The Forgotten On

I reconverted my 89 to R134 after it suffered a bad conversion which killed the original compressor and sent metal shrapnel throughout the entire system.

It was shocking how bad a professional shop can do something and bodge it so badly. They didn't even replace the o rings with the correct green ones.

Don't take chances, and do the system over to make sure it is right and running perfectly.

I highly recommend you flush they system completely and "flush" the compressor by draining its oil and filling it up with new oil and cranking it over until the oil comes out clean and you can be sure what was in there is gone.

Most people that have not flushed the system between multiple fills and services end up with way too much oil in the system which kills performance massively. The system needs very little oil truthfully.

You should also have replaced both expansion valves. They are mechanical parts and will fail. But they are very inexpensive and relatively easy to do.

And for anyone wondering Denso Rebuild are held to a much higher tolerance than their new compressors are ironically. Most cores are either recycled for scrap or have a few pieces harvested.

Best of luck with whatever you decide to do going forward.

merchauser

here are the WSM GTS pages for R134a. am I reading something wrong regarding low side? these readings are supposedly after 10 min. @ 2k?

j.kenzie@sbcglobal.net

Hi Merch,
You seem to be reading the diagrams correctly. They are specifying 27 F. evap temps on average, or even less. That suggests to me that they have their freeze switch set much lower than freezing temp of 32 F. That could work in low humidity regions, but elsewhere you will get a block of ice in your evaporator, blocking air flow, and essentially no cooling. Not sure I understand their reasoning, but it does give great performance except for icing. This is not how I charge my a/c, and I'm guessing not how most others would either. Remember, if you follow their charge by weight spec, and your system is working properly, this is how Porsche says the pressures should run. Puzzling.
Good luck,
Dave

Schocki

Here is a picture of my GTS at idle, ambient temperature 73 F. This was taken after a complete system rebuilt. New compressor, new drier, new evaporator (leak), all new o-rings. Verve console at full cold showed about 35 F.

j.kenzie@sbcglobal.net

Thanks, Schocki. Those are exactly the kind of numbers I would expect. Don't know where the WSM numbers come from.
Dave

merchauser

^^^thanks for that picture. my readings are similar, but I am not getting the correct temps at the center vent. with current ambient at 78, at idle 22 on low side and 175 on high. at speed, on the highway, dash temps are a little better and are around 45'. does that suggest an issue with my condenser? fans are new and working properly, but at speed with higher flow across condenser, temps are a little lower, but not where they should be. .

not certain if I am using my temp gun properly? should I be getting the WSM dash vent temps at the plastic vents? (there I get 58') if I shove the gun as far in as possible and the laser hits the evaporator, I get the 45' reading. in the driveway, dash temps are about 52' (at the evap) does this point towards main expansion valve? or could I have some air in the system?

I won't be happy until I start from scratch, but no time for a week or so. will order a new expansion valve regardless, but would like to know what is actually wrong so I only do this one more time.

Speedtoys

Are you comparing his idle numbers with the high rpm WSM numbers?

Idle data is marginally helpful, but may be all he has to show..which is fair, but is still trying to recharge AC via Goggle.

Speedtoys

It started here, and then dragging it out over a # of days...you cant fix most AC repairs later in the process after missing the clean execution of an earlier step. Although it's been a good discussion of what's going on in a working system.

" might have made a mistake in having a good intentioned neighbor help with the recharge"

Go back before that step, and do it right. You probly got air in the system, or undercharged, or overcharged...you're spending a lot of emotional energy guessing. Gauges are only important to observe after everything else is done right.

Pull a vac for a few hours. See if it holds overnight _exactly_, and fill it with the documented amount.

Check system temperatures -at- 2200 rpm, not idle. Do not check google.

If you have concerns about a good fill, stop your own work at the vac pull and check, and have a shop fill it with a machine..they operate by weight...take the list I posted they should be familiar with 134 conversion math too..their machine will confirm vacuum and seal, and fill it.

dr bob

The temps and pressures in the tables are for the compressor running fast enough to emulate road conditions, so off-idle at minimum. The standard I learned was at least 1500 RPM, and I usually set the throttle higher than that. 2500 is good for me. The fans running gives enough airflow to look like driving it, so no worries there.



The IR temp gun is not a good tool for measuring air temps. Grab a few side-read digital thermometers from the kitchen good aisle at WalMart or your favorite grocery store. One goes in the center vent, one measures ambient, one measures cabin temp on the floor on the passenger side. You can get away with just the vent temp initially, but will want the other two if you get in to serious system performance measurements.

The IR "gun" has a beam bloom that grows as the distance to measured surface grows. The laser is deceiving, as it really identifies the cone center rather than what looks like a single test point. The tool needs to be pretty close to get a valid small-area reading. It also depends on a hard surface as a source of heat, and can't read gas or vapor temps accurately.



Truth be told, expansion valves very seldom actually fail in service. I wouldn't break the system down just for that, at least not yet.

The later cars use an expansion valve(s) common to other European cars that have Behr refrigeration, so they are not at all expensive in the big picture. Do get genuine Behr if you can. Failure happens when the system is left open and moisture causes corrosion in the precision needle and orifices. Or they get plugged with debris in the system, jellied lubricant from a system with the wrong oil, etc. They usually come out for o-ring replacement as part of a R-134a conversion, so it's easy and convenient to just put new ones in. The valves supplied today have the correct curves for R-134a too; Not a big deal as the R12 valves will still work fine with R-134a if they are clean.

If you do decide to replace, do the rear valve at the same time. The rear system includes o-rings where the rear piping connects to main piping, the bulkhead fittings where the lines come through the floor under the passenger seat, the solenoid valve also under the passenger seat, plus the fittings at the expansion valve ends at the rear evaporator. All would be replaced is part of any reseal or conversion effort.

-----

With the freeze switch jumpered, I regularly see under 20º F center vent temps at steady cruise and 90º ambient conditions in our desert climate. Folks who live in more humid climates will not see those temperatures because of the amount of heat removal needed to condense water vapor. Your better weather app will list the "dew point" temperature for the local relative humidity. Most car systems are hard pressed to drop center vent temps much lower than that number especially on hot days. The system spends a lot of capacity on drying the cabin air by condensing water from it, and may run out of capacity before it can cool the air much beyond that point. Use this reminder to manage your system performance expectations; With the right thermometer(s), you may find that the system is performing just fine for the current conditions.

j.kenzie@sbcglobal.net

Good point. I would expect the 2000 RPM numbers to drop and rise respectively, compared to idle, until the evaporator cools to 32 F., then as the compressor shuts off, they should return to similar to idle, then the compressor turns on again. If that's what the WSM is trying to describe, with shutoff by freeze switch, then the numbers would oscillate back and forth each time the freeze switch cycles. I've read somewhere that a charge that causes cycling of the freeze switch is considered a great way to get the coolest vent temps. Maybe that's what the WSM is doing. This would require a slightly lower charge than not cycling the freeze switch. IDK.
Thanks,
Dave

928FIXER

In 25 years of ownership of a dual a/c 928,I too have found that the use of the rear unit decreases the efficiency of the front unit,increases discharge temp of the front unit and raises the operating head pressure.I too also find it of little actual use,except to increase the amount of refrigerant that I have to purchase to keep it full.IF I use the 928 on a sunny summer day(unlikely as my 400 dollar beater blazer blows ice cubes in a matter of a minute),I do use the rearr just to move more air thru evaporators to bring interior temps below 90 degrees and then switch off the rear so that the air discharge temp drops in to the mid 30's at the center vent.
This vehicle,like a lot of Porsche vehicles designed in the 70's have a/c that is really just an afterthought.Any one here ever see the head pressure on a air cooled 70's,80's 911 sitting still on a 90 degree day ?Frightening! 350 maybe 400 Lbs on the Hi side gauge.

j.kenzie@sbcglobal.net

Hi Fixer,
You are right on both counts. The limiting factor is usually condenser capacity, which involves surface area, design and air flow exchange. The evaporator fan in the 911 is also undersized.
Both problems can be partially addressed with parallel flow heat exchangers.
Good luck,
Dave

merchauser

I thought the AC compressor for 928's is always on, and does not cycle, like most cars?

GregBBRD

No idea where you got that idea....not remotely true.

merchauser

oh boy...… a long time ago, when I noticed mine did not cycle, I asked someone of much greater 928 knowledge and was told the clutch and compressor are always engaged and always on with the dash AC button depressed.

what component is responsible for cycling the compressor?