Hey guys -

My AC and PFC have been cooperating less and less recently. This is a common problem due to a number of factors - the fan speed switch aging, bad grounds, etc. The PFC is more picky than the stock ECU on this and a simple relay can solve the problem.

The ECU has control of the AC, this lets the ECU shut off the AC at full throttle and high RPM. You wouldn't want to bypass this, most likely would cause damage to the AC compressor.

Using a relay can improve the ground and also keep the ECU control working properly.

To determine if you have this problem, turn the car to On (doesn't have to be running) and pull up the Sensor Check screen on the Commander. Turn on the AC button and go through each of the 4 fan speeds. Typically speed 1 and 2 will work and 3 and 4 won't - you can tell by the AC light on the sensor check screen. It should be on with all 4 fan speeds, if not you may need this fix. Mine was on at speed 1 and flaky at speed 2, nothing on 3 or 4.

To do this you will need a relay. I didn't want a large relay, this is a very low amp circuit, no need for a giant 30 amp relay. I found this small relay on Amazon -


It's designed for use with car security setups. Nice and small, built in harness, even has a mounting tab.

For hookup, the instructions here hold true -

https://www.rx7club.com/power-fc-for.../#post11503128

The new relay comes with an instruction sheet showing which wire is which. There is a 5th wire you don't need (orange) that can be snipped off. MAKE SURE to insulate that bare connection, I wrapped it and the whole harness in good electrical tape.

As stated in the instructions, wire 1E on the ECU is the one you need. You will also want to use 1B which is a switched 12v.

So, when you go to wire it up -

- Red tapped into ECU wire 1B
- Yellow to a ground point, I put on a ring terminal and grounded to a chassis ground.
- Black wire to ECU side of wire 1E (you cut that wire)
- "Brown" (the wire looked grey to me) wire to the harness side of wire 1E

I would recommend doing a test connection of everything before buttoning it up. With everything connected, do key on again and test each fan speed. Should have the AC light on with each speed. If so, make sure all your connections are solid and insulated and put everything back together and test one more time.

Picture of my harness. I labeled my relay with a label maker so I won't wonder what the heck it is 10 years from now




Dale

 

Great info as always from the RX7 GURU

 

Even though this info is old, it is needed periodically for those newbies that do not do searches first.
It was invented by Freddie from Houma La. back around 2000.

 

I just did this on a customer's car and had to reverse the Black and Brown wires to make it work.

 

I know you guys like a riddle
Got a very very clean, unmodified 2000 JDM FD in for some work : basically a Greddy Vmount, a PFC and a tune.
I searched a lot before writing here.
I have the opposite problem : either with stock ECU or PFC, AC is always on, even though the button is OFF. As soon as turn fan speed on either position, AC compressor turns on.
I do have the ON-OFF cycling problem with the PFC that i don't have with the stock ECU.
On the PFC commander, as soon as i turn fans ON - with or without AC - the A/C light comes on in the Sensors/SW menu.

Should i bother trying this relay mod or my problem is not related ?

Thank you for your help.

 

Get out the manual and follow the wiring for testing. After all it is either off or on which is either no voltage or voltage.

 

I just did this and I’m stumped. I’m using the same relay that Dale Clark linked, wired the exact same way. I have a solid 12v at 1B with the key on. Factory ECU plugged in, AC works flawlessly, but with the Power FC I’m getting this:

Engine off/key on: sensor check shows working on 1,2,4- dead on 3. Engine running, works only on 1, dead on 2,3,4. I’m grounded to the 10mm nut on the ECU bracket. What am I doing wrong?

 

Dale, I think there's a mistake. The black wire is the "coil" in which the trigger signal should be from the harness side of 1E. The brown wire is part of the contact in the relay which should bring 1E from ECU side to a ground (yellow wire).
Banzai Racing is correct...

 

I agree with Billy, I used his pictures on the older thread and a/c works fine now, thanks for the pictures btw Billy. Much easier than reading. I'm visual.

 

You're welcome!

 

I wired it up Billy's way and nothing changed, so I pulled the switch back out and cleaned it again. It works better now, but still not consistently. I'm 99% sure my switch is the problem here (even though it worked perfectly on the stock ECU).