jeff spahn

Okay- I have searched but not found this answer.

This is my first summer with my 928. Just drove it round trip to Atlanta, Pensecola and back to Iowa. 2350 miles door to door in three days. Wowzers. Met Wally P. Super nice guy. Showed me an great deli that makes a mean BLT.

On to the A/C question. I am wondering how well the a/c works in a 928 when functioning well. Mine would run for a few min, then cycle off for 15-30 sec then turn back on. Humidity down south in Mississippi, GA and AL is high to say the least. We get super high humidity here in Iowa so I would love it to work well.

With the A/C cycling on and off it didn't stay super cold in the car in the 90+ heat. I thought with two a/c units, the rear being a recirc unit, it would stay cold. With all the fans on, the voltage at the post was 12.6 or something but the dash said 11. Screw the dash gauge. (Gage in GM speak)

Does this mean I should look at a recharge and leak test possibly or is that what I can expect from the system? I haven't found any records in my pile that say it was converted to R-13.

I am used to my 2010 Jetta that is like a meat locker.

Any thoughts would be appreciated.

Nicole

The 928 AC is not as strong as it is in most modern cars today. I, too, have found it a bit weak in hot climates. I also find that if have the rear AC running, the front one gets weaker.

In consider the HVAC system one of the weakest points of the 928.

jeff spahn

So put the jumper back on the relay right by the windshield. I took it off when i was at Greg Brown's shop. Ron had it jumpered. Bad Idea or good idea to put the jumper back in? Probably getting it recharged can't hurt though.

Rob Edwards

It's still running R12. We put ~2.5 lbs of R12 into that car at my house in February (?) of 2009, which improved the cooling capacity somewhat. It improved the function of the system at the time, but since A) we had to add R12, and B) its performance isn't 100% now means of course that you have a leak somewhere. The fact that it was cycling on and off means the freeze switch was kicking in (?) I recall a discussion about jumpering the freeze switch to keep it from cycling but in a humid environment you'd freeze the evaporator solid with ice.

There will be varying opinions about keeping it R12 vs an r134a swap, but in either case you should probably figure out where the leak is and get the system re-o-ringed, flushed, and recharged with the right oil and a new receiver-drier. The rear A/C complicates matters a bit but it's still do-able. Dr. Bob's r134a conversion will freeze your hands to the wheel, so you can have killer A/C in a black car.

jeff spahn

So still getting it recharged might not be a bad Idea for the trip to Roger's shop in Dallas? I think I'll put the freeze jumper back in too. Can always take it out. Man, it was humid on that trip. I am on track to put about 25k on this car this year alone.

dprantl

If the system is cutting off because of the freeze switch (and the actual evaporator temp is really 32 deg F) when it is above 90 deg F ambient and humid, it is working optimally. If you want it colder, you will need to bypass the freeze switch, but then also watch out for evaporator freezing and reduced airflow into the cabin. And at that point, unless other cars have a much higher fan speed/flow, they will not cool any better either.

Dan
'91 928GT S/C 475hp/460lb.ft

WallyP

The short-cycling might not be the anti-freeze switch. If the Freon is low, the low-pressure switch will cut off the compressor as you describe. A significant part of the oil in the system circulates with the Freon. No Freon circulation, no oil in the compressor, pretty soon, no compressor.

Cycling due to the anti-freeze switch is usually much slower than what you describe.

First thing that I would do is to put a set of gauges on the system and see what the pressures are. Do some leak-testing, and see if the problem is the usual hardened O-rings, or another leak somewhere.

Enjoyed your visit, and envy your road trips!

dr bob

I'll chime in some since I was somewaht involved in the freeze switch decision. Keep in mind that we live in a relatively dry coastal desert climate, and most trips aren't long enough to seriously ice up the evaporatr with the freeze switch in the circuit as designed. Ron was taking short trips, and the car was sometimes out in the sun long enough to get heat-soaked inside. I recommended that he jumper the switch to get max available cooling to remove the most heat possible under those specific circumstances. I generally leave my freeze switch jumpered for regular driving, and only remove it for trips that might be over an hour or two with AC running. If I forget, the evaporator does ice up to the point where airflow is restricted; Then it takes a while wth AC off to thaw it out and drain all the water out. During that while it's way hot and humid in the car. Bottom line is the freeze switch is there for a good reason.

Echoing Dan and Wally's comments above, you'll want to get a set of gauges on the system and see if it's undercharged at this time. Low-side pressure is the telltale, with less than about 20-30 pounds there a time to start looking at things more seriously. A slightly undercharged system will get colder at low evap airflows and therefore low load on the system. That leads to faster icing if the freeze switch is jumpered, and with low load it will ice up pretty fast anyway.

I'll also mention that there are some freeze switches that are colder than others. I had one that cycled between 40 and 48 degrees, hardly optimal for black car in SoCal summers. I tweaked on it to get it to cycle between 35 and 39 degrees, but it was not as repeatable as I think is necessary. So I got another and it works colder and is more consistent. As Rob mentioned, my car has had R-134a in it for the last 10+ years. With the freeze switch jumpered in our dry climate, center vent temps below 20º are common with the fan on second speed and outside temps at 90º, cruising at 60-70 MPH. It will freeze fingers on the steering wheel. Keys to this include the very low humidity we regularly see here in SoCal in the warmer months. Since the AC is not spending capacity condensing water, it has more capacity remaining to cool the air flowing through the evaporator. Condensing water eats up hundreds of times the capacity that simple air cooling requires on a per-pound-cooled-a-degree basis, so apparent system performance can be way different under different humidity and temp loads. Good news is that dehumidifying is a significant part of the apparent performance of a car system in a humid climate, stripping 20+ apparent degrees. 70 humid degrees for many is uncomfortably warm, while 70 dry degrees for many is uncomfortably cold. 70 dry degrees is warm for me, and humidity is a real challenge. If it wasn't for that darn poverty, I'd live in the mountains and be a ski bum. Cool dry climate for me, please.

jeff spahn

Thanks Dr. Bob. I just had it evac'd and converted over to R134. Cold so far. We'll see how it goes to Dallas on Thursday. Windows are also being tinted today so that will help as well.
We have a lot of humidity here to freeze up is a real possibility. I'll keep the jumper off for now and put it on when I go through the desert Southwest this winter on my way to get the stroker put in. Now, as long as my new career goes well that will be a whole 'nuther thread on the stroker motor.

Larry Kohler

Dr. Bob, I notice that you live in Glendale, I am in Agoura Hills. I have been struggling with my '86 928S A/C and several other electrical problems. Can you recommend a good A/C repair shop in the SF Valley, as well as a good 928 general repair shop that is at all resonablly priced? Thanks,

dr bob

Jeff--

I spent a total of almost two days all in doing the R-134a conversion, of which about 4 hours was getting the hoses rebuilt. A long day in the car itself. When I see someone post that they "just had it evac'd and converted" I worry that you may not have gotten all the work that's required for a full conversion. Every o-ring in every connection, two expansion valves, the hoses, a drier, flush and new oil charge in the compressor, and finally evac'd and recharged. There are some who will tell you that they just need to add a "conversion" oil charge, new conversion fill fittings, and fill it with R-134a. Those are the same folks who scratch their heads when you complain about not enough AC. The good news part of that is that a poor conversion will never tax the freeze switch. Bad news is that you may not be real happy with the performance.

Be sure to share the results when you get it back.

dr bob

Larry,

The locals here take their cars to Jules Hauer at Hauers in Van Nuys. Reasonable and pretty good work. Further from you is the master, Greg Brown at Precision in Anaheim. Reasonable and excellent work.

We'll probably be at the C&C next Sunday morning in Topanga area. Got time to join us?

jeff spahn

You may be right Dr. Bob. I am planning on replacing most of the AC bits this fall when i plan to start the new engine process so perhaps it will be a moot point.

JCP1990S4

My car Black/Black has had the entire system redone. Freeze you out 85 degrees out side and lower. 85-95 OK. 95 and above barely noticeable. I've given up. I have spent about 3k on it so far. My Prius boring as hell to drive is ice cold at 110 degrees.

dr bob

JCP--

Not sure what you are fighting. Perhaps the car is not charged correctly. Key to a good R-134a conversion may be that you really need to undercharge the system by weight compared with R-12. If you charge by weight, think in the 90-92% range compared with R-12 weights. My goal on a 80º day is a suction pressure around 25 PSI with engine at 1500 RPM. This keeps cruising center vent temps in the 40º range at 100º ambient. It's a balancing act for sure. R-134a needs all the condenser capacity available at the higher temps, but getting the system to maintain liquid at the expansion valve while keeping pressures as low as possible is a challenge. The compressor can't keep high-side pressures extra high for good condensing on a hot day, and suction pressures low for low evap temps at the same time. The expansion valve is intimately involved in this too, but it can only do so much. Good fan airflow is critical too.

There are other considerations like expansion valve performance, possible air in the system, worn compressor, partially plugged drier (correct dessicant for R-134a and the oil you are using?) and the amount and type of oil used in the conversion. Many "conversion" kits include a bottle of PAG oil, for instance, and that stuff turns to slime if there's still any trace of mineral oil left in the system. The jellied lubricant coats the insides of heat exchanger surfaces and doesn't do such a great job lubricating the compressor either. Lots of possibilities. Do you have gauges to look at system pressures?

If you want to continue the discussion, I suggest that you start a new thread specific to your conditions since it's different from what Jeff is looking at.