jwillman

I have got the system back togather and will throw the vacuum on it tomorrow as I reinstall the HVAC control head and test the vacuum lines and electrics up to the compressor clutch.

What I have done so far.
1. removed old bosch compressor and replaced with a tested good 6E171 and 280ml of ester oil.
2. had ac lines rebuilt with barrier hose material.
3. replaced condensor with good condition used, flushed and pressure vacuum tested.
4. replaced dryer with new one.
5. replaced expansion valve with new one.
6. Flushed evaporator.
7. replaced all o rings with green ones.
8. air pump system delete.

I have an aircraft AC system servicing vacuum pump so that should be able to get out all the moisture and test for leaks. I have put on the R134 adapters and have a R134 manifold gauge set. I know I need to charge to 85% of the R12 fill of 1050g. I have read some conflicting charging processes and wanted to get the 928 preferred method.

I have read procedures that call for adding the R134 as a liquid, can upside down, using the vacuum to suck in the first can in and then start the car, AC on high and suck in the remainder. I have also read to start the car, jump the low pressure switch and add into the low pressure side as a gas, can upright.

Is one better than another? What pressures should I see for R134 when serviced properly? Anything I have missed, sure there is!?

WallyP

Charging as a liquid is much faster, and doesn't require jumpering the low pressure switch. You must be VERY careful to avoid charging so that liquid can be sucked into the compressor - this is usually called "slugging the compressor", and can lead to its instant destruction. The refrigerant must boil to a gas before it enters the compressor.

You can introduce the liquid into both the low side and the high side before starting the engine, but must be ABSOLUTELY CERTAIN that the high side valve is closed before starting the engine. You must never have the refrigerant can exposed to high side pressure.

dprantl

For me there are only two ways to safely charge an auto A/C system:

1) Charge as a gas only via the low side with the car and compressor running. You should not need to jump the pressure switch because when you first let in the refrigerant (with the engine off), the system will be in a deep vacuum and that combined with the pressure in the can/tank will pull/push in enough refrigerant to get the compressor running once the car is started. You may want to fill a bucket with hot water to submerge the refrigerant can/tank to speed up the charging process.
2) Charge as a liquid via the high side with the car off. I have only recently tried this, but it works well and is much easier on a 928 due to the inconvenient location of the low side charge fitting. A bucket of hot water will also help here.

Charging liquid with the compressor running via the low side is risky as it can hydro-lock and destroy the compressor as Wally has said above. Then there is charging by pressures or by weight. I always charge by weight as it is extremely accurate. Whichever way you go, make sure to purge the charging lines of air before charging the refrigerant.

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

jwillman

Thanks Dan. So you let the first can into the low side under system vacuum only, engine not running and then start the car and compressor and continue with servicing as a vapor through the low side?

smyth1

use the sight glass on top of the reciever as a guide as well.the second the bubbles disapear you are fully charged.

Landseer

Subscribed. Excellent thread.

WallyP

Using the sight glass can be helpful for adding refrigerant to a system charged with R-12, but it is little help with R-134a. Charging an empty system is always better done by weight.

If you charge liquid into a non-running system, it is quicker to charge into both low and high sides. If you charge into low side only, the refrigerant must flow thru the expansion valves to get to the high side. Never open the high side valve on a running system.

smyth1

the sight glass is the most accurate way to charge any sytem,doesn't matter which freon you use.a refrigeration cycle relies on a liquid line with no gas in it to work to its full potetial .

pcar928fan

Jim, if you are doing R12 and have any left over lets run a can in to EURO85...

IrishLegend

Good info, it's all Latin to me though. ;-)
I need to get on my A/C soon!

Ducman82

Ester? if your going to R134a, you need PAG oil.......

dr bob

smyth1--

In theory, charging to the bubble is ideal IF AND ONLY IF the conditions you are charging match the conditions you'll be driving in. So temps in and out of the car, airflow through the condenser and evaporator, and of course compressor speed must all be the same as what you'll see when driving. Absent that ability, charge-by-weight is needed, especially for R-134a systems in warm climates.

Jim,

I charge as liquid into the high side, using my hands to warm the can slightly and 'push' the liquid into the system. Engine is off, system and engine are still cold. Charging into the high side means that any liquid that remains will have to go through the expansion valve before it gets to the compressor, pretty much eliminating any chance of slugging the compressor with liquid. Even so, it's a good idea to wait a few minutes before sarting the car. In those minutes you'll close the valves on the charging manifold and disconnect the can. Verify that nothing is in the way of anything moving, and you can start the engine, let it warm just a bit, then turn on the AC. Put a thermometer in the center vent, fan speed at '2'. Then engine to 1500-2000 RPM and verify that you are getting cold enough air at the vent. The low-pressure gauge has equivalent temperature readings on it for the refrigerant you are using, so you are looking for lowest pressure possible there to get the lowest temperature possible in the car. 25-30 PSI there is a good target number, giving you maximum mass through the system with an evaporator temp that's barely freezing. Keep an eye on the high-pressure gauge while you do all this, with something around 325 PSI or so a practical high limit. High-side pressure is an indication of how much heat is passing through the system and how much is being shed to the outside air by the condenser and fans. Not enough air or too-high ambinet temps or load on the system will make the high-side pressure go up. R-134a high-side pressure will "run away" faster than R-12 systems if airflow isn't adequate, so make sure yourfans are working correctly. The later GTS cars that came with R-134a factory fill came with a dual pressure switch, so it protects the compressor from low refrigerant charge levels like the others, but also protects against excessive high-side pressure. It takes a little extra wirig to make it work on the originally-R12 cars but is a very worthwhile modification IMHO. Slow traffic, hot day, high AC load, it will go a long way toward protecting the system from high-pressure excursions and subsequent damage.

HTH!

dr bob

Polyolester is generally better for 'conversion' cars where the system has not been completely flushed. PAG turns to jelly if it mixes with the original mineral oil. It escapes me why plug-and-charge "conversion kits" come with PAG.

smyth1

dr bob that is incorrect.doesn't matter what type of system you are charging the line coming from the condensor must be 100% condensed into a high pressure high tempurature liquid in order to flash properly through the expansion valve.if you have gas in liquid line it will not flash properly and you will lose evaporator efficiency.i've been doing this stuff for 25 years.

jwillman

Recommedation I got from other knowledgable memebers was to use Ester oil as it is compatable with both R12 and R134 and the old R12 oil. You can't flush the compressor and will likely never get the other lines 100% clean you are likely to have some residual oil left in the system.