I'm curious if there's been any success in fixing the PowerFC problem with the AC working. My understanding of it now is that the A'pexi unit has some higher resistance parts in there which are sensitive to any resistance built up from the engine harness. If your harness is old, you may have some higher resistance in there which will cause the voltage drop that results in the AC not working.

So, my question revolves around fixing this voltage drop issue w/out replacing the engine harness. Even with the Mazda Motorsports discount, the engine harness is still expensive ($475!!!). I'd like to save that money for race rubber, if possible .... Any thoughts?

 

what are the symptoms that the system is not working?

 

There is no fix, just hope yours works in at least a couple of speed settings. Mine doesn't work in speed 3.

 

speed settings? elaborate...

 

What typically has happened is that the AC will work on 3 of the settings, but not the 4th. I've heard of it not working on number 2 or on number 3. That's all fine and dandy ... my AC doesn't work, period. The compressor doesn't kick on. The simple root cause is that there isn't enough voltage being supplied to the compressor for it to sense 'on'.

What really irritates me is that there is fresh R12 in the system (from last summer) and in a couple of months, I'm really going to want to use it. So what I'm asking is if anyone has come up with a creative solution that doesn't involve buying a new engine harness ...

 

3 of the settings? settings for what?

rpm settings?

it seems to me that you could wire the switch directly to the a/c clutch

 

Run a search of this topic. Almost every month, a PFC newbie asks the same old question.

You don't even have to do that, just go back 2 months and check the titles.

 

well that'd be nice if this site archived things

where is the how-to on hardwiring it?

 

There's a thread on the bottom of this forum that basically says the same thing. New engine harness will fix the problem. I was wondering if anybody has figured out a way to fix it WITHOUT buying a new engine harness. I guess that's the only solution, huh?

 

From a thread that is still listed out there here was my reply. I think I first posted this in 2000 or 2001.

PFC AC FIX:

I claim no responsibility or liability for anyone who follows my instructions.
You are solely responsible for any changes you make to your car. Do not attemp
this if you are not experienced with wiring.

Following is details from the manuals and how to make your AC work with the PFC.
Three ECU AC functions will be lost with this conversion. If the AC is on, it will not be temporarily disengaged when starting the engine. When accelerating hard, the AC will not be temporarily disengaged. The idle speed will not increase when the compressor comes on. The AC will function normally with all AC components working, except with no ECU control.

Reference materials:
Mazda RX-7 1993 Body Electrical Manual (BEM)
Mazda 1993 RX-7 Work Shop Manual (WSM)

BEM page G-12 AC wiring schematic:
At the left side it shows the AC compressor wiring; +12V - ignition/acc switch - AC fuse - AC relay - AC pressure switch - AC magnetic clutch - ground. From the relay control coil is a Y/B (yellow with black strip) wire that goes to the ECU. It is marked as connecting point 37 and continues on the WSM B-1d schematic. On WSM B-1d, point 37 goes to ECU connector 1L. The ECU basically grounds out this connection to energize the AC relay to turn it on.

In the middle of BEM page G-12 it shows the AC control wiring; Ground - fan speed switch - AC switch - AC thermal (anti freeze) switch - and a wire colored V (violet) going to the ECU. It is marked as connecting point 36. On WSM B-1d, point 36 goes to ECU connector 1E. The ECU basically senses ground through this wire when you turn on the AC.

Cut the two wires at ECU terminals 1L(Y/B wire) and 1E(V wire). This is the bottom connector with 11 pin columns. 1L is the center pin opposite the connector lock tap. 1E is third from one end. Connect the harness side of them together thus bypassing the ECU. Leave the ECU side open. I placed snap-in connectors on all four wire ends so that they could be easily reconnected back to stock.

The cooling fans are operated from another wire that attaches to the AC relay and is not affected by this mod. In fact if you turn on the AC with the two wires cut but not connected together, the fans come on but nothing else happens. The ELU unit will still increase idle speed a little when the fans are turned to 3 or 4.

 

i can't find the body electrical manual anywhere

i took it all apart, and i'm looking at the 11 pin connector, i found the yellow wire with the blue stripe, but i'm not seeing the violet wire....

 

I've seen your post about non-ECU controlled AC fix before, Chuck. Thanks though. I don't like the idea of the compressor still being on when the engine is under full load, so I opted not to do the wiring fix. Ah well ... guess I won't be buying fresh race rubber this summer.

 

There has to be a way to determine what the difference in harnesses REALLY is, and then modify your existing harness so that it works. Hell, run dedicated wires for those particular connections if need be, but you shouldn't have to buy a whole new harness just for this.

jds

 

That is why there is a botton labeled "AC" that is so easy to push. This does turn off the compressor when wanting to go max power/rpms, and leaves the fans running.

Most harness are in very bad condition by now.
I know as I have helped in pulling and rebuilding a few engines in the last three years. The plastic insulation is so poor that many wires are internally very oxidized.

Just like anything else, how safe and strong do you want your engine to be?

 

Yeah, but I've been stupid enough to do things like push the turbos before the engine is fully warmed up (yes, I learned my rather expensive lesson) ... so I'd rather have the failsafe working than burn up the compressor because I got overzealous on the highway in midsummer. So, I will be buying the new harness.

Perhaps, the new harness will solve some of the minor quirks I notice as well.

 

Many shops are starting to require a new harness when they do a rebuild. I have also seen FPD(fuel pulsation dampener) starting to leak, and even oil injector lines cracking/breaking when taking them off.

If your engine is high mileage, it is worth it to replace a few extras even if it will cost about $1500 more.

Do you realize that our primary injectors run at twice the rate of injectors on a 4 cycle engine? One each rpm
as compared to one every other rpm. This mean more wear.

 

I for one would not shell out over a grand of my hard-earned cash for a standalone ECU that cannot do something so simple as handle the AC controls properly. That said, it is reasonable to require a proper electrical connection...I have just not seen anything definitive stating what about the harness is problematic.

jds

 

My A/C works fine...oh yes...I put in a brand new engine harness when I had my KDR/Malloy motor installed (eventhough my old one was perfectly fine--subsequently sold the old harness)...

 

Mine acts a little weird. You know how the engine would rev up when you turn on the AC. Well my idle revs up a little bit and hunts a little when I turn on the AC. It then stays normal. Then when I drive and come to a stop, it repeats itself. Weird...
I do feel some cold air blowing though..

 

Well, you could always spend more to get the Haltech, which can't even run the oil metering pump, a vital component to a 13B-REW. Again, I think it's funny that a system designed specifically for the rotary can't run a vital engine part.....

 

The same can be said about the many screws up by Mazda with the FD RX-7. You must be laughing all the time with those.

I think it is time for the PFC mod to kill this thread!

 

Some should consider taking their fan speed selector switch apart as well. I have a bad one, that, if I pull it apart and tweak the contacts and realign everything, will work where it once didn't. Additionally, there was a noticeable short or overload in one of the wires, indicated by the black charred melted stuff on that particular contact. Essentially there are five wires coming off the back of the fan switch. Inside the fan switch are two spring loaded contacts on one side and five flat copper contacts on the other. If one of your wires overloads and overheats, it can inhibit the movement of the spring loaded contacts. Viola, no a/c or blower motor on that setting. FYI, the a/c indicator light will be on but you won't have any fans. After manipulating things internally, the fans would work and I had a/c on settings I did not previously (ie. no light, no fan, no nothing). It's only a temporary fix however and once it's been tweaked, fixes won't last long. Best to find a used one in good condition or a new one.

 

hmm

why can't we just remove the patch harness since we have the powerFC...we can control fan speeds with the ECU, no?

 

My statement was referring to the Haltech system, not the PFC....

BTW, yes, some of Mazda's decisions do make me laugh. The kind of laugh you hear someone make right before they go postal....

 

For what it's worth, I used to have fan settings 1, 2, and 4 work, as verified on the PFC sensors page (3 would cause the AC light to flicker). Just had my wiring harness replaced and now only setting 1 works. So a new wiring harness isn't necessarily a definite fix....