ebfox

While working on my heat problem, I discovered that my AC compressor clutch is not pulling in. Last fall when I bought the car it was a little low on freon; I put in one can of R-12 and it worked like a charm. This winter I cycled it on a couple of times but I did not verify that the compressor was turning.

Here's what I have done so far; put the gauges on the system and I have a residual pressure of 25 psi. Fuse in the rear fuse block is good. I put my meter on the wire going to the compressor and there's no voltage, (with engine running and AC button engaged) so I suspect the relay which is located in the rear fuse box.

I couldn't find any other fuses or relays associated with the compressor, so it's got to the be the relay or the CCU? Am I correct that if the compressor itself was kaput I would have a control voltage on the lead?

Thanks for any help, advice, moral support, etc.

E. B. Fox
91 C2 Targa

Porschedude

I think you already have the information that you need to determine what is at fault. It sounds like it is low on freon again.

There is a pressure switch up front that is called "3-LEVEL FREON PRESSURE SWITCH". It has two bellows in it and thee contact sets.

One side of the switch controls the cycling on-off of the clutch as demand and reserve change before the expansion valve. It is like your shop air compressor. When the normal upper limit is met, the pump shuts off. It is called "MEAN EFFECTIVE PRESSURE". It is set at (17.5 bar, or 257 psig).

The other switch is the "HIGH/LOW PRESSURE". The HIGH/LOW switch protects the system from hurting itself. If the system pressure went above the designed value, for instance, the "MEAN EFFECTIVE PRESSURE SWITCH" stuck closed and the pump clutch didn't dis-engage when it reached the normal cutout value, the upper limit side of the "HIGH/LOW PRESSURE" switch would drop the clutch out at (27 bar or 397 psig).

The LOW side of the same switch will prevent the clutch from being engaged if there isn't enough freon in the system. It is set at (2.7 bar or 40 psig).

You said that you only had a static pressure of 25 psig and the there was a leak previously and you added a can of freon and it fixed it. It sure sounds like the leak didn’t go away and you are low on freon again.

To prove the low freon charge, you can power the clutch with another source using a jumper wire and see if the clutch engages and the system cools to some degree. You will not hurt the system with the charge you have, but don’t us the jumper as a fix, just to trouble shoot.

ebfox

Keith,

Thanks for the tech info. I was thinking about jumpering the clutch and that's on the agenda for tonight.

Looking forward to finding the leak- it's a bear to get to that condensor.

E. B. Fox
91 C2 Targa

Adrian

Dear Ebfox,
The relay for the compressor clutch is in the engine bay electrical panel. Easy to get to. The relay receives an earth from the control unit switch via the three way pressure switch. The wire to the clutch provides 12 VDC which is powered through the relay contacts. What you do is remove the relay and jumper the contacts that supply power to the clutch.
A common problem is the actual connector inside the clutch assembly. I would again check to see if you are getting 12VDC on this line at the clutch connector and make sure you are using a good contact and have a good contact with earth.
There is another little trick you can try. Turn the heating on and select defrost. If the outside air temp is higher than around 10C then the aircon should be activated by the control unit using a different circuit. Recently I helped somebody with a similar problem and they had lost the earth to the switch in the control unit.
By the way I agree with Keith on the failure based on statistics. However it is worth checking other areas first.
Ciao,
Adrian
911C4

PS: The condensor mixer is not that difficult to get to it is the evaporator that is the pig.

bj

EBFox

I had a similar problem last year although I still had enough freon in the system that allowed the compressor to engage. Take the time to visually inspect the lines from front to back - especially at connections. Mine was leaking at the connections just in front of the drivers side rear wheel - under the rocker panel and at the drier. Each connection simply needed a 1/4 turn to snug things up. I also looked at the condensor for any leaks by visually inspecting at least one side of the condensor. The leaks tend to acumalte dust because of the oil in the freon. I also took it to the local AC shop while it was all opened (i.e cuts down on labour time) up and had them put the sniffer on it.

Bill
90C2

ebfox

Well I have coolage again but still haven't found the leak. I ran a jumper from the positive battery terminal to the compressor and it started right up; put a can freon in, removed the jumper and the compressor ran as normal; Keith's comment about the low pressure cutout was on target. I have a friend with a sniffer that I will see in a couple of weeks so hopefully it will hold charge till then.

I am thinking about converting to Freeze 12; other than the negative stuff written by the guy that sells a competing product, it sounds like a reasonable alternative. My R-12 stash is almost depleted. Would be interested in any Freeze 12 comments.

E. B. Fox
91 C2 Targa

Randall G.

Hey E.B.,

I tried to put Freeze 12 in my car last summer, but couldn't find an installer in my area (Southern Califoria, for crying out loud!). And, you can't buy the stuff directly without being certified (or, something to that effect).

I wound up converting to R-134a, and am quite happy. You've probably located my old posts on the conversion. My car blows cold air per the factory specifications for an R-134a equipped 964/993. It was actually blowing 40.9F the other morning on the way to work, albeit a cold (for So. Cal) morning. It blew 45.3F on the way home from work that same day, probably 65-70F ambient.

Some big upsides to having R-134a are a cheap, plentiful supply, and never having to worry about finding a shop to recover your refrigerant. If you ever need to have your system purged prior to opening, you may have a hard time finding a shop that's willing to capture your Freeze 12, etc. Likewise, your supplier may not be around in a couple years when you need a fresh charge.

Best of luck, whichever route you choose.

Madridcab

Just had my aircon brought "back to life" after winter, turned out to be a gasket in the compressor, the A/C shop (recommended by my P-mechanic) claimed it was a common leak. Demounting compressor, strip down, change gasket, vacate system, refill with 413a (according to A/C shop, R12 no longer legal in Spain, 413a replacement with no conversion required..??), and new oil, Euro 207 (215US$)I thought was a pleasant surprise... They pressure tested and did not think there were further leaks, time will tell... for now it blows nice and cold.

ebfox

Upon further review- I found evidence of a leak at the condensor; tightened it up and it passes the soap bubble test.

Does anyone have a chart which provides full charge pressures at various ambient temps? At 75 Deg F I am running a low of 20 psi and a high of 160 psi with 50 Deg supply air. Good delta T but the pressures feel low to me.

E. B. Fox
91 C2 Targa

Randall G.

Hey E.B.,

Suppose you mean your ambient (or supply air) is 75F, and you're getting 50F out of the center vents?

Anyway, I've e-mailed you the applicable diagrams from the shop manual for an R-12 equipped 964.