Fleet Maintenance continues its series based on technician training sessions presented at the Technology & Maintenance Council (TMC) 2004 Fall Meeting. In this issue, Tony Thorne of Index Sensors & Controls and Jim Guthridge of CCI Compressors discuss the care and maintenance of heavy truck air conditioning systems.
The compressor has two functions: to pump through mass; and to build pressure. And it only does what it's told—everything else that happens is just a by-product, because the pressure goes up, the temperature goes up. That's just physics."
With that statement, Jim Guthridge, chief engineer for Decatur, IL-based CCI Compressors, introduced his training session on air conditioning troubleshooting to his audience at the Technology & Maintenance Council (TMC) Fall Meeting.
SYSTEM FITTING LEAKS
To illustrate how easy it is for the A/C system to lose efficiency, Guthridge shook a bottle of Diet Coke and then tried to pour the fizzy soda pop through a narrow tube. To the surprise of the audience, the liquid could not pass, because the foam bubbles blocked the tube.
"The critical point for R134a is 213.9 degrees F, 101.1 degrees Centigrade; above that temperature it is always a gas," he explained. "What the underhood temperatures are getting to these days, 213.9 degrees is not that hot in the engine compartment now. So you keep getting the liquid to 213.9 and above, it starts changing to a gas right then and there; it's no longer a liquid. It foams as bubbles are created.
"When you go up to the expansion device, whether it's a Thermal Expansion Valve (TXV) or a Cycling Clutch Orifice Tube (CCOT), bubbles will block those gaps," he said. "A TXV only opens up about 15 to 16/1,000ths of an inch, and an orifice tube is pretty small, too. So, the reason I used the Diet Coke, is (to show) the difference in mass flow between foam going through a small gap and liquid going through a small gap in the same length of time.
"A TXV expansion valve needs 15 degrees subcooled liquid delivered to it, or you can get bubbles or foam that restricts flow, and restriction will cause a lot of problems," he explained.
"We're not going to get rid of the (underhood) temperatures, but there are places where you have insulation to protect liquid, so it doesn't pick up the heat as it moves through," Guthridge said. "If you have just a rubber hose and a metal fitting, where the underhood temperature is 240, that metal fitting is going to transfer a lot of heat to that liquid that runs through it."
HEAT KILLS
Those high underhood temperatures were behind the problems one of Guthridge's fleet clients had with intermittent cooling from its air conditioners. "He just couldn't figure it out: sometimes it would work fine, and sometimes they just couldn't get cool enough," he said.
According to Guthridge, the refrigerant was foaming before it reached the expansion device.
"You start having poor operation as long as that high temperature condition exists," he said. "Once you've got the bubbles forming there, then you're going to have reduced cooling. You're go-ing to have more compressor operation, because the system's calling for cool, but the bubbles are keeping the flow rate (pounds per minute) down. That runs the compressor temperature up, because you're causing a semi-deadhead condition. Reduced flow means reduced cooling."
Guthridge suggested his client use Tempilabels to isolate the problem. "We stuck them right on the receiver dryer, and each one of those trucks had the heat stickers on when they came back from California, and all those dots fired off, up to 250," he said. "Well, 250 is significantly above 213.9. So, you can see the possibilities."
Guthridge's suggestion for intermittent cooling? Use Tempilabels to identify hot spots, and add insulation and heat shielding wherever possible.
CHECK THE REFRIGERANT
Tony Thorne, engineering manager for Stanwood, WA-based Index Sensors & Controls, cited research his company conducted into A/C failures with Silver State Materials in Henderson, NV, and CR England in Salt lake City, UT. A study of both fleets ashowed that 92 percent of air conditioning system failures could be traced to low refrigerant levels.
"You could see the compressor in one that was just a year old," he went on. "The clutch was already starting to warp from the high slippage rates. It was a three pound system but it was down to only a pound of refrigerant when we tested it. What the research shows is, low refrigerant equals low lubrication equals damage. Once you get the damage, your whole system is now affected.
"So 92 percent of the problems we found were attributed to low refrigerant, and the other eight percent were based on the system, like voltage, current, fuses, wiring or connectors. Most of those failures were attributed to bad grounds, which can drive the troubleshooters crazy trying to find the problem.
"What these guys learned from working with us was, the first thing I'm going to look at is refrigerant," Thorne said. "There are some obvious things, like, if nothing happens, check the fuse. But now it's standard operating procedure for Silver State that when a rig comes in, the first thing they're going to do is check the A/C refrigerant charge.
"I would say whether it's a CCOT or TXV, low refrigerant charge is the most general and common failure that we saw," he said. "The orifice tube and expansion valve are what meter the refrigerant to create the vapor in order to absorb heat to create that refrigeration cycle. So usually when you get debris into the system from a damaged compressor, they seem to work for a while then they start acting up, and yet the compressor seems to be working.
"So, if a compressor fails and is replaced with a new one, you cannot get all the metal debris out of the system, and those are the first things that can plug or restrict the expansion valve or orifice tube, and once you restrict them you don't get the cooling action."
"PHANTOM COOLING"
"On a low refrigerant charge, the air conditioning system will produce cool air, but not real cold air, which is hard to detect everyday use. This cool air—it's called 'phantom cooling'—is apparent all the way down to just under a pound of refrigerant," Thorne explained. "The cycle rate increases up to 34 times per minute below one pound, but you still feel cool air coming out. So by the time they realize that it's not quite performing the way it should be, damage has already started to the clutch, compressor, lubrication, bearings, and even the front seal. This depends on how many times that clutch has been closing, and how that truck is utilized.
"What is the design intent of the A/C compressor?" he asked. "The clutch is made to operate about six times a minute; that's a fully-loaded A/C system with solar load on the cab, window open, under normal operation. When you get up to 15 to 20 cycles a minute, you're actually three times it's design rating. Some clutches were measuring at 40 clutch closers per minute, and you could actually see the clutches were starting to warp from the friction and heat.
"So the damage can be in three different places; it can be in the clutch; it can be in the front seal and bearing, and it can be in the lack of lubrication to the seals inside the compressor itself.
"The notion is: diagnose and repair, or prevent," he explained. "When you have a three-pound system and you're down below one pound of refrigerant—a third of the required refrigerant—the lubrication is carried in the refrigerant. Unfortunately, if you run the clutch all the time and it cycles, a vacuum is created on the front end of the system, and you're never going to get that lubrication through. If you limit that to six times or less per minute the system equalizes, so now you do get a little bit of charge of refrigerant and lubrication through the system. Because all that pressure bleeds back down to normal, the clutch closes, spools back up, draws lubrication back through it, and then limits the clutch cycle again. So the damage that occurs without any protection can happen almost exponentially, whereas when you have an A/C protective device it stays within the design limits without damage."
THE OTHER EIGHT PERCENT
Then there were the other eight percent of the A/C failures; what caused them?
"The main factors we saw were overloading the electrical system at idle, creating low system voltage, or bad grounds, so there was always a low-voltage condition," Thorne explained. "The other problem is high voltage, where either the batteries are bad or the impedance to the battery connection was high, someplace where the alternator was putting out higher than normal voltage, which would overheat the clutch coil.
"The voltage is running down around 10 or 11 volts, and they start up the truck to recharge it, the A/C compressor would kick in, and a low-voltage problem causes the clutch to slip a lot more," he said.
"Low voltage can also be attributed to ground connections, so even through the system's putting out the right voltage, if the ground to your compressor is bad, the compressor may be seeing only half voltage or less. You can hear that as a kind of weak 'THUD' and see it slip when it spools up. We disconnect everything and take a meter measurement and find we have 10 or 15 ohms of resistance where there should be only three or four ohms of resistance. Come to find out a water pump was just replaced, or just replaced something else where brackets and things got moved around and lost the good connection."
SUMMARY
"Another system effect that we had seen was warranty replacement reports on replaced compressors," Thorne said. "After damage had occurred on a compressor, the compressor was replaced, and two or three months later they were having problems again. An investigation found that metal debris had gotten into the A/C system and either clogged the orifice tube or TXV valve. Those are indications that once they go in and replace the compressor it doesn't guarantee that you're going to fix the problem. You can't always get all those metal particles out of there."
"After a compressor failure, properly flushing a system with the correct fluids, replacing with the new compressor, changing the receiver dryer, and then a complete deep vacuum will typically help reduce repeat failures, but still you never really know if all the particles came out of the system. It makes better sense to just prevent the failure event from happening in the first place, rather than trying to fix it later and hope all the residue came out of the tubes, condenser or evaporator.
