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Low side pressure too low at 40-50 MPH
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jvel2012
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Sep 9, 2012, 4:30 PM
Post #1 of 31
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Low side pressure too low at 40-50 MPH
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Hello,
I am a HVAC Contractor and work on residential and
commercial units, but I am trying to fix my own car.
I have a Toyota Sienna 2005 6 cylinder.
The ac works great if you don't go out for a drive.
The car at idle will operate great. But as soon as you
take the car out on the road and get it up to 40-50 miles
per hour, the low side pressure drops to 20-25 psi and the
compressor stops. I think the evaporator is icing up and the
compressor won't come back up until the evap thaws out.
That is my problem. something is happening when I get up
to 40-50 and low side pressure drops.
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Hammer Time
Ultimate Carjunky
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Sep 9, 2012, 5:13 PM
Post #2 of 31
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Re: Low side pressure too low at 40-50 MPH
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20 to 25 PSI is not abnormal for an expansion valve system at cruise speed. You have a thermostat in the evaporator that should be cycling the compressor off when it gets too cold but any freeze up would take considerable time. Are you losing air flow from the blower?
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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.
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jvel2012
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Sep 9, 2012, 10:51 PM
Post #3 of 31
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Re: Low side pressure too low at 40-50 MPH
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The blower in the car stays at the same air flow. When you say it could take a while to freeze up, it takes about 6 to 8 minutes at the high speed and then the compressor shuts off.
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Hammer Time
Ultimate Carjunky
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Sep 10, 2012, 3:09 AM
Post #4 of 31
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Re: Low side pressure too low at 40-50 MPH
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That doesn't sound like a freeze up to me. First off, you would lose your air flow if it was frozen and it would take a minimum of 30 to 40 minutes with the blower turned down. You could however have a problem with the thermostat, the clutch coil or many other things for that matter, depending on whether this is an auto system or manual. It would have to be troubleshooted while the problem is present.
If you need a wiring diagram, tell me if it's auto temp control or manual.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.
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Tom Greenleaf
Ultimate Carjunky
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Sep 10, 2012, 3:37 AM
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Re: Low side pressure too low at 40-50 MPH
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6-8 minutes and full volume of air is too fast IMO for plain freeze up. Just some questions and let HT choose some more tests/observations:
* Can you tell if fan for condenser quits first before it shuts down?
* Would be odd unless messed with but is fan blowing the right way for radiator AND condenser?
* Is engine temp staying at proper temps or noticeably higher at some speed?
Just notes: No doubt this will shut down for assorted reasons as fail-safe for the system. High pressure too high OR too hot. Sorry not to know on this particular vehicle but some are measuring temp at compressor and some high side pressure - either are telling of excessive pressure for any reason. Take note if condenser is clean also. If on construction sites excessive dirt may be a problem??
One more - do you see condensate dripping when just idling (would take some time) as I don't expect you to see that unless you are looking and higher speeds that would be a trick to observe. Unlike some HVAC systems about all MVAC (mobile) A/C just lets condensate drip out to ground. Yes - aware that some HVAC set ups do not if only "window shaker" units,
T
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Discretesignals
Ultimate Carjunky
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Sep 10, 2012, 4:55 AM
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Re: Low side pressure too low at 40-50 MPH
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What is the high side doing while this is happening?
Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.
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jvel2012
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Sep 11, 2012, 12:27 AM
Post #7 of 31
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Re: Low side pressure too low at 40-50 MPH
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It has a knob to set the temperature, so I think that means it is a manual system. There is no auto setting or display settings. I will have to look into the thermostat on this system. Could you tell me where it might be located? I am also going to check out the filter because I don't think it has ever been changed.
Thanks for the help
Joe
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jvel2012
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Sep 11, 2012, 12:42 AM
Post #8 of 31
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Re: Low side pressure too low at 40-50 MPH
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would the condenser fan be the extra fan on the radiator? I drove around this week end with my manifold gauges hooked up while driving. Duct tape everywhere and felt like a redneck. I could also hook a meter to the fan if I knew which one it was.
The engine was pulled out and swapped by a mechanic.
The problem started after we got the car back and the mechanic doesn't know why, he just wants me to go see a auto ac guy. I need to fix this myself. I am a HVAC guy just not for autos.
The engine temp is staying steady and not changing.
I tried testing this system at different levels of Freon and high pressure has not been a problem.
I will be checking the condenser tomorrow to see if it is clean.
Don't see any condensation at all while it is idling.
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jvel2012
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Sep 11, 2012, 12:52 AM
Post #9 of 31
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Re: Low side pressure too low at 40-50 MPH
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when I speed up to 40-50 MPH both sides are dropping.
After running at that speed for 6-8 minutes the compressor shuts down.
I tried first 2 cans of 134 after reclaiming the old and pumping down the system. the pressure was running 40 low side and 200+ high side. At higher speed the above happened.
Tried more Freon about another 1/2 can and got low side up to 50 and high side was close to 300. I didn't want to chance having a problem over 300. At higher speed high side dropped 10-15 pounds and low side dropped to between 20-25 pounds. I felt that would be too low for the coil because the coil (evaporator ) would be below freezing. At least that is the way I troubleshoot commercial HVAC systems.
Thanks for the help
Joe
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Tom Greenleaf
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Sep 11, 2012, 1:14 AM
Post #10 of 31
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Re: Low side pressure too low at 40-50 MPH
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Joe - Don't let the concept of HVAC mess you up. Mobile uses same but deals with the assorted speeds and airflow - stationary doesn't so there's the difference.
134a or any in vehicles want the exact weight of charge. 10% off up or down can mess it all up. A decal should be placed for capacity. Last year I have handy for this is 38oz 0f 134a. Just shooting more or bleeding some off you've lost count and only best way is start from full well held vacuum and weigh in the charge to right on the spot. Then if not perfect the reasons why not come in to play.
Don't let understanding HVAC allow you to trash this system as again, it is very dependant on correct known charge and pure. Air, even from unpurged lines will come into play and mess it up too. PAG (the oil used) is hygroscopic and will change to icky (nice techno word) when exposed to moisture from air.
Careful - what you miss or do wrong can cause massive problems you didn't start with,
T
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jvel2012
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Sep 11, 2012, 11:23 PM
Post #12 of 31
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Re: Low side pressure too low at 40-50 MPH
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Thanks for the tips. I will recover and recharge the system on Saturday............Joe
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jvel2012
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Sep 11, 2012, 11:24 PM
Post #13 of 31
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Re: Low side pressure too low at 40-50 MPH
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Thanks for the help. I will try your suggestion on Saturday...................Joe
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jvel2012
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Sep 20, 2012, 9:59 PM
Post #14 of 31
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Re: Low side pressure too low at 40-50 MPH
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I didn't get around to checking the ac system until yesterday. I did find the blower filter plugged. I can the car without the filter and it didn't make a difference. as soon as I got up to 40 miles an hour the compressor quit. I also thought about what you said about the liquid line and if it lost heat and could be restricted. At the point that the compressor quits, the line has a lose of heat due to the compressor quitting.
Would it be a good idea to replace the dryer and give that a try?
Joe
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Hammer Time
Ultimate Carjunky
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Sep 21, 2012, 3:11 AM
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Re: Low side pressure too low at 40-50 MPH
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Yes, that would be a good idea. That still leaves the expansion valve as a potential restriction spot but the dryer is a real possibility.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.
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Discretesignals
Ultimate Carjunky
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Sep 21, 2012, 4:47 AM
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Re: Low side pressure too low at 40-50 MPH
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I still don't understand this thread. Reading the information it seems to me the system is operating as it should. The compressor is going to cycle even on a txv system when the themistor senses the evap temperature is getting too cold. Especially at highway speeds with the blower on low. Even though the txv can regulate the amount of refrigerant flowing through the evaporator it can't prevent a freeze up.
You never stated what the outlet temperature was and how long the compressor stays off.
Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.
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Discretesignals
Ultimate Carjunky
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Sep 21, 2012, 5:01 AM
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Re: Low side pressure too low at 40-50 MPH
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Would be interesting to know what the outlet temperature is when this is going on to know if there is really a problem. The first post in this thread the complaint was the low side pressure was getting too low, which those pressures look normal. Maybe that is what I am confused about.
Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.
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Tom Greenleaf
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Sep 21, 2012, 5:07 AM
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Re: Low side pressure too low at 40-50 MPH
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No condensate noted either,
T
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Discretesignals
Ultimate Carjunky
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Sep 21, 2012, 5:15 AM
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Re: Low side pressure too low at 40-50 MPH
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Kind of hard to see condensate at 40-50 mph...
How do HVAC contractors determine how much refrigerant to put into a residential or commercial system? Do they go by weight or pressures? If he is using pressures on this system, it is going to be off.
Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.
(This post was edited by Discretesignals on Sep 21, 2012, 5:18 AM)
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Tom Greenleaf
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Sep 21, 2012, 5:35 AM
Post #21 of 31
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Re: Low side pressure too low at 40-50 MPH
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Weak at HVAC - same idea of evaporating a liquid. Known capacities are going to vary wildly. Rooftop condensers and who knows how much line used? That or condenser level with evaporator. My best with my own is temp of return air, temp of output air, no frost ever and this one does make lots of condensate where some are not allowed to so blow that back at condenser or down a drain. Just can't speak for them all at all.
My own - twice up to state of art efficiency does NOT shut down by any pressure problems or icing problems - some do and if the friking thing blows up again will NOT make an iceberg to melt inside the building thank you
I now keep a thermo in an output duct as a clue it is messing up,
T
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Tom Greenleaf
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Sep 21, 2012, 1:55 PM
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Re: Low side pressure too low at 40-50 MPH
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Even the cans empty don't weigh the same. I'll use the 12oz stuff. 3.2oz empty. My brass tap w cap 2.9oz. Total on a good known full can then = 18.1.
Yep - that close and it matters,
T
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