200! Mack Truck

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200! Mack Truck

Postby Mamrak76 » Mon Mar 27, 2017 2:44 am

I have a basic system with Sanden compressor, recover dryer, txv, condenser, and evaporator. I hooked up and while running had vacuum on the low side and 140 on the high side. I thought it was a clog and replaced the txv. It held vacuum for about 20 mins so no leaks as far as I know. I started recharging and the low side looked good. I shut off my supply of r134a and it went back to vacuum on the low side. I just need some suggestions. All the research I did pointed to txv but that's brand new. The compressor isn't making noises and sounds like new. Thanks in advance!
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Re: 2001 Mack Truck

Postby Tom Greenleaf » Mon Mar 27, 2017 6:00 am

Welcome:

Some basics here first: Do you have a system capacity that is known? With a high side of 140 and low pulling vacuum at a glance suggests it's way undercharged.

If you don't know capacities and lots of new parts you do need some educated guess and fill to something credible erring to lower charge than perfect first and it is difficult or takes time with dependable ambient temps to tweak system to some known fill capacity.

You really need to measure amount by the ounce put in from a totally well held vacuum. What did you use to vacuum system and what reading did you get that held?

Just FYI a full vacuum is 29.92Hg at sea level and can adjust one point for each 1,000 ft. of elevation you are at down a point.

Hope he's by soon and well versed in trucks of the sort meaning "Mack" not an automobile set up but much the same principles apply.

This link is also above in the "Knowledge Base" or can click on this..........

http://aircondition.com/tech/questions/ ... Procedures

That's some great info on charging a system that's intact and ready for the most part and worth the read,

Tom
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Re: 200! Mack Truck

Postby cornbinder89 » Mon Mar 27, 2017 12:07 pm

Day cab only or does it have a sleeper unit? I concur with Tom, how much are you putting in and what does it call for? I don't have spec's on stuff that new, but my guess is the same as Tom's that you need more refrigerant. Tx valve systems can take and store more than orifice tube systems, and are a little more forgiving of the amount in the system than are orifice tube.
If you don't know the amount, and it has a sleeper system, run both while adding. 24 oz wouldn't be out of line and it could run even higher.
Another place that can have a restriction is the drier or even the condenser. If the high tap is after these, your high side reading can be thown off by a restriction before the tap. Feel the lines, if the high side is hot out of the compressor, but cools before the condenser, you could have a restriction.
Is the engine fan running when you are taking you readings? it is on all the time or cycleing? Do you have the engine fan in manual or automatic (if it is equiped with a manual override switch)?
Also, what is the temp where you are working? if it is cool it will be hard to get it right without an adequate heat load on the system.
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Re: 200! Mack Truck

Postby Mamrak76 » Mon Mar 27, 2017 12:48 pm

System capacity if 2 pounds 0 ounces. It is a day cab MR688S. 200k miles. The first thing I did when I saw negative low side pressure was add 30 ounces pf r134a and dye just to get a baseline. I had no response with low side pressure. I already had a New txv so I swapped t out and pulled a vacuum and it held. Then I recharged. The high was at about 50 and it came up but the low side stayed below 20 inches.
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Re: 200! Mack Truck

Postby Mamrak76 » Mon Mar 27, 2017 1:01 pm

Also the fan is a viscous fluid and does cycle when the correct temp is reached. It kicks on at about 180 degrees. I felt the lines the only place the lines felt hot was nearest the compressor. I have a proper refridgerant scale (mastercool) and I consider myself knowledgeable. I have seen clogged reciever dryers and accumulators and they normally throw high high pressures like 350. This is throwing me for a loop.
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Re: 200! Mack Truck

Postby Tom Greenleaf » Mon Mar 27, 2017 2:24 pm

Mamrak76: Sorry for the delay in your posts showing. Site set up it 1st post doesn't till approved and I'm the only one here for that!

Cornbinder is well versed at this for specifics will help more than I can.

You were close enough for better readings with that known capacity so yes a restriction/clog and why?

I'd be looking for type of clog in what's in the oil of the receiver dryer for clues. Unfortunately debris is almost always from the compressor, many times just doesn't quit on time when charge is too low oil fails to circulate well enough,


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Re: 200! Mack Truck

Postby Mamrak76 » Mon Mar 27, 2017 10:08 pm

Thanks for the replies I will replace the recover dryer and just for good measure flush out the system. Will update when I get it done.
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Re: 200! Mack Truck

Postby cornbinder89 » Tue Mar 28, 2017 1:23 am

If the high side pressure tap isn't at the compressor, but further down the line, you could have a restricted condenser or receiver, and if the high pressure tap is after the restriction, it can be mis-leading. We recently went thru that with another thread, the fitting was blocked.
With the low side going into inches, the compressor is moving refrigerant, but it seams slow to make it back around to the suction side.
With the correct amount in the system, and the compressor moving refrigerant, it almost has to be a blockage somewhere.
Receivers are supposed to be sized such that they can hold the entire charge within the receiver in liquid form. For it to do that, the pressure has to be high enough to keep it liquid.
If you can feel the hose/tubing, you want to start at the compressor where it is hot, and feel along until you feel a marked temperature change, and that will point to the restriction.
When all else fails, I add ( Piercing) taps to the hard lines so I can take pressure reading at several point in the system, once the problem is resolved, I replace the sections that I added taps too. It seams drastic but can save you chasing your tail and replacing things on a hunch.
If you have a charging scale and an amount, you are way ahead of most people who have problems. At least you KNOW what is in there and what should be.
I will add that I am trucking, and only check in once a day if that, so If I don't respond right a way, I will at some point.
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Re: 200! Mack Truck

Postby Mamrak76 » Tue Mar 28, 2017 2:52 pm

Ok thanks for the help. I also am driving everyday and the repairs are done in the evening. I will use my temp gun and check around. Also the high and low are both about 3 inches from the compressor. My hope is that will give some direction and when I change the reciver dryer, I plan to do an inspection. I had a Ford Escape last year that had an exploded desiccant bag, but it was a simple repair. On the bright side all of the lines and components are very easily accessible and easy to remove. Which is a luxury these days!!
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Re: 200! Mack Truck

Postby Tom Greenleaf » Tue Mar 28, 2017 3:16 pm

Cornbinder will prevail at knowing what you are looking at tons better that I will. Real trucks if you will.
Just some comments: Heat gun - great tool if you know it's active pressure not just stuck temp = the pressure right there. Quick drop offs where there's no reason can be found.

Sanden and the top manifold if so with both ports side by side. Some can be those ball valves stink. Check - spit test or what works they aren't seriously losing charge and another here will say a good cap is expected to be the final seal. Many will just bubble a tad.

Fun (or not fun) facts - PAG oils when exposed to moisture will become acidic. You mentioned an exploded desiccant bag - could be the reason as I never found that very common.

Some other for debris checking: Drill holes in items or cut them open to both see how much oil comes out and the condition.

Said earlier and I'll stay out as you seem to have a good clue. Just know oil with low charge doesn't move if it can't make cool it's NOT making the loop back to compressor. Stinks but can waste a new or good compressor quickly from lack of lube even though you said it was quiet. Your pressures still showed it worked not like nothing so try not to allow wrecking it if possible.

Good luck. I'm doing the heavy looking on mostly, Tom
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Re: 200! Mack Truck

Postby cornbinder89 » Wed Mar 29, 2017 1:34 am

I am a little confused by the low high side pressure reading. I could see a drier clog, but you would expect the high side to rise more if that were the case. Right now it seam to pump all the refrigerant from the low side than stop when there is no more to pump.
It would tend to make the high side go quite high. I wouldn't start replacing stuff until you can empirically test it in the system and prove it bad.
I would want to be sure the high side gauge is really reading what is happening at the high side port.
I suppose, if there was a restriction/plug in the low side line very close to the compressor, the high side wouldn't rise much, as very little volume is being pumped out of the line, between the blockage and compressor (very small volume between the blockage and compressor), so very little pressure rise in the high side.
If you are going to open the system anyway, why not break it completely apart and blow thru each hose/component to check for a plug. Once you have done that, you can either re-assemble and evacuate or replace the drier and than evacuate.
Low high side combined with high low side, I would say a bad compressor, but low high side and low low side, says the compressor is trying to pump, but there is nothing on the low side to pump. In most cases, that would indicate a low charge, but since you measured out and put in the correct charge, there must be something else going on.
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Re: 200! Mack Truck

Postby Mamrak76 » Thu Mar 30, 2017 12:47 am

I changed the reciver dryer and vacuumed down. I added about 25 of 32 ounces and I have -20 low and 90 high. It is about 65 outside. The only place that feels cold on the system is the small diameter high side line on the condenser. Everything else feels neither hot or cold. I am a very confused
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Re: 200! Mack Truck

Postby cornbinder89 » Thu Mar 30, 2017 1:34 am

Inlet of the condenser, or outlet? It the condenser is cold anywhere, it would indicate that there restriction just ahead of where it starts getting cold.
I'm still baffled by the relatively low high side reading, with what appears to be a restriction in the system. Are you sure your getting the refrigerant into the system?
A compressor that was bad may not see much pressure on the high side, but would not be able to pull the low side into the inches of mercury range either.
Do you have a core removal tool? I'm talking about the type of tool that allows you to pull the valve core without discharging the system. I'm wondering if your high side is not reading what is truly happening. Pulling the core and reading the high side without the core in the way would be where I would go next.
If the condenser is truly getting cold, then I would suspect the condenser, but I would want the pressure readings to confirm it before I would replace condenser. This would mean a pressure reading at the inlet and outlet of the condenser;
It is why I suggest using piercing taps on hard lines to check pressures in several places in the system. Once I have fixed the problem, then I would replace any hard lines that were pierced.
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Re: 200! Mack Truck

Postby Mamrak76 » Thu Mar 30, 2017 8:27 pm

Yes I do have a removal tool. I tried to remove the low side valve but it wouldn't come out! I can change the high side. Any suggestions! It will turn and I could feel the threads but it won't remove.
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