A/C High Side Pressure-Generic Question

ktmlew · July 22, 2012

Working in an indy shop. I do mostly drivability and AC work. Everything from Domestic/Jap to BMW/Saab/VW, etc. Why would say an early 2000 Honda Accord blow 32* vent temp at idle, presures at LS 20-40 & cycling, high-side at 170 while a 2000 Malibu blows 40*, 30/250 same day within an hour of each other? Temps and humidity accounted for.

My experience has been the Jap A/C systems are much more efficent...but why?

Dodge mini-vans are scary, 30/325 at 38-40* are common. Spraying condensor with water WILL drop HS by 40-70*. Are these condensors that prone to clogging or do the fans suck <or not well enough>?

Chev pick-ups 00-07. 30/275-325 @ 42*. Water mist drops HS 50-70*. I suggested fan clutch is dying. Shut truck off and clutch almost free-wheels. Had 3 this week in two days.

Where do you guys get "nervous" about HS pressure? How reliable is water misting/pressure drop? Any good A/C message boards? I need what I call product knowledge. I know the Honda is working as designed. But not sure about some of the others. They aren't working the way I WANT them to.

ktmlew · July 22, 2012

Ok. Just found this which I suspect is true.

Quote:

http://www.certifiedmastertech.com/wordpress/2010/02/10/car-ac-diagnostics/

Kirk Fletcher says:

October 17, 2011 at 10:49 am

Hi Mark, Just read this light bit of info that you have posted here on A/C Diagnostics. I just wanted to share some of the things that we are currently teaching in AC Clinics the last couple of years. Myself, I am trying to get more guys looking at temps rather than mainly looking at pressures. Too many of the OEM’s are putting the high and low side fittings in the wrong area of the system to correctly diagnosis by pressures. Many of the high side fittings are located in the liquid line so the high side readings can be much lower than what the compressor is really creating.

We are now on our fifth generation of AC Condensers for R134a systems and the flow passages are now as small as .015 of an inch. When you have a condenser inlet side of just less than 8 1/2 square inches of combined flow path (28 tubes x 20 flow paths per tube x .015 per flow path) the inlet manifold tube of the condenser becomes the primary filter for the A/C system. These small passages become partially blocked and become tiny OT’s. We get substantial pressure drops at these partial blockages and we get corresponding temperature drops. We are teaching tech’s to look at inlet and outlet temperatures of the condenser. We only want to see delta’s of 25 to 35 degrees.

Too little temperature drop and we have an air flow problem through the heat exchanger. Too much of a temperature drop and we have massive amounts of partially obstructed refrigerant flow paths. This industry continues to try to back-flush these new style condensers with Sure-Shot cans and Flush-Guns with zero results. This is one of the major reasons replacement Compressors have very short life spans. The Compressor was not Defective; the compressor was Damaged because of incorrect service procedures. I know there is a need to educate the DIY on proper A/C System service and repair but most DIY’s don’t have the proper equipment to be successful even if they have the proper knowledge.

It is hard enough to get the professional repair shop to upgrade their A/C Service equipment to the current generation and to purchase the dedicated flushing machines that are really needed to flush and clean these new systems. The goal isn’t to go through the motions; the goal is to get the system perfectly clean. How can we expect the DIY to get these systems perfectly clean when only the very best professional repair shops currently do? To me, selling an A/C Compressor to a DIY at the parts counter is like handing a loaded handgun to a five year old and expecting positive results.

I am using a Robinair charging station and standard vac/charge to spec procedure. I wouldn't have a problem with 250/275 at idle-1200 rpm but the 300+ seems a bit high? Our weather is currently 90-95* at 60-80% humidity.

Brad Clayton · July 22, 2012

I never get concerned about the high side going too "high". In the old days the blow off valve would release the pressure (around 425 psi.) and nowadays the high side protection switch shuts the system down.

If I put a set a gages on a system and high side climbs like crazy, I just let it go and see what happens. I treat them like an overheating concern. It's much easier to figure them out if you let them actually "overheat".

I wouldn't be concerned about 300 psi. on a hot day. It takes a lot of air flow at idle in a shop to keep the pressure lower.

ktmlew · July 22, 2012

Cool...

Ya see what I did there...

Posted Image

That is both good and bad news. Plugged condensors and failing fan clutches make me more money. But I hate techs that over-sell repairs. All I want is zero comebacks. If you bat 1K, they will "come back" for other repairs.

Brad Clayton · July 22, 2012

Ah, don't take my word for it, if the car is running as designed and the register temp is acceptable then I consider the vehicle not broke. I would bet that if you had a big ass fan in front of the vehicle the high side pressures would be much lower than what you have been experiencing. There again the car is going to talk to you if there is a problem.

Honda is a strange vehicle in the nature of.....well, I wouldn't use it as a yard stick to measure up how well other makes and models are working.

jimmy57 · July 22, 2012

Then throw in some variable displacement compressors to the mix and the gauge pressure readings take on another measure of weirdness other than that of varying temps, humidity, engine speed, and fan speed.

I do have a concern. A/C diagnosis is not done with an idling engine. I don't recall ever looking up the test conditions for the vent temp performance check for something I was repairing and having it list normal engine idle speed as the test condition. Your GM pickup with an engine fan is not likely to do "normal" high side with low fan performance @ idle. The Malibu likely has a VD compressor.

Honda Seems to have figured out that $1.00's worth of extra condenser is worth a lot in idle cooling perf. The Honda may be a VD compressor too.

VD compressors are a good thing. They offer better idling perfromance with less HP loss at higher engine speed and when vehicle cools down. The first one confused me. It looked like a garden variety compressor and it was fitted with a compressor cycling switch. I revved it and the pressure with low fan and a cooled down cabin would go lower than 28 and it wouldn't cycle. Highest fan was 32 psi with cooled cabin. I was puzzled and then decided I'd quit guessing and look it up, VD compressor.

When A/C goes to YF 1234a refrigerant, VD compressors are supposed to be one of the ways to make that change go over with little other changes components. VD compressor and high side line with a low side siamesed to it for extra chilling on the high side liquid before expansion valve.

Buddy_M · July 22, 2012

Most of the A/C systems I work on are in heavy trucks(Kenworth, Peterbilt, Freightliner systems). Also see a few light/medium duty also. I have seen high side pressures between 300-400, however as stated above, I do have a fan I built that has a high airflow that I will put in front of a light/medium vehicle to move air through the condensor to drop high side pressures, and allows me to check the performance of the system to how it would work going down the road with maximum airflow. With the heavy side, I can usually just flip a switch and engage the fan clutch to get airflow to drop my high side pressure.

ktmlew · July 22, 2012

I do raise the engine speed as suggested but was concerned about high side pressure when the vehicle is in traffic. Also use a large shop fan to help with cooling. I think the main thing is to learn more about the temp delta across the condenser so I KNOW when they are getting plugged.

Did replace the condenser on a 2005 Grand Caravan. The previous tech had replaced condenser due to damage, compressor & dryer. He did not do any flushing or repairs to the rear air. It was blowing 58* at front vents <52/375 at idle, 55/500+ @ 1200-1500 rpm>. The rear was doing absolutely nothing. Ambient temp. I recommended pulling rear panels to inspect for plastic a bag sucked into evaporator and to check expansion valve for contaminates.

Rear expansion valve had lots of metal in it. Opened lines up-stream and flushed throughly. Replaced EV and recharged. Rear A/C blew at 52*. Better but not great. HS would go to 470-500 at 12-15 hundred RPM. So I recommended condenser.

Replaced condenser and now blows 42* at idle and 39* on test drive at center vent. Rear at 48 but I suspect more run time would have improved that. Pressures were better at 40/250-270.

Also realize the Honda/Malbui comparison is apples/oranges but just looking for opinions on differences. I always suspected better condenser/fan ratio.

Anyone suggest any training manuals/videos available on the net?

Fredsvt · July 23, 2012

Most, if not all, late model Hondas have scroll compressors, their compressors can't really take excessive high side pressures. They tend to self destruct, very rapidly if you're seeing idle pressures of anything higher than 275-300.

The Honda units are VERY sensitive to oil charge levels, even a little too much, they lose lots of cooling performance.

The Dodge minivan, Dodge in general issue, they push the limit on condenser size, being too small with poorly setup fans. They run too slow, until head pressure comes too high, and the fans don't always run at full speed either due to burned out secondary windings or a bad power transistor (fan relay). They use dual speed rad fans on most. PT Losers are great for melting out the second speed windings in the rad fan motor, they'll blow off the relief valve if it is, even on cooler days.

The Malibu, sounds normal, especially if the rad fan is a multi speed unit, they usually don't kick on high speed until 325psi. It may have a variable unit.

GM pickups, with the low slung compressor, early ones had a reflash to spin the compressor while cranking to get the oil out so as not to slug the unit on initial activation. I've seen many of these with very weak compressors from lots of oil slugged starts. Low high side, and very high low side when both front and rear units are running at full fan speed.

Anything with a clutchless compressor is variable unit. A stupid design, the guts are always spinning, whether or not you want a/c. They put a shear pin/keyway in the unit, so that WHEN the compressor seizes, the pulley can continue to turn.

Here's a link, you should be able to get information through here and through their links:

http://www.macsw.org/Content/NavigationMenu/Training/MACSTrainingCenter/default.htm

ktmlew · July 23, 2012

Most, if not all, late model Hondas have scroll compressors, their compressors can't really take excessive high side pressures. They tend to self destruct, very rapidly if you're seeing idle pressures of anything higher than 275-300.

The Honda units are VERY sensitive to oil charge levels, even a little too much, they lose lots of cooling performance.

The Dodge minivan, Dodge in general issue, they push the limit on condenser size, being too small with poorly setup fans. They run too slow, until head pressure comes too high, and the fans don't always run at full speed either due to burned out secondary windings or a bad power transistor (fan relay). They use dual speed rad fans on most. PT Losers are great for melting out the second speed windings in the rad fan motor, they'll blow off the relief valve if it is, even on cooler days.

The Malibu, sounds normal, especially if the rad fan is a multi speed unit, they usually don't kick on high speed until 325psi. It may have a variable unit.

GM pickups, with the low slung compressor, early ones had a reflash to spin the compressor while cranking to get the oil out so as not to slug the unit on initial activation. I've seen many of these with very weak compressors from lots of oil slugged starts. Low high side, and very high low side when both front and rear units are running at full fan speed.

Anything with a clutchless compressor is variable unit. A stupid design, the guts are always spinning, whether or not you want a/c. They put a shear pin/keyway in the unit, so that WHEN the compressor seizes, the pulley can continue to turn.

Here's a link, you should be able to get information through here and through their links:

http://www.macsw.org/Content/NavigationMenu/Training/MACSTrainingCenter/default.htm

My shop has a Modis but they seem to have lost the K-25 key needed to access Mopar data. Supposedly ordered. I assume I can see the fan speed when viewing data?

Thanks for the link. I forgot about MACS. Did my first real A/C training through them back in the late 80's or early 90's.

Fredsvt · July 24, 2012

Yes, you should be able to see the commanded fan state. And you can control both low and high with it as well.

On some, you may hear what seems like two speeds, but low is too low and high is barely what a 'correct' low should be. I've run into that on the PT Losers a lot.

On many mopars they use that power transistor for the fan(s) that burns out. They put them in such "accessible" locations.

ktmlew · July 24, 2012

Today consisted of oil changes, A/C diag on a couple more Mopars and an Odyssey van, Diag of a stalling 2006 Volvo S40...the modis doesn't know what a Vulva is...so I have the Vulva and Odyssey waiting for me for first thing in the morning.

Life in an Indy shop...

Keith Browning · July 24, 2012

Ghee, you almost sound like you miss Ford. Posted Image

Fredsvt · July 25, 2012

I didn't yesterday and today.

Old '03 F350, 7.3, oil leak. Customer wants pedestal orings changed.

He won't do it himself, because.... it's got all kinds of Banks add ons, and pulling the turbo was just a joy. Found that the customer pulled turbo a few years ago to put on the banks outlet housing and monster down pipe and the Banks blow-job waste gate actuator. He neglected to tighten the turbo to pedestal bolt closest to firewall.

The turbo ground up the pedestal, and the turbo's seal on the turbine side was shot as housing was filled with oil after shut down.

Get it together to see his EGTs are WAY high. Normal accel (Edge) results in 1200, half throttle close to 1600. And so much smoke, that you can't see behind it. Needless to say, he's got NO warranty on that stock turbo, that he admits pushing 35 to 40 psi boost pressure on a regular basis.