PorschePhD

Ok, I can not find a damn wiring diagram to save my life so maybe someone could lend a hand or been here before. The AC system in my 94...I charged it last night, forced a can in to get the pressure up so that the switch would engage and the AC clutch would not engage. So I jumped it so I could fill it and run some test. I traced back to the relay and the relay works and is powered just not seeing a ground trigger from the CCU. I can ground that wire out at the head and it will engage the clutch but obviously will not cycle since nothing is telling it to. I am certain there has to be an evaporator sensor telling the CCU to send a signal to the AC relay and power up the clutch. Anyone know what the pin outs are to test this or am I off base. FWIW I tried another head, same thing no different.

Colin 90 C2

compressor is fed 12 v thru relay pin 5. other side of contact is pin 3 which goes to 7.5a fuse in engine compartment. ground for the relay is pin 4 and goes somewhere labeled motor left.

you also have the 3-level freon pressure switch which is usually (on most cars) on the evaporator. The switch has a high and low cutout and a mean effective pressure switch. pins 1-3 is the high/low (n.c)(2.5 bar/27 bar) and pins 2-4 9(n.o)are the mean effective pressure (17.5 bar)

I'm thinking that somehow you must also have the condenser blower and relay energized. Although from the wiring diagram, I can't tell how the logic works.

Lucky for you I dug out the wiring diagram book for another post.

If you need anything else, just post it up or pm me. At least I now know where the book is.

maybe we could horse trade some knowledge. I have turbo conversion questions.

PorschePhD

Please ask away on the turbo stuff. I would love to help!!

Ok, yes you are correct the condenser blower and relay is engaged. It wasn't originally but once we sucked down the system and got a can in it the fans and the relay came to life. The AC clutch stayed dead. The more it ran and the higher the head temps got the fans cycled to the higher setting so everything in the system seems to work but the AC clutch circuit. I traced it back the CCU and it was fine. I jumped it into the CCU main plug and ground it out and the clutch came to life. I am not going to lie, I have an clip to ground....it is so nice to have it on in this humidity.

SOOOO the question is this circuit is all tied as one, or is the mean effective pressure circuit in the switch not feeding the CCU info and therefore thinks the evaporator is maybe too cold and shutting it down waiting for the pressure to come down? Sorry, just trying to sort this out loud. I can run a meter across the 2-4 pins to see what I have. The low and high pressure sides show everything was charged correctly and when I pulled vac it held 22 for about 30 minutes.

Colin 90 C2

disclaimer- I am not an a/c tech, but I slept at a holiday in express last week.
anyway, the mean effective pressure switch just turns on the the 2nd speed of the condenser fan, so that's not it.

I am confused by what you did to jump the compressor.
did you apply 12v to the clutch and it engaged. or did you ground the clutch lead?

PorschePhD

I can take a picture tomorrow of what pin it was, but I pulled the CCU and grounded the lead that runs back to the relay. The relay is powered with the key on and awaits the ground signal from the CCU so I just ran a jumper out of the CCU plug to ground since the CCU doesn't think it needs to send the signal for whatever reason. This was just for the clutch relay in the engine compartment.

Colin 90 C2

can you give me the pin number from the ccu and/or the wire color?

PorschePhD

Solid black and it looks like pin 25 on the top CCU harness.

Colin 90 C2

pin 24 from the ccu is black and it goes thru the low/high pressure switch then onto the compressor relay. Also, after the pressure switch, it goes to the DME pin40 on a wt/or wire. Maybe this is where you are missing. you should check continuity between pin 40 dme to pin 24 ccu. I can't see how the logic works from the diagrams, but we'll get there.

two questions"
1. is there +12v on the compressor lead measure to ground.
2. do you get continuity if you go from pin4 ac compressor relay to ac compressor body (effectively ground)

PorschePhD

Errr, DME...this is our problem I bet, the car WAS a 94C4 and is now a 3.6T C4 so in other words there is no DME in the system. So the system should be setup like a C2T. I was under the impression that other than the second rear blower the systems were the same.

So pin 24 runs through the switch, then grounding the pin at the CCU(pin 24) the system still should cycle when the core gets too cold? Based on what you are saying, yes?

Questions: Yes, but only when I jump pin 24 to ground which then in turns activates the relay in the rear panel converting the ground signal from the CCU to 12volts to the compressor.
2.Yes, I have it from the compressor to the relay. I also have checked the black wire(24) from the relay to the CCU. It is fine hence why I jumped it there as opposed to the engine bay.

I am concerned about the DME lead tho. And here is my question. Shouldn't the CCU generate a ground signal on pin 24 when the AC button is compressed? If the signal goes like this CCU>AC Switch>pressure switch>AC Relay>AC Clutch then something in the AC switch in the CCU must be nutty. Ok, so that may be true, but then why does the condenser relay and fans receive the signal and operate as normal. Are two sides of that switch receiving different inputs? If so what is the other pin on the CCU feeding those.

PorschePhD

OK, so if I ground pin 24 the AC clutch relay engages and the compressor works. If I unplug the three way pressure switch the clutch disengages which means that the only thing that it can be is the CCU. Thoughts

Colin 90 C2

I need to think about this a little and my mind is mush tonight.
I will try and scan in the wiring diagram for you.
pm me your email and I 'll get it out tomorrow sometime.

I know the ccu's get flaky from overheating. Any chance of you borrowing a good one?

Wachuko

Maybe a stupid question... but are you sure that the black wire in the back (at the compressor) is plugged?

The one circled in red:

PorschePhD

Jamie,

Yes, I have. But thanks for the suggestion.

Ok, where I am at is it has to be a bad unit. If I ground pin G22 which is the evap freeze sensor the clutch will click on then shut off. I fiddled with it for about 30 minutes and actually got it to engage properly, but it will not stay. This sensor feeds ground to the CCU when the evap is not frozen and when it is breaks contact thus killing K24 pin. For some reason the CCU will not hold that signal. Anyone have a known GOOD unit? If not I have an idea of how a relay and some thought could go a long way here. It also would be a hell of a lot cheaper. Since half the AC system functions off the switch I just tap that into a relay and feed the frozen sensor lead into a cut off relay that is ontop of the jump relay. Follow me

dfinnegan

I have a spare; though I have not tested it so it is not *known* to be good. Still, I expect that it is and would be willing to loan it to you for a test.

If you'd like to borrow it, PM me your address and I'll get it out tomorrow.

PorschePhD

Thank you for the help! I actually have a buddy that has a 95 993 and I swaped units with him. Mine works perfect in his car. WTH!!!