My Condensor fan and compressor were not engaging a couple of weeks ago. I narrowed down the condensor fan problem to a bad resistor which I replaced and it now works when I engage the ac switch. (Thanks Rennlist)

However, my compressor still does not come on. I know that this is probably due to a low regrigerant level. What should the pressure be? If I have to recharge the system, I assume that I would need the compressor working. How do I do this? Any help would be appreciated.

Thanks

Sandman

 

Hello Sandman,

To properly charge your A/C system, you really need the special equipment shops use. This equipment can draw an extended vacuum on the system (remove moisture, contaminants), weigh the amount of R-134a charged and (more importantly) weigh the amount of oil present in the system. I know lots of people just pop in a can of R-134a, but without the professional equipment, I don't think there's a good way to make sure you have just the right amount of oil in the system (not too little, not too much).

All this said ... there's nothing special about the 993's R-134a AC system that should prevent you from taking it to a generic (but hopefully competent) A/C shop for a recharge. Hopefully, you can find one of these in Bombay . You may have to point out where various components are (receiver/dryer, condenser, evaporator), and possibly how much oil needs to be added (I can give you the data from the shop manual, if you need it). The system should be charged with 840g of R-134a, though I suppose they may just add R-134a until they get the best system pressure vs. center vent temp. compromise. In the U.S., a typical charge for topping off the system is around $60-$70.

The center vent vs. system pressure graph can be found here:

<a href="http://forums.rennlist.com/forums/ultimatebb.php?ubb=get_topic&f=3&t=005053" target="_blank">Another AC Thread</a>

Hope this helps, and best of luck.

 

Oops ... correction, Sandman. The graphs in the thread I linked above are for (1) center vent vs. ambient temp & (2) low pressure vs. ambient temp. The graph for high-pressure vs. ambient temp. is not in the thread. I can send you that, if you need it.

 

Hey Sandman,
I think I did/do have a similar problem as you. However, my problem only happened when the ambient temperature was really HOT - like 90+ degrees F. I had two blown resistors in the front wheel wells, so I took care of those first - and it had no effect on the compressor not kicking in. However, I now noticed that when turning the a/c on - the system came on with the front condensor fan cycling on and off - but still no compressor. I verified the compressor by supplying a 12v jumper lead straight to the compressor terminals and the compressor did kick in and cold air was flowing in the cabin. I think the compressor relay in the engine compartment works just fine - but no signal was arriving to the relay.

Now... the weird part is when the outside temp is really hot and I start the car and turn the a/c on, I hear the front condensor fan cycling with no compressor. After I drive for about 10-20 minutes, the compressor kicks in and all is right in the world once again. If it is less than about 85 degrees outside and I use the a/c, it never has any problem and the compressor kicks in just fine. The CCU temperature control seems to work just fine as I have been using some heat lately and the air work as expected. I also know I have some more troubleshooting to do on my side of things before anyone could probably completely diagnose this, but I am going to have to wait until next summer now before the outside temperature gets hot enough again.

Right now, my suspected culprits are:
- compressor relay failing in the heat. (Kind of doubt this, but will need to verify nonetheless.)
- CCU flaking out. (Possibly - but trying to convince myself that this isn't it.)
- Pressure too high or too low in the system. I doubt too high as I checked the pop-valve on the compressor and there is PLENTY of pressure, so I suspect too high possibly - but would the temperature effect this?

How close does this sound to what you are experiencing? I would guess your temps in Bombay are pretty hot - so this is possible. Maybe try the a/c at night or when nice and cool and see if it comes on at all - even after driving around to get some air blowing across things. If anyone has any thoughts on this, I'd sure appreciate it. I love driving with the windows down and sunroof open, but the only times I really WANT the a/c - when it is REALLY hot - I can't have it. Go figure, huh?

 

Hi Brian,

One of the reason some "experts" discourage converting R-12 equipped cars to R-134a is because of R-134a's higher operating pressure, which may cause the compressor to trip off right when you need it most--that is, when it's hottest. High pressure side pressure increases with ambient temp., up to the point where the condenser fan switches to fast-speed. After an initial drop, it then starts to increase again with ambient temp.

&gt;Now... the weird part is when the outside temp is really hot and I start the car and turn the a/c on, I hear the front condensor fan cycling with no compressor.

Is it possible you're hearing the condenser fan cycling between fast & slow-speed? The condenser fan should run continuously in slow-speed with the AC on. If it's not, this could be your problem. Yet, you wrote that you already replaced the ballast resistor.

The condenser fan should switch to fast-speed when the high-side pressure reaches 17.5 bar (253 PSI). The compressor trips at 27 bar (391 PSI).

If the condenser fan is cycling between slow & fast, this means your system is teetering at around 17.5 bar pressure.

I know of one 964 owner (car converted from R-12 to R-134a) that shorted the condenser fan ballast resistor, causing the fan to run continuously in fast-speed when the AC is on. A neat trick!

&gt;After I drive for about 10-20 minutes, the compressor kicks in and all is right in the world once again.

Could it be that air flowing across the condenser is acting as your (potentially in-op) fan, eventually bringing down pressure to the point where the compressor will run?

Hope this helps, and good luck.

 

Hi Randall -

Actually, I am running with a "jumped" ballast resistor, so I have a one speed fan. Now that I try to think back, I can't recall if after jumping the destroyed resistor I get a cycling on/off high speed fan or continuous high fan. I would presume that even sitting for 20 minutes with the fan running, it would eventually cool enough to get the compressor to kick in.

Your notes give me some more optimism that I merely have "high" pressure problems. At one point I read someone saying that they pushed the valve pin in on the fill valves on the compressor to verify pressure. I don't know what normal pressure should do but those things blew a split second jet stream of mist out the back of the car about 10-15 feet. Felt pretty pressurized to me - to say the least. The dealer in FL I bought the car from reported that they checked/filled the a/c prior to sale to get it ready for me. Perhaps a quick fill was a little careless.

On this too high pressure theory... with "safe R134"... could I simply relieve more pressure? Or is this not a safe option (for myself, the system, and the air) to do on my own? Should I really get a shop to get the system back to specs?

Sandman - sounds like this could be similar to your problem - IF your condensor fan kicks on after the resistor replacement or shorting is done. Hopefully we both only have a high or low pressure problem - which would be cheap compared to a dying CCU.

On a side note - thanks Randall. Your expertise on this subject is truly appreciated.

 

Hi Brian,

If you're really motivated, you can buy a gauge that will allow you to measure the system's pressure. Either on the internet, or at a generic parts place such as Kragen. Make sure you get the kit that will allow you to measure both the low and high-pressure sides (the cheaper kits can only measure the low-pressure side).

BTW, you need to take care when working with R-134a to avoid frostbite ... wear gloves & goggles.

 

Thanks Randall. I might look into that - if for no other reason to truly confirm on my own what is going on and when. Look for more from me on this next summer.

I think I looked kind of funny testing the pressure. I had towels covering everything, gloves on, goggles, a hat, and was standing as awkwardly far away as possible to reach the valve while shielding myself with the free hand. <img src="graemlins/icon107.gif" border="0" alt="[icon107]" />

 

I had the same issue when my harness failed. The AC compressor would not engage through the harness. It would however engage when jumped with 12 V directly.

 

Thanks Randall and Brian

I am getting the pressure checked tomorrow and hope that it is just a low pressure problem. I will keep you updated.

Ciao

Sandman