The Diesel Stop banner
1 - 8 of 8 Posts

·
Registered
Joined
·
8 Posts
Discussion Starter · #1 ·
2007 F250, 6.0 I just replaced the evaporator and accumlator. I vacuumed the system and refilled the system with refrigerant. The ac works great, I'm getting a duct temp of 35 degrees. I Decided to check the pressures with my manifold gauge set. Ambient temp is 70 degrees. After hooking them up I turn on the low side and get 38 on the low side and 150 on the high side. I'm getting this reading with only the low side open, the high is closed. When I open the high side the low side will spike to 120psi! When I turn the high side off it goes back down to 38 and 150. Should I be getting a reading on the high side gauge even when the gauge is closed? Are my gauges defective or do I have another problem? I don't think I overcharged it, the sticker under the hood calls for 2.82lbs, I used the 12oz cans so I put in 3 cans and using a refrigerant scale 9oz out of the last can for a total of 45oz. Any help would be appreciated!
 

·
Premium Member
Joined
·
15,107 Posts
The valves on the gauges aren't there to open when you want a reading - they are there to open the high or low side to the inner chamber (the charge port) on a 3 way (two valve) gauge set. So - you wouldn't ever open the red valve on any normal charging scenario. When you open the blue and red valves at the same time, you are effectively shunting high side pressure into the low side port - that's why you're seeing the low side "spike".

Keep both valves closed when you are doing your pressure readings. Only open a valve when you are charging or when you were vacuuming the system and then ONLY when the center (yellow) line is connected to either the vacuum pump or your refrigerant source.

Things get even more fun when you have a four valve setup like mine....
 

·
Registered
Joined
·
8 Posts
Discussion Starter · #3 ·
Thank you for explaining it. This is my first time working on an ac system, this is a new set of gauges and it didn't come with any instructions. I hope I didn't do any harm by letting it spike, it was only for a few seconds? When I'm reading the gauges, which one do I go by, when the compressor is running or when it's not? At 70 degrees I'm getting 38 on the low side and 150 on the high with compressor off and 24psi low side and 175 high side with compressor running. How do these numbers look? Thanks
 

·
Premium Member
Joined
·
15,107 Posts
Numbers are fine. Only compressor running numbers are useful. True charge is based on a superheat calculation, but no one does that on MVAC systems. Going by weight like you did is the best way. When the compressor stops, the values will equalize over time.


Sent from AutoGuide.com Free App
 

·
Registered
Joined
·
8 Posts
Discussion Starter · #5 ·
Thanks alot. I will leave it alone. It seems to be working fine. For future reference what is a good range on the gauges to have.
 

·
Premium Member
Joined
·
15,107 Posts
Lots of factors- temperature across the condenser, wet bulb temperature in the cab, air flow, at both sites- generally those affect the high side values the most. Low side will generally be in the 30-35 psi range. Make sure it isn't cycling too much when you get it on the road. You may have lost a little refrigerant in your lines.


Sent from AutoGuide.com Free App
 

·
Registered
Joined
·
8 Posts
Discussion Starter · #7 ·
How often should it cycle? With it idling it will run for around two minutes and then off for a few seconds and back on. Should I add a little more refrigerant? Seems like the bottom number is a little low. I have about 3oz left in one can.
 

·
Premium Member
Joined
·
15,107 Posts
I'd go ahead and put that in. It won't keep in the can and you probably lost that much in the lines. Hook your can up to the yellow line. Open the can valve. Crack the line at the gauge for a second to vent the air that wa sin the line. Then open your blue valve with the compressor running. Heating the can and shaking it will help get all the gas out.


Sent from AutoGuide.com Free App
 
1 - 8 of 8 Posts
Top