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

[Gordo2]

If it's cool - you're cool.

Seriously though, getting the right weight of refrigerant into the system is what you should focus on. Based on the amount you added (post evac), the vent temps you are reading and the pressure readings you are measuring - I think you are good to go.

Gordo

[wwest]

[QUOTE=COLB;7493561]I'll see if Gordo will weigh in on the Rib recipe!

So today I went out to run errands -- involving about 60 miles of driving in the heat of the day -- low to mid 80s with high humidity.

After driving about 20 miles to get the car up to operating temperature, I checked the A/C to see if my charge is holding. I hooked up the gauges and started the car -- with the A/C off -- and got these readings: 40 psi on the low side, and 200 on the high side.



Thanks, you just inadvertently proved a point for me. I have stated a theory based on the possibility that the pressure on the high side might remain elevated for a substantive period of time post engine shutdown, or in your case post compressor running.

Of special interest, I suspect, to Kuehl and 1 or 2 others.

Could you possibly give us an estimate of the elapsed time betweem when the compressor was last shut down and your measurements, picture?

Everything I see would indicate that you overcharged the system initially and it remains so.

[COLB]

The low side pressure sitting around 25-30 does not indicate an overcharge to me.

If anything, it sounds like a partial blockage in the evaporator -- but the system is cooling quite well.

[david482]

I am looking to do the same thing and was wondering where you got the O ring kit. Are these the o rings on each end of each hose? Just want to make sure I get the right seals in the right place. I have just finished rebuilding my Nippon compressor.

[COLB]

Originally Posted by david482

I am looking to do the same thing and was wondering where you got the O ring kit. Are these the o rings on each end of each hose? Just want to make sure I get the right seals in the right place. I have just finished rebuilding my Nippon compressor.

I got the Santech kit from another site. I try to buy my parts from our host, but the kit they offered didn't have the conversion fittings. The O-rings are the same, though -- the green ones are for R134.

You put an o-ring on each end of the hose, then put a little oil on them.

I looked at rebuilding my compressor -- and if the current bandaid fix fails, I will probably do that. Any major challenges?

[wally509]

I'm watching this thread closely as I need to address my A/C system in my '87 also. It's really nice and refreshing that it hasn't blown up into 10 pages of bickering about barrier hoses and trinary switches

Having said all that...

Where does a DIY'er go from here? The posts I've read seem to indicate doing this will last at best a year or so before needing a topping up. Can a DIY'er with a set of gages do that?

Seems to me if the amount of refrigerant needed goes by weight, he would have to blow all the leftover into the atmosphere, hook up his vacuum pump for a hour, and then put in the 40oz of 134a again...given he doesn't have a recovery machine. If this got to be something that needed to be done a couple times per season that could get kinda long in the tooth. Please correct me if I'm wrong.

Thanks,
Wally

[david482]

Basically I am planning to do what you did, I think my compressor failed about 10 years ago. I just got the seal kit and overall it was pretty easy especially using the tech article. The only challenge was I decided to split the case which should have been easy it just took more force then I expected and the pistons came out. The challenge was just getting everything back on the spindle, I finally put the pistons on while holding the spindle then using a string to wrap around the pistons to hold them on while I got it back in. The other issue was making sure you put the washers on the top and bottom of the spindle before you put the pistons back on.

[wwest]

Originally Posted by COLB

The low side pressure sitting around 25-30 does not indicate an overcharge to me.

If anything, it sounds like a partial blockage in the evaporator -- but the system is cooling quite well.

The low side pressure is primarily a function of the compressor itself, how close might it be able to drive the low side toward/to a vacuum, and secondarily how "closed" can the TXV get in the process of throttling the refrigerant flow into (and thus out of) the evaporator volume.

An earlier test you did indicates that your TXV might well be able to go FULLY closed. You measured a significant level of high pressure quite some time POST having the compressor run/running. Basically that CAN NOT happen absent the evaporator having been previously FULLY CHILLED to the point that the TXV was almost, or even fully closed(well below freezing..??). Some sort of blockage against flow might also do that but since your system seems to operate normally otherwise that appears to not be the case.

Install a pressure switch to control the compressor and you may well find both the high side and low side pressures more within specification.

And...Given that LOW vent temperature you saw then your thermostatic compressor control, cycling, switch may be failed or malajusted. That would result in the compressor remaining enaged well beyond the desired evaporator core temperature....low side pressures PULLED much closer to a vacuum.

[wwest]

Originally Posted by wally509

I'm watching this thread closely as I need to address my A/C system in my '87 also. It's really nice and refreshing that it hasn't blown up into 10 pages of bickering about barrier hoses and trinary switches

Having said all that...

Where does a DIY'er go from here? The posts I've read seem to indicate doing this will last at best a year or so before needing a topping up. Can a DIY'er with a set of gages do that?

Some years ago I decided to convert my 88 Carrera to R-134a mostly because I wished to lower the cost of refilling, having a shop refill the system at least every 2nd year. I purchased the refill kit that included a low side pressure guage. $20 DIY refills vs certified A/C technician/shop, $$$.

Seems to me if the amount of refrigerant needed goes by weight, he would have to blow all the leftover into the atmosphere, hook up his vacuum pump for a hour, and then put in the 40oz of 134a again...given he doesn't have a recovery machine. If this got to be something that needed to be done a couple times per season that could get kinda long in the tooth. Please correct me if I'm wrong.

Thanks,
Wally

"..goes by weight.." But the "weight", volume of R-134a to be filled, can be determined by system functionality as measured by the minimum low side pressure with the A/C operating WOT and the engine at ~2000 RPM.

Basically the cabin as open to atmosphere as is possible, max cooling & blower speed, the system can be filled using the proper(chart) low side pressure in accordance with OAT.
For instance, at 84F OAT and the above circumstance, the system should be charged until the low side pressure is between 45 and 53 PSI. Note: the A/C "load" must be high enough that the compressor never cycles off. This procedure cannot be used with OAT below 65F.

"..couple times/season.." Mine, R-12, was on the average every second year. R-134a should have been worse but the binary switch may have alleviated the leakage entirely, only time and use will tell.

[kuehl]

Originally Posted by COLB

I put in a total of 3 oz. Two directly into the compressor, and one can of R134 that had an ounce of oil in it.

I was hesitant to add more, as I got none out of the compressor, and only a little bit out of the front condenser.

You want 5-6 oz oil for the total system; you don't want to lock up a compressor and go through the procedure to clean the entire system.

[kuehl]

[QUOTE=wwest;7493885]

Originally Posted by COLB

After driving about 20 miles to get the car up to operating temperature, I checked the A/C to see if my charge is holding. I hooked up the gauges and started the car -- with the A/C off -- and got these readings: 40 psi on the low side, and 200 on the high side.

Thanks, you just inadvertently proved a point for me. I have stated a theory based on the possibility that the pressure on the high side might remain elevated for a substantive period of time post engine shutdown, or in your case post compressor running.

Actually what his observation shows is the after the engine is off the high side did not exceed normal operating pressures. And if he grabs a half rack of ribs the low and high side will equalize before he finishes the ribs.

[wwest]

Not going "there".

[kuehl]

"So, at 83F, with a full charge, your high side is 275?" Think! (compare the pressures of r12 and r134a at 83F)

[SilberUrS6]

Originally Posted by kuehl

"So, at 83F, with a full charge, your high side is 275?" Think! (compare the pressures of r12 and r134a at 83F)

Asking wwest to think is like asking a dog to do trigonometry.

[kuehl]

wwest got sent home, the question was to colb,
wwest set up his own forum to argue with me, lol.

[COLB]

Originally Posted by kuehl

wwest got sent home, the question was to colb,
wwest set up his own forum to argue with me, lol.

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.

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!

[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.

EXACTY, very well stated.

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.

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!

The only way I know to determine proper charge kevel is in "fixed", ridgid, conditions.

That's with the system working as hard as it can to cool the airflow passing through the evaporator and at a defined OAT. The cars cabin is fully open to atmospheric OAT, blower = High, cooling = MAX, Compressor NEVER cycles off during the test period. In these defined conditions at say, 85F, the R-134a low side pressure should be
46-55 PSI. PSI at 55 would very close to an overcharge.

[Gordo2]

Here's a good video that shows how you can troubleshoot an A/C system using gauges:
Using AC Pressure Gauges To Fix Car AC Problems - YouTube

Speaking with my bro some more - he primarily uses the gauges to ensure he isn't getting excessive pressures and to confirm that the system doesn't have any blockages or other problems.

As a rule of thumb - he charges based on weight and ensures he isn't getting freak readings with the gauges. Excessively odd readings as per the video - he troubleshoots, normally looking for blockages.

Another rule of thumb - he charges until the low side line feels cold to the touch and sweats.

I think you hit on Kuehl's question - variation in R12 and R134 operating pressures and the difference atmospherics and operations status make a significant difference on pressures.

The bottom line in my mind is this - as you mention there are a boatload of variables, worse yet, it's impossible to represent operating conditions while parked... The pressure/temp tables and weight get you into the ballpark while parked (and keep you from going overboard.

Gordo

[wwest]

Good video tutorial, maybe even excellent.

[Arne2]

Originally Posted by COLB

Agreed on the binary pressure switch. I would have liked to add one, but I couldn't find the right part.

Our host doesn't appear to offer one, and links to the other vendors I found are dead. One vendor sells one attached to a new compressor-to-condenser hose, but that was $160 plus shipping. which put me well over budget.

If I can find a plug N play switch as you describe I will install it next time I have to get into the system.

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