Freightliner mechanic, needs specific help on AC pressure switch

[Rick10]

We added half a can of R134a, got the cycle time up from 7 seconds to 13 seconds and quit, the register temperature was about 57F, with the cab in the sun outside temp was ~80F. High side pressure was low 100 psi, high 125, as it cycled off the low side pressure, 40 psi on, 23 psi off. My truck driver friend is happy with that. If we have any more issues, we will dump the charge, pull a vacuum, and recharge to the 42 oz target. Thank you guys for the help, I can see these truck systems have similarities to car systems, and yet they are different.

 

[mndriver]

I get my cooling to 45°F below ambient temperature. So on a 90°F day, it's still cooling to 45°F

Normal center vent temps are 37-38°F

 

[Injun]

We had a guy here a few years ago who put pipe insulation over all of the AC lines under the hood. He was getting low 40s at the vent after that.

I tried it with my ol' S***Spreader. It did make a difference.

 

[cwby008]

Holy frostbite Batman......

Both my switch's are on my receiver dryer just above my steer axle.

Which switch is the low and which is the high on yours mndriver mine is the same?

 

[389 Hood]

Look at the piping that the particular switch is attached to. The larger diameter pipe is the low side, the smaller pipe is the high side. If the switch is stuck try tapping it lightly. Corrosion gets into the internal contacts. That may get you cold air until you can replace it. You now have to figure out which switch isn't operating.

 

[cwby008]

On the drier they both are small lines going to it only difference in the 2 witches in it one has a plug in on it for my plug the other has a built on pig tail that goes to a switch?

 

[389 Hood]

My bad. You said drier and I was thinking of the block going into the firewall to the evaporator. On that there are two switches and the expansion valve.
I can't think of why there would be 2 switches at the drier. One there and the other on the low side of the system. One may be a switch that shuts the system down if there is too much moisture or the desiccant bag is too wet. I'm not that familiar with the Freightliner set up.

 

[389 Hood]

Found this on a search. We used to have a guy here that was a mechanic turned driver, turned mechanic again.

https://www.justanswer.com/medium-and-heavy-truck/8gjig-freightliner-columbia-a-c-problem-truck-system.html

It seems both switches are on the drier. How, I don't know, but I think one is a compressor cycling switch, the other the high pressure cut off. The low pressure switch will shut off the compressor when the charge is too low to prevent seizing the compressor.

 

[cwby008]

I had a buddy put his gauge on and it spike it showed to much refrigerant. The ac is just blowing warm air. We bought the truck it work fine then when we used it year later the ac it doesn’t work. So I went to Freightliner and ask for the high pressure switch. They gave me a sensor you plug my wire harness into. So I change that one and it does the same thing. So then I look up more and I hear the one with the built in pigtail could be the high pressure but Im not sure so I don’t want to spend another 45 bucks for it being wrong.

 

[389 Hood]

There are all sorts of schematics and diagrams if you do searches.

Why not just have an A/C shop check it out for you? You'll be better off in the long run. Throwing parts at a problem gets expensive. Especially when you don't have the proper diagnostic equipment.

 

[cwby008]

Can’t afford a simi mechanic. This is my rv toter thought I would just figure it out and pick people’s brain see if they might know which is which

 

[389 Hood]

Any A/C shop should be able to trouble shoot the problem.

 

[bluewolftrucking]

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.

Do you need to drain the system to replace a high pressure switch for the two thousandand seventeen freight liner cascade

 

[389 Hood]

I would say yes, to both the high and low pressure switches. They both take pressure readings internally. Unscrew one or the other and you'll get a refrigerant blast in the face.