of the wiring is in place after I adapted the Miata harness to the Molvo.

What caught me off guard is that the AC switch on the console actually does not switch the AC relay on. All it does is turn on a light to tell you that the AC is on. The AC relay seems to be triggered by a signal from the MCU.

At the moment, I am trying to envision how I might actually make the AC button actually do the job. Assuming the thermoswitch is NC  and that the  0.7V forward bias of the diode does not mess things up, I could extend the kight green and black wire that used to go to the MCU t to the the negative side of the AC relay coil. Any idea how critical this thermoswitch might be should I want to eliminate it?

I'm not strong on AC, I'll look to see what that thermoswitch is actually measuring.

Is the thermoswitch in the evaporator?

In reply to Pete. (l33t FS) :

Yeah. Not sure if it is there to detect too hot or too cold.  Gotta go do the headstand yoga and see if I can find it. will run an experiment and see if I can trigger a relay through it. If I go this way and the  thermo switch is a Normally Closed device, then it can continue to do its job.

Putting this here as a place holder. this is described as the two wires that connected to the  mcu. The Light Green/black is the thermal sensor wire. Not sure what the other wire is. 

Working from this statement from 2005:

All you are doing is taking the wire that comes from the HVAC controls to the Miata computer and splicing it to the wire that ran from the computer to the AC relay.

NOHOME said:

In reply to Pete. (l33t FS) :

Yeah. Not sure if it is there to detect too hot or too cold.  Gotta go do the headstand yoga and see if I can find it. will run an experiment and see if I can trigger a relay through it. If I go this way and the  thermo switch is a Normally Closed device, then it can continue to do its job.

It is to detect too cold.  The only reason for low side pressure switches is to keep the evaporator from getting below 32F and freezing over.  The temp switch controls this directly, without needing to infer a freeze point from the refrigerant.  Means you can swap refrigerants without having to change or adjust a switch.

This is the same way RX-7 A/C worked at least for my '80, '81, '84 and '85, for port and factory installed systems.  The pressure switch is likely a high side cutoff to prevent pressure from going high enough to blow out the compressor.  You might be able to use the wiring diagram from one of those, because they were not ECU controlled, strictly HVAC button controlled.

Sorted. Ground out the wire to the thermosensor and the AC switch sends a ground to the plug where the AC relay harness attaches. I can work from there.

Also got the new speakers hooked up in the back and the window- washer -squirter working. All in, a good afternoon in the shop.

Question: Why did Mazda make it so flipping difficult to pull the toombstone off of this car? Has to be one of the more diabolical service hindrance devices on the car.