dr bob

Good job, nice write-up and pictures!


Some more (maybe too late...) thoughts:

-- Evacuate the hoses and the charging manifold all the way back to the bottle using the vacuum pump prior to charging. Use a spare hose to hook up the vacuum pump to the pressure safety switch port, the one you see there in the third picture. This is a little more thorough than trying to purge the lines with gas as you fumble with the yellow hose on a schraeder holding vacuum. Once vacuum is established for a while with all the charge plumbing attached, disconnect the pump while it's still running, and reinstall the switch before charging.

-- You can do the initial fill with liquid from the bottle. Engine is off. Fill to the high side so no liquid will get to the compressor. (must go forward through the expansion valve and evaporator, or backwards through the condenser before any gets to the compressor) Close the high-side valve as soon as liquid stops flowing. Let it set a bit to make sure all the liquid evaporates before you start the engine. Then finish the charging through the low side port in the normal fashion.

-- 21 degrees is pretty darn good, especially with a >80F ambient. Verify that the freeze switch is working if you plan any extended rides with AC on. Otherwise you'll be shutting the compressor down manually every once in a while to thaw out the evaporator box. You'll know when you need to do it when airflow is choked off by the ice block in there.

Bill Ball

As we know, the IR guns are not accurate against some surfaces like the vents, and the real numbers are higher. I've gotten way below zero (-16F) with an IR gun. When I put an analog AC thermometer in the vent, running around on an 85 degree day, I did get 28F. That's still below what the freeze switch is supposed to do (cycle between 32 and 40). Need to look at that.

Rob Edwards

Yeah, 21 isn't real, and it was barely 70 in the garage yesterday. I've got a decent temp sensor that plugs into my multimeter, I should have put that in the vent to get a more accurate reading.

Bob- great tips on charging, I had the hoses vacuumed to the manifold on the gauge but stupidly didn't think to connect the high side hose to the bottle and purge it first. Also a good call on flowing freon in the high side first. All I know about charging I learned from an old shadetree/ghetto method posted on a BMW board about 8 years ago, it's good to hear how it SHOULD be done, even if it's a little late for me.

dr bob

Rob,

I don't know if that's how it SHOULD be done. I sat there thinking a while back about how to speed up the process, and that's what I came up with. Seems to work. Charging liquid moves the flash point down the hose a bit into the car, so the bottle doesn't get so cold so quick. That allows you to charge more before you turn the engine and compressor on. You can speed it up even more if you pre-heat the bottle in a bucket of 100 degree water.

Using the IR thermometer may or may not be the best way to get a good reading. I use a little digital kitchen thermometer with a peak-hold feature. Found it at one of those roadside-mall kitchen gadget outlet stores for about $10. One for in the car, one for ambient.

I get 21f at the center vent with the freeze switch broken, 80+ ambient, cruising at 2k RPM. That's with R134a in a black car in the daytime, after all the heat-soak effect is left in the wind. That's cold enough to freeze your fingers on the wheel. I blame this result on getting more air out with a better-than-average vacuum pump, making sure no air is introduced during charging (see previous post...), and using super-whamadyne expansion valves intended for R-134a. I bought the whole mess of parts from Dave & Jeannie way back when they were first getting going. That's almost ten years ago now since I did the conversion. It's probably time to clean the mud and dust out of the evaporator soon, since it's been a while.

dprantl

Ok here's a question. On a car with rear A/C when the car is off (key out of ignition), is the rear A/C solenoid closed or open? Never had a rear-A/C car until now.

Dan
'91 928GT S/C

dr bob

Rear AC solenoid is closed until key is on, AC is selected, rear AC fan speed any, rear temp setting below in-car temp.

The rear system is of somewhat dubious value, mostly because the air inlet and discharge are pretty close together. I've used it a few times when the rear of the car has luggage in it, just to keep the contents from staying hot after a hot soak someplace. Otherwise it stays off. Others report that they use it to help recover from hot-soak loading. After all the complaints received from air-cooled owners about marginal (or worse) AC performance, the 928 system seems to be more than robust. Makes sense for the flagship car. Perhaps the rear AC gets more use in hotter-climate cars.

dprantl

So that means if I'm vacuuming out the system, I need to have the ignition on with the A/C slider on and the rear A/C **** more than 0 in order to pull a vacuum on the whole system?

Dan
'91 928GT S/C

Rob Edwards

I don't THINK so- even if the solenoid under the seat is closed, the entirety of the rear system is still in continuity via the other line. So you can still evacuate completely although you're pulling a vacuum through the rear expansion valve to evacuate the portion of the line between the solenoid and the rear unit.

You do need to have the rear A/C set to max with the car running during charging.

After changing the pressure line to it, I agree that the rear air is a PITA.

dr bob

Rob's correct. No need to have that solenoid open to get a good evacuation. Stray air and moisture will boil off and pass out through the low-pressure line from the rear system to the low-side port and your vacuum pump.

Rob is also correct about needing that open when charging, but only if you are charging without the scale. If you use the sight-glass method, it gets a little tougher to get it "just right" on dual-air systems when the heat load on the two is different. The rear system really unbalances everything if you charge by the sight-glass or sight-glass-plus-temp-balance method. You almost need to charge the front system, get it balanced, then see how much more it takes to get the liquid line to the rear expansion valve filled, watching the temp balance and the sight glass. Add enough to flood the sight glass with the rear air on, and you'll give up condenser capacity when the rear is off. It's pretty obvious when you have the gauges hooked up.

Rob Edwards

Phew- I had a 50/50 shot on getting that one right......

dr bob

Better odds than I usually give myself... 50% chance of getting it right, 100% chance of getting it wrong.

dprantl

Good, I vacuumed my GT with the car off and charged with both systems on full. I only wondered about it later.

I would never charge any A/C system without using a scale and the factory recommended amount by weight. Charging by temp/sight is precarious at best.

Dan
'91 928GT S/C

dr bob

Charging by temp/pressure/sight-glass is extremely reliable when done right, with the caveat that it's perfect for the conditions at the time it is being charged. Weather changes, engine load changes, heat load in the car changes, then the system balance changes and suddenly it isn't perfectly balanced any more.

Charging with a scale is extremely reliable when done right, with the caveat that it's perfect for conditions that may or may not be the conditions at the time it is being charged. Weather changes, engine load changes, heat load in the car changes, then the system balance changes and suddenly it isn't perfectly balanced any more.


The subtle differences show up when repeatability is a factor, and/or when the knowledge of the person doing the charging is not stellar. Converting the pressures to temps, calculating approach temps, estimating those effects on pressure, reading the press/temp/saturation curves from the chart, determining the mechanical capabilities of the system as it sits, in the typical service in which it will be used, then deciding from all those things if enough is enough... That kind of thinking just doesn't line up with the way we train AC service techs these days. Instead we depend on the scale, we always evacuate/recover/clean the old refrigerant from a system, and fill it from empty because that's the only way a charge-by-total-weight process can be used.

dprantl

Well, it seems that the factory went through a large number of tests, ran through all those calculations, and came up with the best freon weight compromise for all situations. Full evacuation and charging by weight is the only way to go if you really want it done right. It's the way the car came from the factory after all.

Dan
'91 928GT S/C

michaelathome

Rob, thanks for the great write-up! I am just about to give this a try as well.

Let us know how it works out.

Michael