jakehaab

Air duct vents have poor air flow fan works on all speeds PRE AC OVERHAUL

8 posts in this topic

I am getting ready to do a ac system overhaul. I have had this 91 rd688s for 6 years and dont remember if the ac ever worked or not but 6 years of no ac has been enough. A line came off the compressor and the system became open so i am going to start by changing the compressor reciever drier and all hoses and see it it holds pressure...but before i tear into all of that when i switch on the max ac from the control panel and fan on high ( i hear a significant air leak but.. ) the air flow that comes out of the vents is minimal to nothing. What should i check before i start tearing everything apart...this hvac control is relativly new and installed by mack a few years ago. 

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Look close at the air blend doors or their actuators. They direct air flow to the floor, dash vents and the defroster outlets. Some units also divert air from heater/ ac box out side air intake to recirculate settings.

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Also replace the evaporator right away. More than likely it's plugged up with dust, hair, bugs and God knows what else. Keeping the inside of the cab clean helps keep the evaporator clean. You could pull it out and clean it but as long as it's out you might as well replace. Here's a list of what i replace. 

1. Compressor

2. Expansion valve

3. Receiver drier

4. Evaporator

5. Pag oil

Replace lines if needed otherwise flush the lines and condenser with a good a/c flush and nitrogen. I usually put 6 to 9 ounces of oil in the condenser, a few in the drier and evaporator and ALWAYS drain all the oil out of new compressor and put in the right amount. Usually 7-9 ounces. I have seen new compressors shipped empty when there was a tag on them that said filled. I have also seen them with 12+ ounces in them. Remember to oil all o-rings and crown system overnight. 

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Low air flow at one vent could be something simple like the hose came off inside the dash.

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Also replace the evaporator right away. More than likely it's plugged up with dust, hair, bugs and God knows what else. Keeping the inside of the cab clean helps keep the evaporator clean. You could pull it out and clean it but as long as it's out you might as well replace. Here's a list of what i replace. 
1. Compressor
2. Expansion valve
3. Receiver drier
4. Evaporator
5. Pag oil
Replace lines if needed otherwise flush the lines and condenser with a good a/c flush and nitrogen. I usually put 6 to 9 ounces of oil in the condenser, a few in the drier and evaporator and ALWAYS drain all the oil out of new compressor and put in the right amount. Usually 7-9 ounces. I have seen new compressors shipped empty when there was a tag on them that said filled. I have also seen them with 12+ ounces in them. Remember to oil all o-rings and crown system overnight. 


Did you notice where the low pressure cut out switch was when you were working on yours...im hearing its suppose it be on the firewall or on the reciever drier and its in neither of those locations. There is no power going to the wire that goes to the compressor and dont know if that wire comes from the low pressure switch or not. ( havnt done a full investigation into that yet ). But couldnt imaging the system doesnt have one.

Sent from my SM-G920V using Tapatalk

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Low pressure will be either on the dryer on the line around it. High switch is on the compressor. 

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The low switch is no where to be found in the engine compartment..not even wires anywhere that could possibly be to the low switch...could it be that it opperated with out one? I need to trace back the wire that i have plugged into the compressor and see where the heck it goes

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On the passenger side, standing on the ground, there is a rubber sleeve that couples the A/C blower discharge duct to the inlet side of the evaporator, a Phillips screwdriver and a few minutes time removing the inlet duct(slide the rubber to the right or left to allow the removal of the duct, can't remember which way) and your staring at the inlet side of the evaporator, which I'll bet a donut on, is nearly 100% plugged. Clean it with a vacuum or wash it in place. One of my old drivers likes riding with the window down, A/C on, dusty air through a cold evap coil, damp from the S.Louisiana air makes a muddy mess! I have to clean his every few months, complaint from him? A/C not hardly blowing

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