A/C Forensics

Hugh R · 08-20-2003, 10:49 AM

On my 84 ROW Targa with the Seiko Rotary compressor,

I recently had my A/C charged (the Freon leaked out over the years with the PO). According to the A/C shop, I've got 45 psi on the low side and about 135 on the high side (deck lid closed). I stuck an accurate A/C thermometer in the dash vent and I find that on the open road with 100 F outside temp I get consistently 50 F out of the vent. I hear the thermostat cycling about every ten seconds to keep it at 50 F. When I'm stuck at a light or very slow traffic, I get 60-70 F temps. out of the dash vent, then it drops again to 50 F as I get RPMs and MPHs up.

Two thoughts, 1) can I pull the freon-filled brass or copper thermostat sensor out of the evaporator to make it think its not as warm as it is, thus cycling longer and hopefully lower temps, and 2) if I install auxiliary electric fans on the condensor, will it help on preventing temperature rise when I'm stuck in traffic.

I don't think I should try the Duracool or other R12 replacements, until I can establish whether I can get the temperature lower by making the thermostat cycle londer and keeping the vent temperture low when my RPMs and MPHs are low.

sammyg2 · 08-20-2003, 11:31 AM

The thermostat is there to keep your evaporator from icing up. If it is already cooler than 32 degrees making it cooler will probably not help. On humid days the moisture in the air will condense and freeze on the evaporator and could possibly damage other components. I'd try and measure the evaporator temperature first to make sure the thermostat is functioning properly before removing it.

The old saying is, you can never have too much condensor. Anything you can do to increase the effectiveness of the condensors will increase cooling. Having said that, the fan on your engine pulls in quite a bit of air, if all that air is indeed going through the rear condensor i can't see additional fans helping much. You might want ot try and put some foam weatherstripping between the condensor and the deck lid to make sure all the air is flowing through the condensor. Make sure you don't have any big leaks in your engine sheet metal where hot air from underneath could be sucked back up into the engine compartment.
Not only will that reduce the efficiency of the condensor it will tend to make the engine run hotter.

Also make good and sure all the aluminum surfaces on the condensors are clean and free of built up oxidation that can interfere with heat transfer and make sure the fins are reasonably straight. You should already have a fan on your front condensor, make sure it's working.

If your vent temperatures fall when you are moving that either means you are not getting enough air flow through your condensors at idle or it could mean you are not pumping enough with your compressor. Try to duplicate the test again but this time set your idle to 1500 rpm to see if that makes a big difference. If it does that means your compressor isn't doing a good enough job. Maybe it's worn out, maybe it's just under driven.
If it doesn't make much difference it could mean a lack of air flow through the condensors.
There's a company out there that sells and additional condensor that goes under the body. I don't know much about it but I think it's sold by griffin or something like that. it may be just for early cars, I dunno. I'm sure others will chime in with more details, I've heard that the additional condensor makes a big difference.
Habe you modified your vent in front of the center console yet? I just pulled that plastic plate off mine and the increase in air flow was surprising. Maybe the 84s already have better ventilation that the SCs, if so never mind that last part.

ruf-porsche · ruf-porsche's Garage

Hi side pressure seems kind of low. What was the ambient temperature when you took all these readings?

Did the shop find and repair the leak? Did they replace the drier/reciver? Did they evacuate the system down to remove any moisture that might have gotten into the system? If they didn't do any of the last two items; that is your problem. You might have moisture in the system and it is freezing at the expansion valve.

david914 · 08-20-2003, 04:48 PM

If you're only getting 50 degree temps at the coldest, and your t-stat is cycling, I would think that your problem might be (at least partially) in your t-stat. If it's cutting out prematurely, then you won't get maximum cooling. You might try jumping out the t-stat temporarily to see if you can get lower vent temps. Remove the A/C control panel from the console and short the two wires on the back of the t-stat together. Stick your thermometer in the center vent, put the gauges to it and see what happens.

Hugh R · 08-20-2003, 05:20 PM

The system was purged and leak tested and apparently was acceptable so the tech refilled with freon. Great suggestions on cleaning the condensor and evaporator coils and trying to by pass the t-stat. I'll check into the front condensor and fan (I've only had a car a few months). Thanks let you know what I find.