First Project thread: 88 Cab A/C retrofit on the Cheap

[wwest]

Originally Posted by Arne2

Here's a link to the one I am using as I go through my A/C. The switch and charge port end up in slightly inconvenient location on a Nippondenso, but they look like they will work. Will post a picture once I get the compressor back in the car, if I remember...

Binary Switch w/ R-134a Discharge Port 7/16" - A/C Pressure Switch

Male Trinary Safety Switch For Electric Fans - A/C Pressure Switch

Trinary switch, same firm.

[kuehl]

Originally Posted by COLB

The pump quickly pulled a vacuum to over 30 microns -- which is better than the spec on the box (visible on the blue, low side/suction gauge

OK. Let's assume your equipment is accurate for the moment. But FYI: Analog gauges measure in inches of mercury on the low side. You need an electronic gauge to measure in microns, millibars or pascals. Most analog gauge reference 'ticks' printed on the scale are difficult to discern whether you are at -30 or -29. Depending upon the ambient temperature that can mean the difference between 'boiling' the moisture and not. I've had 3 various brands analog gauges in the past, one said the low side was -30, it was actually -28, with another the needle fell off the pin, and I had a high side gauge that said it was 50 psi higher than actual. Before we commence a vacuum procedure we test the pump and insure the oil is not contaminated with moisture, if there is moisture you have to run the pump through a few cycles to evaporate it out. Fortunately you are starting off with new oil.

Originally Posted by COLB

I cranked up the engine RPM to 2000 for a minute or so, and the pressures dropped quickly. I finished the third can, and part of another, bringing the charge to around 40 ounces -- with the pressure reading hovering between 40-50 while I was adding the refrigerant, then stabilizing at 35 low/blue and 275 high/red when I finished adding and closed the blue valve at the gauges and monitored the system while it ran for about 10 minutes. Part of the issue is that the variables driving the pressure are too varied and unmeasurable to isolate.

Although the "rule of thumb" for R134a is you start with 80-90% (weight) of the original R12 factory charge (47 nominal for your year car), it is a rule of thumb. The final say is Pressures and Temperatures. A high side of 275psi at 82F, if this was at idle, with R134a means you are either overcharged or you have residual air in the system.
The worse case condition for the system is idling. At idling the deck lid condenser see's less air movement. So, the smarter preference is to check the pressures and adjust them for "idle", not at 2000 rpms. The 2000 rpms is a 'check'. Designing or charging a system that can maintain low high side pressures will extend the life of the compressor.

Originally Posted by COLB

Temperature, for example -- the ambient air temperature outside my garage was 82. Two inches above the Condenser grill -- where some sources say you should measure -- would have been considerably higher over a parked, running car -- closer to 100. Further, the recommended ranges vary by up to a hundred PSI depending on source.

It is true if you take ambient air readings over the black top and then over a grass lot adjacent to your work area you can find a wide swing in temperatures. The ambient temperature you want to monitor is near the deck lid condenser, 1 to 2 feet above it is acceptable.

Originally Posted by COLB

My understanding is that R134 operates at a higher pressure than R12, but I don't know what you are driving at -- other than a lower charge of R134 by weight will yield comparable pressures to a full charge of R12.

Up to, say 80F, R134a and R12 have nearly identical P&T's. Moving high on the Temp side of the chart R134a does run at high pressures with stock systems; if you have more condenser it can be considerably less.

Originally Posted by COLB

But overcharge doesn't seem to add up. Perhaps a weak compressor.

"overcharge" can be a catch-all phrase. You can have higher than normal high side pressures if you have air in the system, moisture or trapped gases in the the refrigerant oil. Excessive air raises pressures. Moisture can ice up inside expansion valves. Residual R12 trapped in residual refrigerant oil can throw off gauge readings.
2 common ways residual air or moisture remain in a system are the method or procedure used for evacuation or failure to purge the refrigerant charge line prior to charging (swapping cans is a common point where air gets into the charge line).

Originally Posted by COLB

By the way -- I do appreciate the insights!

Your quite welcome.

[wwest]

Originally Posted by kuehl

OK. Let's assume your equipment is accurate for the moment. But FYI: Analog gauges measure in inches of mercury on the low side. You need an electronic gauge to measure in microns, millibars or pascals. Most analog gauge reference 'ticks' printed on the scale are difficult to discern whether you are at -30 or -29. Depending upon the ambient temperature that can mean the difference between 'boiling' the moisture and not. I've had 3 various brands analog gauges in the past, one said the low side was -30, it was actually -28, with another the needle fell off the pin, and I had a high side gauge that said it was 50 psi higher than actual. Before we commence a vacuum procedure we test the pump and insure the oil is not contaminated with moisture, if there is moisture you have to run the pump through a few cycles to evaporate it out. Fortunately you are starting off with new oil.


Although the "rule of thumb" for R134a is you start with 80-90% (weight) of the original R12 factory charge (47 nominal for your year car), it is a rule of thumb. The final say is Pressures and Temperatures. A high side of 275psi at 82F, if this was at idle, with R134a means you are either overcharged or you have residual air in the system.


The worse case condition for the system is idling. At idling the deck lid condenser see's less air movement. So, the smarter preference is to check the pressures and adjust them for "idle", not at 2000 rpms. The 2000 rpms is a 'check'. Designing or charging a system that can maintain low high side pressures will extend the life of the compressor.

A bit of a conundrum...***


It is true if you take ambient air readings over the black top and then over a grass lot adjacent to your work area you can find a wide swing in temperatures. The ambient temperature you want to monitor is near the deck lid condenser, 1 to 2 feet above it is acceptable.


Up to, say 80F, R134a and R12 have nearly identical P&T's. Moving high on the Temp side of the chart R134a does run at high pressures with stock systems; if you have more condenser it can be considerably less.

"overcharge" can be a catch-all phrase. You can have higher than normal high side pressures if you have air in the system, moisture or trapped gases in the the refrigerant oil. Excessive air raises pressures. Moisture can ice up inside expansion valves. Residual R12 trapped in residual refrigerant oil can throw off gauge readings.
2 common ways residual air or moisture remain in a system are the method or procedure used for evacuation or failure to purge the refrigerant charge line prior to charging (swapping cans is a common point where air gets into the charge line).


Your quite welcome.

*** When recharging an "ordinary" car using the OAT and low side pressure the condensor pressure is moreso "in check" with OAT airflow and little radient input.

Not so much so with our cars having a condensor hanging over a HOT engine. Start with a COLD engine? Otherwise 2000 RPM/no load sounds like a good idea, keep the condensor closer to the OAT reference.

[wwest]

Originally Posted by COLB

I'm not sure what you are asking me.

Yes -- the high side was 275 at that point. At other points of measurement it was lower and higher, while the outside temp was relatively constant.

Part of the issue is that the variables driving the pressure are too varied and unmeasurable to isolate.

Temperature, for example -- the ambient air temperature outside my garage was 82. Two inches above the Condenser grill -- where some sources say you should measure -- would have been considerably higher over a parked, running car -- closer to 100. Further, the recommended ranges vary by up to a hundred PSI depending on source.

My understanding is that R134 operates at a higher pressure than R12, but I don't know what you are driving at -- other than a lower charge of R134 by weight will yield comparable pressures to a full charge of R12.

When I said it was a "full charge" -- that was based on the pressures we measured when we initially filled the system (45 low side), and the total weight added: 40 ounces (85% of the owner's manual capacity for R12).

Given that, high side pressures have generally remained at the upper ends of the range, while low side pressures have been 20-30, depending on how warm the car was at the time -- not the outside temperature.

Vent temps have stayed satisfactory throughout.

What means this..?

It was/is my understanding that when using the OAT and low side pressure to determine (re)charge level the engine RPM must be elevated, and there must be "free-flow" through the evaporator. In other words enough HOT airflow to consume ALL of the system's cooling capacity. I also seem to remember that should the compressor cycle off then the references are not valid.

So if there is a clear diagnosis here, other than Porsche A/C systems are Rube Goldberg machines, I don't know enough to make it.

But overcharge doesn't seem to add up. Perhaps a weak compressor -- but that would presumably be felt in warm vent temps.

My way forward here is to enjoy the cool air and see what happens -- I believe the downside risk is pretty low, as the worst thing that can happen is damaging a 25 year old compressor that I would rebuild or replace anyway if I later choose to go with the full Griffith package in the future.

By the way -- I do appreciate the insights!

Kuehl?

[kuehl]

Gee Wwest, you gotta be be bored to be up so early quoting in quotes.
Me? I'm just passing time while a program tries to resurrect a corrupted VM file.
But, to answer you:

Vent temps have stayed satisfactory throughout. What means this..?
It means he's happy with what he got.

"The worse case condition for the system is idling. At idling the deck lid condenser see's less air movement. So, the smarter preference is to check the pressures and adjust them for "idle", not at 2000 rpms. The 2000 rpms is a 'check'. Designing or charging a system that can maintain low high side pressures will extend the life of the compressor." A bit of a conundrum...***
'conundrum' = There is no riddle or pun.

"It was/is my understanding that when using the OAT (=outside air temp) and low side pressure to determine (re)charge level the engine RPM must be elevated, and there must be "free-flow" through the evaporator. In other words enough HOT airflow to consume ALL of the system's cooling capacity. I also seem to remember that should the compressor cycle off then the references are not valid."
Simply using a low side pressure reading will not tell you the level of charge because you could have a very cold core temperature, blockage in the TEV (moisture typically) or other "stuff" going on.
Raising the rpm has always been a standard procedure in Porsche's procedure and some other mfg's, however its a check. Systems are always check with the engine running, windows closed, AC on, and preferably no outside air mix (mute point because these models are recirculate only), and its common sense you don't take readings when the clutch is not engaged; at idle is the gold standard for worse case conditions, such as sitting in traffic.

Here is a very good basic reference source Snap-On, Technical Training Systems, Basic Service Training Series, Book ACT279B or later editions, or, Mobile Air Conditioning Society (MACS), SAE International, Automotive Air Conditioning Refrigerant Service Guide or their Automobile Air Conditioning System Theory, Diagnosis and Service Procedures book or later editions; MACS use to produce videos as well.

[Discseven]

Nice post Brad!

I went for the"Ice Man Project" some years ago --- pulled all the lines and put the "ice man" hoses in and the additional condensor in the driver's rear wheel well.

... never did get to the 33 degree temps that were promoted.

I'm going to revisit my system based on your post!