Freightliner mechanic, needs specific help on AC pressure switch

Hi, I'm new to the site. I'm looking for help finding a cycling pressure switch for a 2006 Freightliner Sterling, with a Mercedes Benz engine. Looks like the attached photo....any help will be appreciated. I can get the system to charge with R134A, but the low pressure side goes too low (10 psi). I can only get the compressor to come on by shorting this switch on the connector wire side.
 

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mndriver

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Holy frostbite Batman......

Both my switch's are on my receiver dryer just above my steer axle.
 
Thanks for the feedback. This switch is on the passenger side. I think the tank is an accumulator, low pressure side reservoir. All I need is the switch. I'm not sure if I take it off if there is like a Schrader valve underneath it so I don't blow the refrigerant charge. You are up early, have a good day.
 

mndriver

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There's no shrader valve in them that I'm aware of. Mine were actually part of the actual dryer.
 

GAnthony

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Hi, I'm new to the site. I'm looking for help finding a cycling pressure switch for a 2006 Freightliner Sterling, with a Mercedes Benz engine. Looks like the attached photo....any help will be appreciated. I can get the system to charge with R134A, but the low pressure side goes too low (10 psi). I can only get the compressor to come on by shorting this switch on the connector wire side.
i have read a few threads in the past where some people can get these parts at NAPA or even AutoZone.....

as so many times, these are just standard A/C systems

take the switch off, and take it to them?
 

r3gulator3

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Sterling parts are ford parts. You could have luck matching it up at a ford dealer. That system runs a low pressure switch on accumulator and a high switch/ fan switch on the high line.

Sterling was fords heavy duty line until Freightliner bought them in the late 90s.
 
More info. The label says this system takes 42 oz of R134 A. I got a new low pressure cycling switch at Premier Truck in Knoxville, $17. Took the old one off, and it did not drop charge, because it was mounted to a Schrader valve. It worked good for a long time, pumping lots of cold air, and cycling between 43 and 22 psi, in a 15 second off to off cycle time. The high pressure side was reading about 125 psi, which from my experience is pretty low, but there is a huge plastic fan pulling in lots of outside air, so I assumed it was just an efficient system. Then we added 3 oz of oil at the recommendation of the driver's son who used to work on AC compressors. From then on, the suction pressure was fixed at about 50 psi, and not cycling on the suction side. Still putting out lots of cold air at the registers. The the compressor continued to cycle, but you could see now that it was responding on the high pressure side, like going from 100 psi to 125 psi and shutting off. Then on again at 100. The suction pressure just remained at 50 psi. The high pressure gage tap is on the condenser outlet, very close to the evaporator housing entrance. I assume the expansion valve is inside that evaporator plenum, just a little downstream of the hi pressure tap. Then the son said he thought it must be overcharged, because the compressor was being controlled by the high pressure switch. The high pressure switch is close to the compressor outlet, and the high pressure refrigerant gauge tap is AFTER the condenser up by the firewall.

Overall, we did add about 12 oz of refrigerant, because of the high pressure side being so low, but now we are thinking the system may have always had enough charge, just a bad low pressure switch.

I am totally confused, tempted to leave it alone since it is putting out good cooling, but I don't understand how adding the oil seemed to end the suction side cycling, and why the high pressure side always stayed so low, on the gage. Also why the compressor was still cycling, obviously off of the high pressure switch, which is probably not as intended except to protect the compressor from too much pressure. Maybe that high pressure switch is bad too. I never had these strange events working with cars and small trucks. If anyone can make any conclusions from this data, let me know soon, as the driver intends to use the AC system as it is now, cycling off the high pressure switch.
 

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r3gulator3

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The oil has created a restriction in your orifice tube. I would recover the system and put in a vacuum for a few hours then recharge with 42oz. I wouldn’t put anymore oil in it. The pressure you had once the system started running initially was perfect for that set up. The ac clutch cycles off of low pressure. Also if this system hasn’t been ran in a while there could be other stuff clogging the orifice tube. So if vac and recharge don’t get ya back where you need to be then it would be time to get at the orifice tube and likely replace it.

I worked for Freightliner for a lot of years and also worked for Wick’s Sterling trucks as a technician.

R Rick10 the reason the oil changed things is because you created a restriction. And the high pressure being at 125 is completely normal on that system. Everything is ran off the low side there. Only reason for the high switch is to cycle the fan if necessary and to prevent the system from exploding when the orifice tube gets completely blocked or a reed valve sticks in the compressor.
 

mndriver

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Thought these ran off an expansion valve, not an O-tube.
 

r3gulator3

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Thought these ran off an expansion valve, not an O-tube.
Freightliner is expansion valve, sterling being a ford product is orifice tube.
 
r2gulato3, Thanks for sharing your experience and expertise. My trucker friend got the message to not use his AC today, even though it is a very hot day here in Tennessee. I did not like the idea of putting 3 oz of oil in the system, because I have watched it in deliberate lab experiments at the auto company I worked at, where we added oil and measured the system BTU cooling deterioration rate as we added water, and the separately added oil. But that was 45 years ago when I worked on car and light truck AC. That was back when compressors had oil sumps, and little pumps to suck oil up into the swashplate area. I think compressors today don't have oil storage areas, they just make sure to mount them low on the engine, so the oil migrates to the compressor, where it belongs.

Should I consider a system flush, or maybe open the system at the high pressure gage port, while sucking on the low pressure port with my AC vacuum pump, to suck some of that oil out? Although that might be hard on my vacuum pump, which is designed to suck gas, not liquid. I could rig up a jar to catch liquid before it got to the vacuum pump.

I hope I do not have to get into the orifice tube, which must be inside the evaporator plenum.
 

r3gulator3

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r2gulato3, Thanks for sharing your experience and expertise. My trucker friend got the message to not use his AC today, even though it is a very hot day here in Tennessee. I did not like the idea of putting 3 oz of oil in the system, because I have watched it in deliberate lab experiments at the auto company I worked at, where we added oil and measured the system BTU cooling deterioration rate as we added water, and the separately added oil. But that was 45 years ago when I worked on car and light truck AC. That was back when compressors had oil sumps, and little pumps to suck oil up into the swashplate area. I think compressors today don't have oil storage areas, they just make sure to mount them low on the engine, so the oil migrates to the compressor, where it belongs.

Should I consider a system flush, or maybe open the system at the high pressure gage port, while sucking on the low pressure port with my AC vacuum pump, to suck some of that oil out? Although that might be hard on my vacuum pump, which is designed to suck gas, not liquid. I could rig up a jar to catch liquid before it got to the vacuum pump.

I hope I do not have to get into the orifice tube, which must be inside the evaporator plenum.
When you hit 28” of vacuum the oil starts to boil off with the residual refrigerant. That’s why I said pull vacuum for a couple hours. That will boil off quiet a bit.

Your sanden compressor does have a sump. If you install a new compressor it actually comes with all the oil your system needs in it. So when installing a compressor it’s imparative to either flush out everything or to oil balance the system. However the sump works as you think the compressor is mounted below the accumulator so it can catch oil to run back through the system.

I wouldn’t open the system unless a repair is needed. Which may be the case if something other than oil plugged up your orifice tube.
 
r2gulato3,
After the truck was driven for 2 days with the AC off, we thought we would check it again with the gauges before we did anything. Surprisingly something seems to be better, it is cycling off of the low pressure again, instead of the high pressure. Maybe the oil settled out of the orifice tube area down to the low side of the system. We did take some charge out the last time we were diagnosing it, maybe too much, because while it is properly cycling off the low pressure side (on at ~41 psi, off at ~23psi, the on to on cycle time is only about 7.5 seconds, and the register temperature is about 68F. I would call this rapid cycling. Does that suggest too little charge or too much? We had like a 15 second cycle time before we added oil, and blew off some charge.
 

r3gulator3

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Depends on relative humidity, also is this being checked in the sun or the shade?

I would advise at 68 degrees with a relative humidity less than 70% that duty cycle is ok. It is very likely that not operating the system for a couple days did allow the oil to settle. That excess oil should collect in the accumulator over time. But may gum things up down the road.
 

mndriver

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Any system since 1990, about the time the industry switched from R-12 to R-134, I make it a point to charge by weight, any system I work on.

They've just gotten that sensitive to charge amount. A very small amount over or under charged can make the system not functioning like it should.

Because of the inherent problem of R-134 systems not being able to seal well from the size of the molecules in the freon, I've even just gotten into the habit between 18-24 months, and just having the system evacuated and then recharged in the spring before the cooling season really starts.

Yes, I do all my own a/c work on my Century as well. O-tube system I haven't worked on since 1991.
 

r3gulator3

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Yeah o tubes are wierd. Hey by the way mndriver mndriver I put in a call to one of my friends at Wick’s. So sterling actually had both the ford system with the O tube and the Freightliner system with the TXV (thermal expansion valve). Change over was 07 MY. When they started shoe horning DD15s in for Conway and ABF which had Denzo compressors and a completely different electrical system.
 

mndriver

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I just can't justify the expense of a $3000 a/c machine sitting around the shop for a single truck.
 

r3gulator3

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I just can't justify the expense of a $3000 a/c machine sitting around the shop for a single truck.
Set of gauges and a vacuum pump are all ya really need man.
 

r3gulator3

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Ok fine, and a scale. Lol
 
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