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So i had engine problems ( overheating) and had ATS RACING swap in a GEN4 engine into my GEN 2 MR2 (1991). The AC worked before the swap. ATS kept the car a while and finally gave it to me with a bunch of problems (smoking randomly, gear box cables falling off leaving me without 2,4 and reverse, rattling heat shield, loose downpipe nuts, and improperly installed accessory belt).
Well the AC doesn't work either. I thought it was low refrigerant but a local Toyota dealer tested the system and it was full and not leaking. They told me the compressor wasn't engaging. So i found a cable dangling that leads to the ac compressor and connected it. Still nothing. I rarely drive the car and time passed. Today i decided to troubleshoot. I found where the cables lead to and where they are supposed to be connected. SUPPOSED TO BE...
I found th cable that turns on the compressor is not connected to the ECU. I looked up the wiring diagram and tried myself but all it did was disable my tachometer. Does anyone know how to fix this? I disconnected the wiring to the AC and the tach started working again.


 

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The A/C wire going to the ECU doesn't turn the compressor on it just tells the ECU that the A/C is on and to raise the idle. I had to use a relay to flip the signal to my gen5 ecu, it either needed to see ground or 12v to know the clutch was engaged, I don't remember which. This won't prevent the A/C from working.

What kept my A/C from working was the A/C amplifier not seeing a engine RPM signal. The A/C amplifier controls the compressor clutch and if it doesn't know the engine is running it won't engage the compressor clutch.
 

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Well the AC doesn't work either. I thought it was low refrigerant but a local Toyota dealer tested the system and it was full and not leaking. They told me the compressor wasn't engaging. So i found a cable dangling that leads to the ac compressor and connected it. Still nothing. I rarely drive the car and time passed. Today i decided to troubleshoot. I found where the cables lead to and where they are supposed to be connected. SUPPOSED TO BE...
I found th cable that turns on the compressor is not connected to the ECU. I looked up the wiring diagram and tried myself but all it did was disable my tachometer. Does anyone know how to fix this? I disconnected the wiring to the AC and the tach started working again.
I've just finished installing, recharging, and diagnosing my AC system.
I have a gen 5 engine and when I had finished installing my AC system it didn't work.

Things I've done
  • Recharge AC system to proper pressure, depending on whether. It was about 85F out in SoCal yesterday so I charged it to 45psi. I had to jump the AC relay to have the AC clutch turn on.
  • AC switch wasn't working, so replaced
    The AC clutch didn't turn on.
  • Checked AC relay, good
  • Tried a 91 AC amplifier BUT the AC clutch turns on as soon as i have the fan blowers on regardless of the AC button on or off. SO I kept use of my 93 AC amplifier
  • Check the wiring from the AC amplifier to connector IE4
  • Then check that the AC amplifier was getting a TACH signal, which it did not.
    On the body harness, I had splice in the TACH signal to the amplifier from the TACH signal that leads to the TACH meter from connector IE1
After doing all this, it's now working like it's suppose to.
USDM 93 NA car, JDM 93 body harness, gen5 3S.


The A/C wire going to the ECU doesn't turn the compressor on it just tells the ECU that the A/C is on and to raise the idle. I had to use a relay to flip the signal to my gen5 ecu, it either needed to see ground or 12v to know the clutch was engaged, I don't remember which. This won't prevent the A/C from working.

What kept my A/C from working was the A/C amplifier not seeing a engine RPM signal. The A/C amplifier controls the compressor clutch and if it doesn't know the engine is running it won't engage the compressor clutch.
On the factory system, it send 12V to the clutch and ECU.

I had the same issue as yours, amplifier wasn't seeing an engine RPM signal.

What I'm doing now with the AC idle up is to splice into my engine harness wiring for the AC clutch and create a single, separate circuit to my LINK ecu so I can control the idle up.
 

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This is a general overview of the a/c. The a/c system is autonomous and does not rely on the ECM. You do need a wire connection going through the body harness and engine harness from the a/c relay near the glovebox all the way to the compressor clutch. The wire in the engine harness should have been done by whoever did your harness. If push comes to shove you can run a wire from the a/c relay under the doorsill and through the firewall to the compressor. The part in the body harness is stock wiring. The relay is triggered by the a/c amplifier, which is behind the glovebox. IIRC, the a/c amplifier gets the rpm signal from the instrument cluster, which gets it from the ECU.on a gen4 or from the ignitor on a gen2. The a/c amplifier is a dedicated ECU that takes inputs from various sources including pressure sensors, temperature sensors, and the hvac control panel, and operates the compressor and the radiator fans. A/C amplifer failures and hvac control panel failures are not uncommon. Your car's original factory service manual shows all the wiring and the troubleshooting procedures for the a/c.

Incidentally my a/c operates correctly with a 2GR swap. It really does not matter what engine/ecu you put in there.
 
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