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This spring I modified my R1 MANA system to use the Denso/R134 drier, and replaced my original leaking condenser with a larger modern design coil.
The low and high pressure values are OK for a R12 designed system.
Now the A/C cools better than it had new, when moving.
Since my e-shaft has an under-drive pulley, it never has cooled that well at idle. But once the rpms increase over 2000 it does great.
My pressure switch went bad about 10 years ago and it was replaced with a Miata switch.
But lately in Houston high summer air temps, high 90s+, I have a new A/C problem. It does not happen in the 80s or cooler.
When at a long stop light, with the interior fan on high, and the engine cooling fans on speed 3, and rpms at 1500; the pressure switch turns off the compressor.
It then takes a long time for the switch to turn back on do parts finally cooling down. I bypassed the switch and the A/C runs like before it switched off.
The fluid is full with no bubbles in the site glass.
Could the high air temperatures and resulting higher engine components temperatures be causing too high of a pressure for the pressure switch?
But lately in Houston high summer air temps, high 90s+, I have a new A/C problem. It does not happen in the 80s or cooler.
When at a long stop light, with the interior fan on high, and the engine cooling fans on speed 3, and rpms at 1500; the pressure switch turns off the compressor.
It then takes a long time for the switch to turn back on do parts finally cooling down. I bypassed the switch and the A/C runs like before it switched off.
The fluid is full with no bubbles in the site glass.
Could the high air temperatures and resulting higher engine components temperatures be causing too high of a pressure for the pressure switch?
Are you using R12 or R134A? And do you know what the specs were on that replacement Miata pressure switch you installed? (i.e., low & high pressure thresholds that the switch is supposed to open at)
R134A pressures run significantly higher than R12, especially when the car is sitting still and heat soaked with hot & humid ambient conditions described. So my best guess is the switch is doing what it was designed to do - cut off the compressor to protect it when the pressures get too high (or too low, but that's not your problem here). Best thing to do is hook up a manifold gauge set, run your A/C and try to replicate the heat soaked conditions in your driveway, and report back what high & low side pressures you're seeing when the compressor cuts off.
Using a propane base refrigerant from Envirosafe. Low pressure about 50 with high pressure about 195. Cools good but has a lot of bubbles in site window.
Low pressure about 52 with high pressure about 205. No bubbles in site window, this is what I am using.
Specs for an equivalent from 4 Seasons say off below 30 and above 390.
Chuck, it sounds like you have a misbehaving pressure switch. I checked the pressure chart for that refrigerant and those numbers are within spec for +95° and over 40% humidity. If you charge it to where the bubbles disappear and it froths for a second or two when you shut the compressor off then clears chances are your refrigerant amount is correct. I know you said the cooling fans are on the high level but what is your condenser position? Do you have a front mount? Also, I would hook my gauges back up and take it to 2k RPMs for another reading. The FMS calls this a performance test. The low number should drop and the high number should increase. I would imagine it’s one of two things since it seems to work in cooler temperatures, and your refrigerant amount seems ok. Lack of airflow to the condenser not being able to exchange the heat or your compressor is getting tired. The high side seems a little low for the high ambient temperature but with your Underdrive pulley this would become pretty apparent as you stated. I would put the regular pulley back on but that’s just me. Record those pressures @ 2k rpm and get back to us.
I have a Blitz front mount which does not block the front inlet as it is set back 5" which also allows air flow around the sides of the IC.
I will get my gauges out, drive the car until it is heat soaked, and do the failure test again.
Houston air temp 97F, just came back from a get the engine compartment hot and parked the car with AC on. The AC turned itself off and the pressures were weird.
Low side above 350+ psi and high side 195 psi. Normally with a cold engine both pressures are around 120 psi.
WTF?
If the suction line has fluid trapped in it, exposure to high temps could easily raise the pressure this high. Some expansion valves will check reverse flow, effectively closing the suction line back to the compressor. Assuming the system isn’t severely overcharged, the pressure will be directly related to the temperature in each isolated part of the system.
Let temps settle overnight and check them once everything has normalized At ambient temp and compare it against the pressure vs temp chart for the es refrigerant.
I had a similar problem , my dryer was clogged.
the ac was working well when the car was moving but it would raise the pressure on idle and very low speeds and it wasn't cold enough. Something worth looking into. The dryers are pretty cheap
That determines that your pressure switch is working, and that’s mounted on the high side line. Chances are the high side was up there too but bled down after it tripped out. ( if the pressure got high enough some refrigerant could’ve came out of the blow off valve on the compressor ) If the system is not overcharged with Refrigerant which is common for this issue, and you’re sure you don’t have air in the system, you have a blockage. Could be the expansion valve plugged up but chances are it’s the new condenser / dryer you installed. I agree I would let it cool down overnight and I bet it will return to the normal pressure equalization. You need to catch it with the gauges hooked up as it heat soaks. Hook them up on a cold car and then start it and watch what the pressures do as it heat soaks. If the high side climbs before it trips out you know it’s the expansion valve, if the low climbs first, you have a blockage in the condenser/dryer and or line.
Let's see if this clears up the facts better. New condenser coil and new drier.
The low side pressure gauge goes up to 120 psi then has a black zone after that saying RETARD and where it ends says 350 psi.
When the system is cool or warm, both gauges will read about 120 psi with the AC not working.
Just went to run tests with the engine compartment only being warm.
After about 10 minutes engine warm up with outside air temp now 98F. Engine cooling fans on speed 1.
2000 rpm: 62 psi low, 250 psi high
1300 rpm high idle: 85 psi low, 200 psi high
Turned off the AC and the low pressure gauge pegged the needle stop which is highest it can go,
Removed the gauge hoses from the AC system, and when I open the pressure release valve to vent the freon to ATM much oil came out.
Could the mechanic who rebuilt my system put in too much oil?
First thing I'd do is evacuate all of that Envirosafe propane based crap out of the system and replace it with a proper refrigerant. So if you still have the system lubricated with a mineral based lube, go back to R12; if you flushed & replaced the lube with a PAG based lube, charge it with R134A. If you used an Ester based lube, you have the choice of using either R12 or R134A, as Ester lube works in both. Your refrigerant is why your pressures are so wacky - you should never see pressures as high as you're seeing on the low side; normal low side pressures for R12 are in the neighborhood of 22~28psi, and for R134A the range is 22~35psi
But to your question about too much oil, yes too much oil in the system will bugger things up too. As will allowing ANY air/moisture into the system while servicing - for example, if your mechanic didn't bother to bleed the charging hose of air prior to charging, it can introduce air/moisture into the system.
The low side shouldn’t be much over 50-52 psi at that temp @2k rpm. When you rev it up from 1300 to 2k it should drop the low side and raise the high side. 85psi on the low side is way high with only 200 on the high @ 1300 rpm. There’s not enough differential . That’s a characteristic of a a tired compressor or an overcharged system. They could’ve overfilled the oil in the system but generally that bogs down the compressor / engine and limits the refrigerant causing less than desirable vent differential temperatures. There’s a variance on oil capacity that differs in the FMS depending upon where you look it up. But I’ve always found that 6 ounces on a empty clean system works perfect and that’s about middle of the road for total oil volume. You could check with whoever did it and see how much oil they put in the system. Maybe there can tell you how many oz’s they filled too. Sometimes new parts are faulty. An easy test would be to remove some of the refrigerant and see if the low side falls while the high side remains in margin.
My experience for pressures have differed from Pete’s. Especially when the ambient temperature is up as high as you’ve quoted. But I agree you’re way high on the low side. Here is the technical data right out of the book. The second picture is a r134 chart which will run higher than r12. If it were me, I would be going back to r-12. No reason to reinvent the wheel.
My experience for pressures have differed from Pete’s. Especially when the ambient temperature is up as high as you’ve quoted. But I agree you’re way high on the low side. Here is the technical data right out of the book. The second picture is a r134 chart which will run higher than r12. If it were me, I would be going back to r-12. No reason to reinvent the wheel.
~ GW
I just pulled those pressure figures from the FSM procedure for checking A/C system pressures on the FD - see excerpt below for the specific details on test conditions.
I just pulled those pressure figures from the FSM procedure for checking A/C system pressures on the FD - see excerpt below for the specific details on test conditions.
I’ve seen that one. During the high rpm test ( 2k performance test ) that’s the numbers I generally see with r12. When at idle it does go up a touch closer to the mid/ high 30s on the low side and down a touch on the high. If I remember correctly, you’re running an under drive main pulley on your fd with the ac system you retrofitted? What are your numbers at idle? They may be closer to what chuck should see.
I’ve seen that one. During the high rpm test ( 2k performance test ) that’s the numbers I generally see with r12. When at idle it does go up a touch closer to the mid/ high 30s on the low side and down a touch on the high. If I remember correctly, you’re running an under drive main pulley on your fd with the ac system you retrofitted? What are your numbers at idle? They may be closer to what chuck should see.
~ GW
I'm running R134A in a resto-modded stock Denso system (i.e., aftermarket condenser, dryer & custom lines), so the refrigerant capacity is not exactly the same as OEM, but my L/H pressure figures as measured exactly per the FSM procedure (@1500 RPM and with ambient temps between 86~95*F), I'll see low side at 15~30psi and high side at 190~210psi. If I bump up my RPMs closer to 2000, to account for my under drive e-shaft pulley, the L/H pressures will both move up a bit and fall right within the FSM ranges. Cabin stays plenty cool, and vent outlet air temps typically range between 40~45*F, depending on how hot it is out.
One thing to note is that when you do this test in your driveway is it doesn't take long to start heat soaking the condenser, since the car isn't moving and getting all the airflow it needs. When that happens, the pressures on both sides will also go up, and you'll notice less pressure differential between them. So you'll need to do your measurements quickly before things start to heat soak, or have a hose handy to spritz some water on the condenser to help cool it down.
One thing to note is that when you do this test in your driveway is it doesn't take long to start heat soaking the condenser, since the car isn't moving and getting all the airflow it needs. When that happens, the pressures on both sides will also go up, and you'll notice less pressure differential between them. So you'll need to do your measurements quickly before things start to heat soak, or have a hose handy to spritz some water on the condenser to help cool it down.
I use a big floor fan pointed at the nose to keep them cool. Lack of airflow really does impact them greatly. Gotta make sure “both” cooling fans are operating also. Either way he shouldn’t see such a high low side @ idle. I’ve never used the propane base stuff, so I can’t speak to the characteristics of that refrigerant. Like you said, he needs to check it quickly why it’s cold and see what happens.
As stated earlier, my AC is using a propane base refrigerant and would cut off after idling a long time at a red light when the outside air temps where in the 95s+ range.
This is with the AC/heater fan on speed 4 and the engine radiator fan on speed 3. I use the FAN SWITCH MOD which allows either engine shut down cool off of 10 minutes or run at 1 speed higher than normal.
I dump my refrigerant to reduce the amount of oil due to symptoms of having too much oil. Then when refilling the system, filled up to 100 psi not running as compared to the previous 120 psi amount not running.
This reduces operating pressure by about 5-10 psi. The 120 psi is a fully loaded system without any site window bubbles.
Went for a test drive today and the AC still works fine. But today's air temp is around 90F and the long idle test failure did not happen. Either not hot enough or the changes worked.
Will do a test the next really hot day.
Chuck, that’s a very peculiar way to charge an AC system. I’m assuming you did this to remove what you think was excessive oil. Then charged it slightly less than it was before. If you just wing it by charging a static pressure for the initial charge you’re gonna have a bad time. It should be charged by weight, or until the sight Glass bubbles JUST stop. I’ve seen some people charge to the lowest vent outlet temp, but I only do that on a system with unknown capacity. If you’re seeing completely clear refrigerant through the site glass and it does not froth when you turn the compressor off, you have to much refrigerant. Chances are this was the case. It still may be an excessive amount, but it looks like you’re on the right track.
Chuck, that’s a very peculiar way to charge an AC system. I’m assuming you did this to remove what you think was excessive oil. Then charged it slightly less than it was before. If you just wing it by charging a static pressure for the initial charge you’re gonna have a bad time. It should be charged by weight, or until the sight Glass bubbles JUST stop. I’ve seen some people charge to the lowest vent outlet temp, but I only do that on a system with unknown capacity. If you’re seeing completely clear refrigerant through the site glass and it does not froth when you turn the compressor off, you have to much refrigerant. Chances are this was the case. It still may be an excessive amount, but it looks like you’re on the right track.
It is harder to do it by weight with a propane base refrigerant. I been using Eviro-Safe for over 20 years after R12 became hard to find and expensive.
On the can it states: 6 oz is equivalent to 16 oz HFC-134a or 18 oz of CFC HFC-12. The Arctic Air recharge special fluid contains 4 oz or 12 oz R12..
After you charge a system and let the L & H pressure equalize, and cool: you end up with a base pressure.
You can then recharge or top up by this pressure without actually running the AC.
21.18 oz is what the manual states as full charge in the 1993 manual for R12.
Just a different method.
This week I replaced my Shane Racing one piece under-drive e-shaft pulley with the stock pulley. Had to also buy a stock A/C and P.S. belt from MazdaTrix.
Going from the smaller to larger pulley increased the speed of the AC compressor by about 20%.
This has helped idle AC cooling.
Thus the problems with my AC rebuild were: too much oil, refrigerant overcharge, and low rpms.
08-02-22, 10:27 PM
