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AC was cold then not. Troubleshooting help needed

Old 05-13-2019, 09:58 PM
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Default AC was cold then not. Troubleshooting help needed

Hi all,

So yesterday it was 4:00 pm, 85' F and as I started towing my boat back from the launch my AC was working fine. I turned it off before climbing my big hill (1,700' in 6 miles). The temps of the transmission and engine coolant never went above 205'. But when I tried to use the AC again at the top of my hill the air temperature was no longer icy cold and instead only blew straight air. It stayed this way the last 20 miles of my 35-40 mile trip home. It is an Automatic Climate Control system.

When I got home I checked under the hood and the accumulator was warm, not cold and the AC compressor clutch was not engaged with the compressor not rotating.

This evening with the truck having sat in the driveway all day, the outside temperature about 70', and just a bit before sunset (i.e. no sun shining) the AC blew icy cold air.

Almost two years ago in early June of 2017 I replaced the compressor, accumulator, all lines and hoses, and condenser along with doing a very thorough flush of the evaporator. I also added some UV dye to the system. The system has worked fine since then. Yesterday was the first time I tried using the AC for an extended period of time this spring.

So, it is "kinda" working. Any advice on what to check and how to diagnose what is going on?

Is it possible there is some control circuit that is on purpose or mistakenly disabling the AC? Could my symptoms be explained by me having a small leak? Something else to check?

Old 05-13-2019, 10:59 PM
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First think I would check is the pressure in the system. If it's looking lower than it should be, then you know you have a leak somewhere, and the pressure sensors could be shutting the system down and not allowing it to function. Gases in a closed system such as the AC system fluctuate pressure with temperature changes, so if the pressure is borderline where it needs to be and low, it may act all sorts of crazy and work sometimes and not others.

Short of that, I would look at the low pressure sensor next.

Then possibly the outside air temp sensor since I know those auto climate control units use one.

The actual climate controls in the dash would probably be the last thing I look at.
Old 05-13-2019, 11:31 PM
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When you have a call for cooling but no compressor engagement, check to see if there is 12v being sent to the compressor clutch. if not, make sure that there is 12v on pins 2 and 3 and that pin one is being pulled low on the compressor clutch relay.


Last edited by GeorgeLG; 05-13-2019 at 11:53 PM.
Old 05-13-2019, 11:50 PM
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Typically an auto ac system will have a receiver that has a sight glass on it's top. When the compressor is running on max ac and the temp control set as cold as it can be, the sight glass will be clear of bubbles, indicating a full charge. If bubbles are constant it's time to start looking for that dye. You might also check the ac condenser coil to verify that it clean of all debris. A dirty coil will drive the high side pressure up to a point that a high pressure switch will cut out the compressor. Are you able to hook a scanner up to see if any codes are given? Some rigs have a blend door that will move to the heat position even when calling for ac. There are a lot of possibilities.
Old 05-14-2019, 07:18 AM
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I have had oil getting in the low pressure switch on the accumulator causing erratic compressor operation in mine. I've replace this switch twice now, and each time it will work for a couple of years and then start giving trouble. Each time I removed the switch, I got a fair amount of oil out of it. Looks like someone put an excessive amount of oil in the system. Good think these switches are dirt cheap, plentiful, and you don't have to discharge the system to replace them.
Old 05-15-2019, 12:46 PM
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Quick update:

I had an auto parts store try to read any codes but we could not find any for the AC. Then I rented a UV lamp to look for leaks (I had put in some dye when I repaired it) and also a pressure gauge manifold set form the parts store.
  • No leaks detectable by the UV lamp.
  • Comparing to the table of pressures for the Hi and Low pressure side of the compressor I saw that I was at the low border for enough refrigerant.
  • I tried to put more refrigerant in the system but I discovered that I had mistakenly kept the can tap I had gotten from GPD (pn 5811248) that had defective threads for the can side. Argh. Then I went to the parts store to get another can of refrigerant and bought the wrong can tap (this time for the resealable cans). By the end of the afternoon the outside temperatures had fallen so that the pressure readings were questionable. (This was something new I learned).

By late this afternoon the forecast has the outside temperatures just high enough to do a final check. I suspect I'll be fine with the additional refrigerant I put in but it will be probably a few weeks before it gets back to the 80's when I could see if it is working fine.

BTW, I found a couple really useful tutorials on on how car AC systems work. One is a lecture from a community college and one is from an AC equipment vendor.

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