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AC pressure readings and problem with dealer.

fastline

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Well documented! I have to get to a 'thing' right now, but I will review all the data. You did a good job and this should help. If your cab was not cool, that compressor should not be cycling. I don't like it's pressures but it does have some subcooling.

As for the water, I think it is unrelated and there is a drip pan inside and I think the tubes are projected up, not flush with the bottom of the pan, so the pan can hold water. this may explain no water coming from unit, and water in the vents as the water level comes up where the blower fins can contact it.

Either way, it seems pretty obvious that the cover of the unit in the cab has to come off the figure that one out. may find other issues there as well. I want to know why it is cycling.
 

XSKIER

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That "no dripping until I shut it off" deal happens on my 908M as well. Started at hour 1, now a year and 280 later same thing. I just assumed that it had something to do with the "pressurized cab" nature of design. The cooling performance is adequate, so the system pressures have not been checked. This same basic cab runs on all CAT small wheel loaders over the last 10 years, so if the "hold it till you shut 'er down" condensate feature was much of a problem, I'm sure we would have seen it here?
 

Doug580l

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Well documented! I have to get to a 'thing' right now, but I will review all the data. You did a good job and this should help. If your cab was not cool, that compressor should not be cycling. I don't like it's pressures but it does have some subcooling.

As for the water, I think it is unrelated and there is a drip pan inside and I think the tubes are projected up, not flush with the bottom of the pan, so the pan can hold water. this may explain no water coming from unit, and water in the vents as the water level comes up where the blower fins can contact it.

Either way, it seems pretty obvious that the cover of the unit in the cab has to come off the figure that one out. may find other issues there as well. I want to know why it is cycling.

I did some more testing today. First in the morning when it was about 80 degrees. Let me know if you want any of that info. It first started cycling after about 3 minutes and then kept cycling about every 70 seconds.

Testing done at 94 degrees. 125 degrees inside the cab. I put a thermometer on a wire and hung it at about head height in the cab to test the cab temps.

It didn't cycle for the first time until after running 77 minutes. After it cycled the first time the next cycles occurred at these intervals: 6:45, 5:30. 1:47, 5:40, 2:48, 4:55, 3:45, 1:48, 5:40, 5:50 and 3:17


After 5 minutes running full throttle. Low pres. 24, High 195, vent 61, Liquid Line 114, cab 92

After 15 minutes Low 22, High 190, Vent 54, LL 111, cab 84

After 40 minutes-fairly consistent. Low fluctuates between 19-22, High 180-185, vent slowly got down to about 50, LL 109-112, Cab 74

After 55 minutes turned engine down to idle for about 20 minutes. Low 30-31, High 206-208, LL 117-119, Cab 76

Turned back up to full throttle. Started cycling after about 3-4 minutes.

Averages over the next 45 minutes or so:

Low, 20-23, High 185-195, Vent at beginning of cycle goes up to about 59, then down to about 50. LL 108-109, Cab 76

Hope this all means something. I was kind of surprised that there were better pressures at idle. They were much more consistent too, very little fluctuation.

Thanks
 

fastline

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Sorry man! I had some obligations this weekend. I am still reading through all the test data but everything I can see, it really looks like low charge!!! I will explain.
1. Head pressure lower than typical of MVAC 134 system at those ambient temps
2. Compressor cycling way before cabin temps are cooled down.
3. High vent temps indicating higher than normal evap temp
4. Not much condensate further indicating warmer evap.

though it would be nice to know what the evap or suction line temp is running, everything so far would indicate the cycling is not being caused by low temps at the evap (freeze/etc), but low suction pressure. In almost all cases if you have low suction and liquid pressure, the system is undercharged! But how could that be when using the factory spec charge? I just don't know, I am just calling it like I see it.

Because the clutch is computer controlled, it could be a HVAC glitch causing the cycling, but otherwise the system appears to be "close". IE, I see pressures and temps that I would consider in range for a good functioning system that just needs a little more charge. I am not saying "go charge it" just yet, just sharing what I see right now.
 

fastline

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I looked at some of your values for liquid line temps. I do 'prefer' temps after the receiver, but gotta work with what ya got. Temp is not likely to be much different. I see subcool that initially was around 20*, but dropped to less than 10. In MVAC it is pretty normal to see 30* of subcooling. The way we achieve that is higher head pressure. Basically this is ensuring nothing but liquid goes to the TXV and the receiver you can consider a 'liquid reservoir' where liquid is on tap and read to go to the TXV. This is because of the variability of RPMs and inconsistent conditions. This is why subcooling is usually much higher than any stationary system.

To be honest, I would be very surprised if you added 134 to this system and did not see anything change. I don't see anything yet that really confirms a bad compressor. The condensate issue needs to be addressed and it needs to be another test point to monitor. Part of the reason for AC is to remove humidity as latent heat so yeah, it should be puking water when running. If there is no or minimal condensate, I already know the evap is not getting much below dew point.
 

Doug580l

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I looked at some of your values for liquid line temps. I do 'prefer' temps after the receiver, but gotta work with what ya got. Temp is not likely to be much different. I see subcool that initially was around 20*, but dropped to less than 10. In MVAC it is pretty normal to see 30* of subcooling. The way we achieve that is higher head pressure. Basically this is ensuring nothing but liquid goes to the TXV and the receiver you can consider a 'liquid reservoir' where liquid is on tap and read to go to the TXV. This is because of the variability of RPMs and inconsistent conditions. This is why subcooling is usually much higher than any stationary system.

To be honest, I would be very surprised if you added 134 to this system and did not see anything change. I don't see anything yet that really confirms a bad compressor. The condensate issue needs to be addressed and it needs to be another test point to monitor. Part of the reason for AC is to remove humidity as latent heat so yeah, it should be puking water when running. If there is no or minimal condensate, I already know the evap is not getting much below dew point.
It appears that the problem isn't going to be easy to diagnose, which is probably why the dealer isn't fixing it. They seemed to be focusing on the refrigerant levels too.

I know that you already addressed it, But I did add refrigerant before I brought it in the first time and it made no difference to the pressures. If they don't fix it again when it's brought in this week, I'll consider adding the 12 oz. like you suggested. Because at that point I will probably start replacing components myself. Starting with the thermostat and after that the expansion valve and compressor. Don't know what else to do.

As far as the water coming out of the vents, that never happened until the machine was serviced the second time. I did get colder vent temps and slightly higher pressures after the second service also.

If it will make a difference diagnosing the problem, I can look at the liquid line again and see if there's a way to check the temps after the receiver.

Thanks for the help.
 

fastline

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I am not as worried about testing after the receiver as I am getting accurate data. I see guys try to use a meat probe! You really want a good thermocouple firmly attached for accuracy. That is really just one data point though.

I would not go in blindly replacing things. An underfeeding TXV will typically have low suction but high head pressure. You don't have that.

When we talk about 'vent temps', you must realize we are looking more at the delta T across the evap. That is the difference in temp of air going in, and temp of air coming out. Perfectly normal to see higher vent temps when it is trying to pull down a hot cab, but vent temps should steadily decrease. It is also a function of airflow across the evap so if I want to see if I can get vent temp to drop, I will lower the fan speed. That 'should' drop vent temps at the sacrifice of air flow.

If you can get access to the suction line close to the evap anywhere, it would be good to feel that. maybe it is possible air is bypassing the evap, which WILL cause low suction, cycling, etc, yet not get cold vent temps. Not likely, but..... this one is a brain bender.

Don't ever blindly add 134 based on weight in this case. Add some, while weighing what you did, and monitor the system. 12oz really seems like a LOT, but honestly, if the system took it and head pressure does not budge, I'd be looking hard at the compressor. The system info does not really suggest a severe undercharged system, so I probably would not make it to 12oz unless I start seeing response in the system, which would be increased head pressure, and better performance in the cab. Water coming out of the vents has to be addressed. That is not normal!

But really, this is practically a NEW machine!!! And if it can't even maintain a comfortable cabin temp, something is wrong. Tell them to give you a different machine. I would make them fix the problem! That should not be your burden.
 
Last edited:

fastline

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I should reiterate that adding charge beyond what is specified is NOT typical, and I'd probably want to talk to the Bobcat engineering group to discuss. What can happen is a larger condenser, receiver, lineset, etc, could have been used, which would require a different charge, which 'maybe' was not documented.

In any case, adding refrigerant is going into the wild and these systems cannot be charged looking for a fixed value like subcool. Won't happen because those values are moving targets!

I don't think you tried this per my previous recommendation, but if you are hooked on again, put cardboard over the condenser and see if that causes head pressure to rise. You should see liquid line temp increase and head pressure.

Also remember every time you hook on, you are losing refrigerant. I use smart probes so I don't have loss but most use gauges with hoses.
 

Doug580l

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But really, this is practically a NEW machine!!! And if it can't even maintain a comfortable cabin temp, something is wrong. Tell them to give you a different machine. I would make them fix the problem! That should not be your burden.
I agree, but first I have to get them to admit there is a problem. First time they sent it back when the vents were blowing 60 degrees, they said that was normal. In order to get them to look at it the second time I had to tell them what the service manual said, vent temps should be between 36 and 53. We'll see what happens this time. But if they don't/won't fix it I will contact Bobcat.
 

Doug580l

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I don't think you tried this per my previous recommendation, but if you are hooked on again, put cardboard over the condenser and see if that causes head pressure to rise. You should see liquid line temp increase and head pressure.
I will do this the next time I test.
 

mitch504

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Did I see you say that when the compressor cycles off, there was still a big difference in the 2 pressures?
 

Doug580l

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Did I see you say that when the compressor cycles off, there was still a big difference in the 2 pressures?
Not 100% sure what you're asking. When the compressor cycles off, over about a 5 or 10 second period of time, the low pressure rises to about 46 and the high pressure falls to about 120 before the compressor kicks on. Once the compressor kicks back on again, the pressures are a few psi higher than the average for a few seconds and then they drift down a little and hold fairly steady. So, for example when the compressor kicks on, the low pressure might be 24-25 initially then it falls to around 20-22 and the high pressure goes from about 190 to about 185.
 

fastline

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Not 100% sure what you're asking. When the compressor cycles off, over about a 5 or 10 second period of time, the low pressure rises to about 46 and the high pressure falls to about 120 before the compressor kicks on. Once the compressor kicks back on again, the pressures are a few psi higher than the average for a few seconds and then they drift down a little and hold fairly steady. So, for example when the compressor kicks on, the low pressure might be 24-25 initially then it falls to around 20-22 and the high pressure goes from about 190 to about 185.

I think Mitch might have been trying to assess leakdown at the compressor. However the TXV in these does not fully shut off plus the receiver would null that test in any case. It is by design that when the compressor kicks off, refrigerant continues to circulate at some capacity. As the compressor cycles, pressure will start to equalize, SH will go up, evap temp will go up, until it cycles back on again.

Doug, keep us in the loop. I'd love to be on this job. I love the ones people can't figure out. Easy ones are just too easy.....lol
 

mitch504

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I thought you said when you just walked up to the machine and the compressor was off, there was that much difference in pressure, thanks for clarifying that.
 

Doug580l

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I think Mitch might have been trying to assess leakdown at the compressor. However the TXV in these does not fully shut off plus the receiver would null that test in any case. It is by design that when the compressor kicks off, refrigerant continues to circulate at some capacity. As the compressor cycles, pressure will start to equalize, SH will go up, evap temp will go up, until it cycles back on again.

Doug, keep us in the loop. I'd love to be on this job. I love the ones people can't figure out. Easy ones are just too easy.....lol

I did the cardboard over the condenser test.

It was about 98 degrees out and humid. Heat index 116.
Before the cardboard. Low pres. 24, High 197. This is after about 15 minutes with the gauges on.
After 18 seconds, Low 28, High 300
After 45 seconds Low 30, High 350. Still slowly rising at this point, but I removed the cardboard. I didn't know how long to leave the cardboard on.

Is this the results you were expecting?

Also, I ran the machine for about 6 hours before doing the test. For the first hour or so, the vent temp averaged around 56. After that, by 930 or so I needed the blower on full and the vent temps varied from about 59 to 62. When I quit running the machine, around 330pm, the vent temp was pretty close to 60 and the cab temp was around 84.
 

fastline

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I did the cardboard over the condenser test.

It was about 98 degrees out and humid. Heat index 116.
Before the cardboard. Low pres. 24, High 197. This is after about 15 minutes with the gauges on.
After 18 seconds, Low 28, High 300
After 45 seconds Low 30, High 350. Still slowly rising at this point, but I removed the cardboard. I didn't know how long to leave the cardboard on.

Is this the results you were expecting?

Also, I ran the machine for about 6 hours before doing the test. For the first hour or so, the vent temp averaged around 56. After that, by 930 or so I needed the blower on full and the vent temps varied from about 59 to 62. When I quit running the machine, around 330pm, the vent temp was pretty close to 60 and the cab temp was around 84.

HOLY ****!!!!! I guess I forgot to mention to just try to get the high pressure to climb to a normal pressure. I would have pulled it at 300! Oh well, nothing blew up I hope???? LOL What we know now is that compressor is likely good and that is another card towards "it needs more charge". It really seems odd so either the shop can't weigh a charge properly, the OEM has specified the wrong amount, or some other factor.

With that info, I would feel more confident in adding some charge. Still pretty crazy that the amount could be wrong, so if you decided to go that route, remember to add, then wait, then add, then wait. When you get in a rush, you overcharge equipment. It can take several minutes for things to balance out.
 

Doug580l

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HOLY ****!!!!! I guess I forgot to mention to just try to get the high pressure to climb to a normal pressure. I would have pulled it at 300! Oh well, nothing blew up I hope???? LOL What we know now is that compressor is likely good and that is another card towards "it needs more charge". It really seems odd so either the shop can't weigh a charge properly, the OEM has specified the wrong amount, or some other factor.

With that info, I would feel more confident in adding some charge. Still pretty crazy that the amount could be wrong, so if you decided to go that route, remember to add, then wait, then add, then wait. When you get in a rush, you overcharge equipment. It can take several minutes for things to balance out.

Lol, oops. I didn't think about how high the pressure would go or how quickly once it was blocked.

I think I will wait until the dealer works on the machine again before I try adding any refrigerant to it or replacing anything. They were supposed to look at it this week but now say it will be next week before they service it.

Thanks
 

fastline

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Lol, oops. I didn't think about how high the pressure would go or how quickly once it was blocked.

I think I will wait until the dealer works on the machine again before I try adding any refrigerant to it or replacing anything. They were supposed to look at it this week but now say it will be next week before they service it.

Thanks
I was just in mine and remembered something. My system data tag is in kg. Maybe someone doesn't math real well? You might run the math on yours and confirm how much they are adding to see if it pencils out.
 

JD955SC

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I’ve seen OEM tags wrong, OEM service info wrong, and machines go wrong (get out of calibration)

One memorable machine I worked on listed 2.5 lbs on the tag, 3 lbs in one section of the manual, and 5.5 lbs in another section.
 

Doug580l

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I’ve seen OEM tags wrong, OEM service info wrong, and machines go wrong (get out of calibration)

One memorable machine I worked on listed 2.5 lbs on the tag, 3 lbs in one section of the manual, and 5.5 lbs in another section.
I will add that to my list of questions and things to check for them when they look at it next week. I don't know as much about ac's as most people here, but it seems that the symptoms all point to low freon levels.

I was using the machine a couple days ago and it was in the low 80's out. After the first hour or so the vent temps would vary between about 50 and 60 degrees. So I had to keep changing the blower settings as it got warmer or colder inside. In any of my other machines I would of had the blower on the lowest setting and maybe adjust the temp control here and there. But with the bobcat it's always set on the coldest setting unless it's 70 outside.
 
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