dr bob

Andrew-

Does the drier have a sight glass? With conditions Bill mentioned (1500 RPM, fan on 2, center vent temps monitored, cooling fans operational) on your 85º day, you'll want to have a few bubbles streaming in the glass, no more no less. The humidity affects the center vent temps but doesn't affect the condenser action or the condensing temp noticeably. (Humid air is less dense so there's a very slight difference in condenser perfomance.)

The risk of overcharging on the system is that the high-side pressures will run away at ambients over 95º if there isn't plenty of airflow through the condenser. By 105º ambient it gets way too serious to ignore. Undercharging slightly causes the evaporator efficiency to drop off at high loads, and the resulting reduction in heat load will help prevent that high-side runaway.

The R-134a cars, late '93-on, use a dual pressure switch arrangement that protects the system from over-pressure sdamage by cutting off current to the compressor clutch. It takes a little wiring change but is probably worth your while.

AO

The later cars do not have a pusher fan like the early cars, but yes, the fans are running.

I measured at 1500~2000 RPM and it seems steady there. Unfortunately it started raining right after I did this and then I had to take the kids to their piano lesson, so I haven't driven it since I did this. Hopefully the weather is clear for tomorrow and I'll drive it a bit to see if there's any improvement.

Almost no oil came out. Maybe an oz or so.

This wasn't supposed to be a total vac and replace, but it's looking like it might become that, but it'll have to wait until after SITM. I did purge, but maybe not enough? How long does it take? I gave it a couple bumps and thought I was good to go.

No sight glass on the drier. Understand about the high side pressures running away, that's why I wanted to address the overcharge. I'll look at adding an over-pressure switch too.

Bill Ball

Andrew - check the drier again. The sight glass is almost always there but is often painted over.

AO

I know it doesn't have one. When I replaced it last year, I was in a pinch and replaced it with what I could get locally = cheapo w/o sight glass. Oh well.

dr bob

Andew-

FWIW, I have a three-stage vacuum pump and leave it on overnight. It goes lower than my gauges can read, plus the extra time means that any free moisture in the system is boiiled out. Commercial places say that 15-45 mins is enough, but I'm not in agreement at least when it come to my own cars. If you want it to be COLD, you can't have ANY air in there.

AO

Where does one find such a wonderous tool? Is it expensive?

dprantl

To purge air in the charge line I usually loosen and let it hiss for 3 seconds or so. You're not really losing that much refrigerant since most of it should be a liquid still in the can. A 3-stage pump is of course best, but the cheapest you could probably get away with for a proper job is something like this:

http://www.harborfreight.com/two-sta...112&zmap=66466

25 microns is very low and does a good job of boiling water out of the system fast. I myself only leave the pump on for an hour or so because I'm too lazy to do it longer and the pump is at my brother's house so I can't just leave it running in the garage overnight.

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

AO

So are you recommending that I purge, vacuum, and refill the system? I think I can do that. Just time and money.

dr bob

Andrew-

I bought it at an industral auction 20+ years ago. It was part of a lot of supplies and equipment from a QC department. I was looking at industrial ovens, and ended up with a bunch of lab cabinets and other equipment as a fringe benefit. Look around where you are for similar auctions. Stuff sells for pennies on the original dollar. I didn't have an appreciation for how good it was until I used it on a car and it tried to bend the low-pressure gauge needle against the pin. compare with the average one- or maybe two-stage pump that gets down to an indicated 28-29" best.

dr bob

The refrigerant and oil are cheap. The time with the pump attached is free, if you just let it sit overnight in your own garage.

For the 928, I connect the vacuum pump to the Schraeder valve connection where the pressure transducer sits. The charging manifold connects to the car normally, with the gas cylinder on the middle yellow hose. That way the whole system is evacuated, including the manifold and hoses all the way back to the cylinder. No air is introduced during the charging process.

AO

dr bob-

That some great advice. I think, however, I'll tackle it after SITM. The HF pump isn't available locally, and I like you idea of sourcing a 2nd hand pump. Then hooking it up so I evacuate the lines and all is a really great idea.

BTW, I started the car up this AM and it seems to be blowing cold air out the center. So maybe there's hope.

hans14914

I just did Katie's truck yesterday. Left the vacuum pump on for about an hour to clear the system. HF has one on sale during the summer months. I was at my friends shop, using his nice snap-on tools though. After the prep work (vacuuming) proper filling only took a couple minutes. Man its nice having the right tools.

Tony

Thats why its the low side!

123quattro

I recharged my soon to be father in laws Acura this weekend. It was mid 80s. With the compressor running and the AC full on I was trying to get 30-35 psi on the low side per the instructions and temp table.

Randy V

You won't get a true understanding of how it's working until you drive the car with the AC on.

Take it out on the freeway for 15 mins with the front on full cold.

Keep the rear AC turned off. - always. It only reduces the cooling efficiency of the front as it is a shared system.

I have a feeling you'll be fine if you can get the lowside pressure within spec.