AC pressure readings and problem with dealer.

I've had my Bobcat E50 to the dealer twice trying to get the air conditioner to work properly. So far there's been very little change. The machine has a little over 100 hours on it. I don't know a whole lot about ac's.

The low side is at about 20 psi and the high side at best is 170 psi. Before it was brought in the first time, I added about 8oz. of refrigerant. It made no difference to the pressure readings or the temperature at the vents. According to the service manual the temperature at the vents should be between 36 and 53 degrees, depending on humidity. And the pressure readings at ambient air temperature of 86 to 101 degrees should be: Low side 15-33 psi and High side 210-265 psi.

When it was being serviced the first time they said that everything was fine and it was blowing 60 degrees out the vents which it was but that's not cold enough to keep the machine comfortable. I then checked the pressure readings and they were off so I had it serviced again. I also told them what the service manual says regarding vent temperature.

Before it was brought in the second time I told the service guy that the High side pressure was too low.
I also told them what the service manual says regarding vent temperature.

When I got it back yesterday he said they put it on the ac machine and pressure was where it's supposed to be and they checked everything out. Put a dye in and no leaks. They said a water drain line was clogged and now it works better. It doesn't. High side pressure is still low and it's maybe a couple degrees colder at the vents.

Any ideas what's wrong? I've tried searching the web and can't seem to come up with a low pressure side low or normal and high pressure side low scenario other than low refrigerant which they claim they removed it, vacuumed the system and put the proper amount in, twice now.

Thanks,
Doug

I would be checking air flow through the evap core . Going strictly by pressures is also not very accurate. Too many variables.
I don’t know how that machine is set up but if it has blend doors in the ac heater unit that could be an issue. Low evaporator core air flow will cause the high side to be lower.

You can Google a 134a pressure/temperature chart to know for sure that your system is working correctly. As the ambient temperature changes so will the pressure. The readings you gave seem on the low side. Most a/c systems when working right will be blowing 30 degrees lower than the ambient temperature. Some are better and some are worse. If it has a red dot type a/c system in it. It will never cool enough. Those have to small of a condenser and evaporator cores to be efficient. Also if the heater bypass valve isn't closing then that will negate any cooling. Most a/c systems blow through the heater core first.

Mobiltech said:

I would be checking air flow through the evap core . Going strictly by pressures is also not very accurate. Too many variables.
I don’t know how that machine is set up but if it has blend doors in the ac heater unit that could be an issue. Low evaporator core air flow will cause the high side to be lower.

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I'm not exactly sure how to check the evaporator core air flow. I'm including a picture of the evaporator area with the front cover off. I think it draws air from the cab and from under the hood by the hydraulics.

Doug580l said:

I'm not exactly sure how to check the evaporator core air flow. I'm including a picture of the evaporator area with the front cover off. I think it draws air from the cab and from under the hood by the hydraulics.

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Oops, forgot the picture.

BigWrench55 said:

You can Google a 134a pressure/temperature chart to know for sure that your system is working correctly. As the ambient temperature changes so will the pressure. The readings you gave seem on the low side. Most a/c systems when working right will be blowing 30 degrees lower than the ambient temperature. Some are better and some are worse. If it has a red dot type a/c system in it. It will never cool enough. Those have to small of a condenser and evaporator cores to be efficient. Also if the heater bypass valve isn't closing then that will negate any cooling. Most a/c systems blow through the heater core first.

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I checked out a chart and the service manual. The high pressure side didn't seem to change much between 85 and 90+ degrees. At 85 degrees out the high pressure was about 165-170 psi and at 90+ it was about the same. It should be well over 200 psi. There isn't any scenario in the troubleshooting section of the repair manual for the High pressure side being low except for low refrigerant which I'm pretty sure is right where it should be. I do think that it's strange that when I added about 8oz. of refrigerant it did not affect the high pressure readings. My understand is that if there was excess refrigerant the high side pressure should be too high rather than too low.

Good functioning ac system is about flow and conversion. Close off the heater core lines if possible so you are not blending evaporator cool air with the heater core hot air. When the condenser is cooling sufficiently the high side will drop. On average, your high side pressure will be around 7 times the low side pressure.

If you are getting a reading of 20psi on the low side and 170psi on the high side @ 90*F. Then you probably don't have enough freon in the system or the compressor is weak. Make sure that the belt is tight as well.

Are you checking pressures at 1500 rpms? Rarely are a/c systems on compact equipment much to brag about. It is functional, but not optimal.

BigWrench55 said:

If you are getting a reading of 20psi on the low side and 170psi on the high side @ 90*F. Then you probably don't have enough freon in the system or the compressor is weak. Make sure that the belt is tight as well.

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Vetech63 said:

Good functioning ac system is about flow and conversion. Close off the heater core lines if possible so you are not blending evaporator cool air with the heater core hot air. When the condenser is cooling sufficiently the high side will drop. On average, your high side pressure will be around 7 times the low side pressure.

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I'll see if I can figure out how to close off the line tomorrow.

BigWrench55 said:

If you are getting a reading of 20psi on the low side and 170psi on the high side @ 90*F. Then you probably don't have enough freon in the system or the compressor is weak. Make sure that the belt is tight as well.

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The machine has been to the dealer twice in the last week and a half. Both times they said they drained, vacuumed and refilled the system so hopefully the right amount of refrigerant is in it. Don't know what I can do about the belt. It's tucked away pretty good. I poked it with a screwdriver to see how tight it is, didn't seem loose. The counterweight has to be removed in order to get to it if it needs to be tightened.

Questionable wizard said:

Are you checking pressures at 1500 rpms? Rarely are a/c systems on compact equipment much to brag about. It is functional, but not optimal.

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Yup, checked at full throttle.

I have never in 25 years of working on ac systems almost daily had a properly working system at 20 psi on the low side charge it until the low side is around 30-35 if it doesn't go up to that near the recommended amount of Freon you have a restriction in the suction side

Hooking to an ac machine means nothing only that they dont know how to read guages and bought some time by pushing buttons and walking away for 1/2 hour vacuuming boils out water thats it there is no water in a sealed system when no parts have been replaced
Ask them the exact amount of refrigerant removed and how much put back in and compare to the specs maybe the same guy read it wrong twice...

Also find another dealer its not going to cool with 20 psi on the suction side

Jonas302 said:

I have never in 25 years of working on ac systems almost daily had a properly working system at 20 psi on the low side charge it until the low side is around 30-35 if it doesn't go up to that near the recommended amount of Freon you have a restriction in the suction side

Hooking to an ac machine means nothing only that they dont know how to read guages and bought some time by pushing buttons and walking away for 1/2 hour vacuuming boils out water thats it there is no water in a sealed system when no parts have been replaced
Ask them the exact amount of refrigerant removed and how much put back in and compare to the specs maybe the same guy read it wrong twice...

Also find another dealer its not going to cool with 20 psi on the suction side

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I thought restriction also, but with the high side being low too. I think it's low on freon or failing compressor or belt slipping.

Txv system in TB180FR, 48°f vent temp, engine at operating temp, 90°f outdoor temp. At idle.

BigWrench55 said:

if the heater bypass valve isn't closing then that will negate any cooling. Most a/c systems blow through the heater core first.

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That and the internal cab air filter- I see that the machine has low hours, but if a squirrel built his nest in your ductwork, you aint getting the flow the engineers designed.
That on its own will lower pressures.

Jonas302 said:

I have never in 25 years of working on ac systems almost daily had a properly working system at 20 psi on the low side charge it until the low side is around 30-35 if it doesn't go up to that near the recommended amount of Freon you have a restriction in the suction side

Hooking to an ac machine means nothing only that they dont know how to read guages and bought some time by pushing buttons and walking away for 1/2 hour vacuuming boils out water thats it there is no water in a sealed system when no parts have been replaced
Ask them the exact amount of refrigerant removed and how much put back in and compare to the specs maybe the same guy read it wrong twice...

Also find another dealer its not going to cool with 20 psi on the suction side

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According to the service manual, the system holds 23 oz. of refrigerant. Before I had it brought in the first time I added around 8 oz. of refrigerant. The gauges readings did not change. Doesn't that mean something? I expected the pressure to go higher, but nothing. They claimed that the machine had the right amount of refrigerant when it was brought in. So either they didn't check it or it was low before I added to it. They pretty much said that everything was fine as it was. They sent the machine back without contacting me. I called them and told them that the service manual says it should be blowing between 36 and 53 degrees at the vents and that the pressure readings were low. He admitted they never checked the pressure readings so I sent the machine back. This time they said there was a clogged drain line causing the issues and it was fine now. I asked about the pressures and he said they were fine now too. Nothing has changed. He also said that they put dye in to check for leaks since I told him about adding refrigerant to it.

I don't have much choice as far as dealers, it's warranty work and the next closest dealer is almost 3 hours away and part of the same dealership group.

So have you had the machine in the summer before Had it worked well and then not as well ?

The only official way to check charge is to recover and weigh back in charge. If you have a thermocouple you can insulate and attach to the liquid leaving the condenser, you can check subcooling and that is also a valid way to do it but more art and judgement required than the weight method. Otherwise you are somewhat shooting in the dark but it can be done that way too.

westerner said:

That and the internal cab air filter- I see that the machine has low hours, but if a squirrel built his nest in your ductwork, you aint getting the flow the engineers designed.
That on its own will lower pressures.

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I checked both filters, they are clean and no obstructions.