A/C only cools 58 out the vent

Tiny
KHERBIG73@GMAIL.COM
  • MEMBER
  • 1997 MITSUBISHI MIRAGE
  • 1.5L
  • 4 CYL
  • 2WD
  • AUTOMATIC
  • 172,000 MILES
I replaced all major A/C components, compressor, condenser, drier, evap, exp valve. I pulled vacuum overnight and it still will not get colder than 58 out of the vent. I was not able to replace the evap temperature sensor as I could not locate one to buy. That and the pressure switch are the only 2 items I did not replace. A/C cools just fine but when it gets to that temperature the system cuts off for 5 - 15 seconds then comes back on and repeats. I am certain it is not the pressure switch. Oh, and both my low and high pressures are perfect while it is running. I think I have narrowed it down to that temperature sensor but I do not know what to do since I cannot find a new one to buy. I am not sure if I can get one for another vehicle that would act in the same manner and if I can I do not know which vehicle to choose.
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Monday, September 6th, 2021 AT 8:44 AM

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Tiny
CARADIODOC
  • EXPERT
58 degrees is very respectable. You'd be very uncomfortable if the entire car got that cold.

A/C systems regulate to prevent the evaporator from dropping below 40 degrees. The humidity in the air condenses on the evaporator and drips onto the ground. If the evaporator were to get down to 32 degrees, that water would freeze into a block of ice and block air flow.

Once the evaporator gets down to around 40 degrees, remember you're blowing considerably hotter air through it. That air will never get down to the same 40 degrees. In fact, the goal of automotive A/C systems is to lower the air temperature by 20 degrees. Considering the really large volume of air, that's a big accomplishment. The comfort comes from removing the humidity. If you're starting with anything higher than 78 degrees outside, I'd consider the system is working normally. You should see the duct temperature go a little lower when the fan speed is reduced so there's less volume of heat to remove from the air.
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Monday, September 6th, 2021 AT 12:39 PM
Tiny
KHERBIG73@GMAIL.COM
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Thanks for the reply, but I was able to easily pull out the evap temperature sensor, and I just let it hang under the dash just to see if the system stayed on, and it does. I was getting 42 out of the vent. I am guessing when I reinstalled the new evap I had placed that sensor to close to it and it was getting to cold to quickly. I was able to move it over some and it finally works properly. I do disagree with your 20 degree temperature difference from air. From what I understand that is what a house system is supposed to reach, but your car's A/C system should put out in the 40's. In every car that I have been in, it is impossible to keep the cars A/C running on max the full time, it will freeze you out of the car. Before I found the fix, while 60 degrees does sound great, it did not come out of the temperature but for a few seconds at that temperature because that sensor was getting to cold to quickly and the system would shut off. So on a 90 degree day, 60 degrees coming from the vent for 5 seconds then it shutting off for 15 seconds, does not cool the car down at all.
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Monday, September 6th, 2021 AT 2:43 PM
Tiny
CARADIODOC
  • EXPERT
A house air conditioner has a much easier job. The industry-standard for automotive A/C systems is definitely 20 degrees, so if you're achieving that, you may not have a problem.

When we see 40 degrees duct temperature, we won't allow a customer to go that way. That is way too low and is going to result in evaporator freeze-up. To get the air that cold you have to start with an evaporator that's down in the low 20s. That is asking for problems.
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Monday, September 6th, 2021 AT 3:18 PM
Tiny
KHERBIG73@GMAIL.COM
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I'm not trying to make you look bad, but you should do a Google search and just look at the reputable sites to see what temperature should be coming out from your A/C vent. 20 degrees from ambient is definitely not correct. Every car brand is different but most consider 40 degree difference is what should be coming from the vent. My issue was that my system would shut itself off at 60 for 10 to 15 seconds, come back on for 20 seconds or so until it got back to 60 rinse and repeat. So while I was getting 60 degrees out of the vent, it was only for a few seconds then I got no ac benefit for the next 15 seconds until it turned back on so my car would not push out 60 continuously, it did this no matter what temp it was outside, from 95 to 75, 60 was it. I have researched what the temps should be a lot, and 98% say in the low 40s, could get lucky with high 30, and maybe low 50 if it is really hot outside. The 20 degrees you reference relates to house A/C's, just do another google search and look at the reputable places, 99% will say the inside temp in your house from the vent should be close to 20 degrees less than outside air temp. I do appreciate help, but the information you are giving out is totally not correct and I ask that you research before giving out bad information again.
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Wednesday, September 8th, 2021 AT 12:36 PM
Tiny
CARADIODOC
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Well, I've been teaching Automotive Technology for years, and I attend every class when we borrow our classroom for manufacturer-sponsored classes for dealership mechanics. I'm not going to change what we have all been teaching for years. Bare in mind too what you read in service manuals or online often doesn't agree with real life. Every week I worked at a new-car dealership we got a handful of new pages to stick in our service manuals. Those were to correct mistakes and to update them for mid-year design changes. Car owners who ordered those service manuals from the training centers don't get those update pages.

I can understand you may not be getting the results and the performance you did in the past, but you have to look at the huge volume of air you're passing through the evaporator. You can burn your hand on a lit match, but it isn't going to heat even a tiny room in your house.

One of the main purposes of air conditioning is to remove the humidity from the air. 85 degrees can be quite comfortable if the humidity is real low. To do that, you know the cold evaporator is being covered with that condensed water, and you know that water will freeze and block the air flow if the evaporator gets down to 32 degrees. All manufacturers regulate it to keep it near 40 degrees. Those are design facts that aren't open for dispute.

The hot air coming in isn't in contact with the evaporator's fins long enough to draw all of the heat out. Air does get that cold in a refrigerator but that's with air that's stagnant and not being changed constantly. Mitsubishi does indeed say duct temperature should be 37 - 40 degrees, but that's after 20 minutes, and only in the "Recirculation" mode. In that mode, cooled inside air is run through the evaporator repeatedly with only a little fresh outside air being introduced.

Your compressor is supposed to cycle off if the air leaving the evaporator gets down to 28 degrees, and it sounds like that's what's happening. Any chance that temperature sensor is mispositioned?

There are other potential causes of insufficient cooling, but if you pulled the system into a vacuum, and you said the pressure switches are working, I would have to assume you put new refrigerant in, and I have no reason to doubt you put the right amount in, so there's no point in looking in that area for a problem. You might consider a sticking blend air door that is allowing some heated air to mix with the cooled air.

From everything you've said, it sounds to me like the refrigerant circuit is working as designed and that is not the area to be looking. Can you see frost forming on either of the hoses under the hood where they pass through the firewall? Expect to see a little on the low side suction hose, but if a lot is forming, that is where the cooling is taking place instead of in the evaporator. That points to the system is over-charged. If you see a lot of frost forming on the high-side hose in one spot, that suggests there's a restriction at that point.

Also be sure the condenser fan is running. If it is not, the more common symptom is the high-pressure relief valve will blow, but before that you are likely to get good cooling while driving and natural air flow keeps the condenser cooled. We can rule out a butterfly collection in the condenser since that is new. Likewise, the receiver / drier likely isn't plugged since that is new.

You also found the high and low side pressures to be perfect. That means there's nothing to fix there. Many owners have complained about some Honda models not cooling well since they were new, and testing has shown duct temperature only gets down to the mid 60s. Honda says that is normal and was how those systems were designed. Of course they improved their systems after a few years of hearing those complaints.

What I would do at this point is first to verify water is dripping from the condensate hose after the system has been running a while. If you see that, we need to look at other possible causes like the blend air door. If you don't see water dripping, either ice has formed on the evaporator from over-cooling, or the system is low on charge, which I doubt. For over-cooling, you know the refrigerant system is working. It's working too well. That's where I'd look at the placement of the temperature sensors.

One last comment of value. You can always have a failed temperature sensor, but my frequent comment is those are the last things to suspect. Temperature sensors have just one component inside them, so failures are very rare. It's much more common to find a broken wire or corrosion on the connector terminals. Ford did have to prove me wrong by having a huge rash of coolant temperature sensor failures in the early '90s, but that was due to a manufacturing problem. It's also real uncommon to find a sensor that is out-of-specs. We don't even teach testing them for resistance since that is a waste of time and therefore the customer's money. All manufacturers provide typical specs in their service manuals, but those are only for training purposes. If you were to test a dozen temperature sensors from properly-running systems, you would find near half of them read out-of-specs, so what's the value in testing them? The exception is if a temperature sensor has three or more wires. That means it has power and ground wires to power additional internal circuitry. That circuitry certainly can fail. Mass air flow sensors with built-in intake air temperature sensors are a good example.
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Wednesday, September 8th, 2021 AT 3:55 PM
Tiny
KHERBIG73@GMAIL.COM
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My problem was the temperature sensor. When I reinstalled the evap box it had popped off and I tried to get it back on in the same place, but evidently I had it way to close to the line or fin stack because it kept telling the A/C to turn off at 60 degrees, and once it hit 60 degrees at the evap it would never stay on to cool the car, it kept turning off for 10 to 15 seconds, temp go back to 63-63 turn back on for a few seconds until it hit about 60, and just kept repeating. I was able to remove the temp sensor very easily, I had direct access to its penetration hole at the glove box. I left it out temporarily just to test if the system would stay on below 60, sure enough it did, it gets to high 30 low 40s now. So I am attempting to get the temp sensor back in a reasonable place so that my lines don't freeze, truthfully I don't drive the car more than 10 to 15 minutes at a time and not everyday so I don't think my lines will freeze up if I left it out. Plus it gets so cold that Ill either turn it way down or just turn it off for a few. I am extremely happy with my outcome. I paid total $345.00 for all the component's off of rockauto. I did spend another $150.00 on a vacuum pump and manifold gauges, if I had taken it to any shop I would have spent well over a grand probably $1,500.00. It's an old car so I wasn't about to put that kind of money into it, and it took me awhile to even buy the parts I did. But it had no leaks from the old equipment, no leaks with the new, the engine still sounds fantastic, so I decided to put new tires on it to celebrate, lol they got 45,000 tread warranty, but it would take me 10 years or more to put that kind of mileage on it, by then I will have an additional car. Thanks
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Thursday, September 9th, 2021 AT 3:35 PM
Tiny
CARADIODOC
  • EXPERT
Dandy. Very happy to hear you solved it.

I was visiting a friend today who owns a body / repair shop, and was in the middle of ordering A/C parts for a crashed vehicle, so the discussion came up. He was of the opinion it's normal to expect duct air to be lower than 58 degrees, but many newer cars won't achieve that. My truck, for example, is comfortable on a rare 90-degree day, but the air feels not nearly as cold as it was on my older vehicles. I didn't stuff a thermometer in it, but I know it isn't as cold as I'd expect it to be.

The good news is this is handled. Please come back to see us with your next problem.
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Thursday, September 9th, 2021 AT 5:11 PM

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