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A/C pressure...

Avery4jc

Avery4jc

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The A/C in the k3500 could use a little help and I thought the root of my problem was that it seemed low on R134 (the compressor cycles on and off pretty often) so before I took it in to an a/c shop I decided to give one of those cans a shot...

I fired up the truck and turned the A/C to full blast (like it said) and then connected the hose to the low pressure port...

It starts at 20psi and builds to 45psi... once it hits 45psi the compressor kicks on and it drops back down to 20psi and builds back up, etc.

The can says 45psi is at the high side of where it should be... but does that mean it should be there all the time? When it kicks on and drops to 20psi should I fill it or do they mean if it never reaches the "full" zone (like 30-45psi) that you need to fill it until it can build pressure to be in that zone?


Wow I hope that made sense...

:confused:


Basically everything is physically working on the A/C system but on hot days it never really gets cold (colder than the outside temp but not nearly as cold as the C2500's A/C system gets to and they're literally identical)...
 
The system is still too low. When the compressor kicks on the pressure drops down to 20 and cycles it back off, right? Don't pay any attention to the pressure without the compressor running, it doesn't really matter. Add more 134a (WITHOUT stop-leak) until the low-side pressure is right around 30psi, maybe a little higher. At this point, the air should be cold and the compressor should not be cycling off very often. If you can't get the low side pressure to come up some, there is probably a clog somewhere in the system and the high side pressure will be too high.
 
Ok I'll add more in the morning... thanks for clearing that up.

On a side note I've been hearing a lot about the in cabin filter... aparently they get really clogged up over time and drastically reduce the performance of the HVAC system...

I know the newer trucks don't even run an in cabin filter but I imagine mine does... does anyone know where it'd be? (Treat it like an '88-'98 truck)...
I unbolted the cover in the passenger side floor area and all I found was the heater core...
 
88-98 trucks do not have them. the newer style trucks do from what i remember. but you still got the old school body. so im guessing you dont have one.
 
To correctly evaluate your AC system you also need a high pressure AC guage. When the AC is running the high pressure reading should be 2 1/2 times the outside temperatures. Example: if the outside temperature is 70 F the high pressure reading on the R134A guage should be 175 psi ( 70 + 70 + 35 ). If it is higher than 250 psi on a 70 degree day you have a blockage on the high side. The sysetm has a low and high limit safety circuit that cuts off the compressor - to keep the compressor from self destructing. Some systems also use an electric fan that that turns on at 200 psi and off at 150. If your system has an electric fan , is it turning on ? If the fan does not turn on, the AC will quickly hit its upper limit pressure and cycle off. Check the fan and its relay ( maybe bad ). For fuel economy reasons the factory uses the smallest compressor possible and works the heck out of it! Bottom line - get or borrow a high pressure guage. If your truck does not have an electric fan and the pressure too high - high side restruction ( piece of rubber hose , dirt in system, moisture in system) Have it fixed by a person familiar with AC systems. It's not worth getting hurt for.
 
nerraw117 said:
88-98 trucks do not have them. the newer style trucks do from what i remember. but you still got the old school body. so im guessing you dont have one.
Click to expand...

Ok

Mike D said:
To correctly evaluate your AC system you also need a high pressure AC guage. When the AC is running the high pressure reading should be 2 1/2 times the outside temperatures. Example: if the outside temperature is 70 F the high pressure reading on the R134A guage should be 175 psi ( 70 + 70 + 35 ). If it is higher than 250 psi on a 70 degree day you have a blockage on the high side. The sysetm has a low and high limit safety circuit that cuts off the compressor - to keep the compressor from self destructing. Some systems also use an electric fan that that turns on at 200 psi and off at 150. If your system has an electric fan , is it turning on ? If the fan does not turn on, the AC will quickly hit its upper limit pressure and cycle off. Check the fan and its relay ( maybe bad ). For fuel economy reasons the factory uses the smallest compressor possible and works the heck out of it! Bottom line - get or borrow a high pressure guage. If your truck does not have an electric fan and the pressure too high - high side restruction ( piece of rubber hose , dirt in system, moisture in system) Have it fixed by a person familiar with AC systems. It's not worth getting hurt for.
Click to expand...

Hmmm I'm starting to think A/C work should be left to the pro's. :)
I might run by Air Tech this week and let them check it out... I didn't notice a fan kicking on and off but I also wasn't watching for it.

daleearnhardt01 said:
Avery while these trucks do not have a cabin air filter, it is common for the conditioned air to not be as cold as possible if the evaporator core is plugged up with a bunch of trash. It is under the dash with the heater core and blower motor. Not hard at all to check and clean if necessary.

Read ... http://www.fullsizechevy.com/forums/general-discussion/tech-articles2/272657-improve-obs-c.html
Click to expand...

Thanks, I'll check it out.
 
Well it was a busy night...

I finished pulling stuff apart (thanks to the link Chris posted) and went in through the opening in the ducting where the blower motor goes and cleaned out the evaporator... the center area was pretty dirty and the edges were completely caked up with who knows what (looked like dried up mud) :)

I also ran down to Kragen and picked up a new blower motor (figure since I was in there I might as well)... cha ching! $$ that sucker was $50! The one in my k10 was only like $14 when I replaced it... but whatever it's done now and one less thing I'll have to worry about.



Ok onto what the thread is about... it hit me tonight that I have another A/C system identical to this one sitting right in front of me... the black crew cab. So I fired it up and hooked my guage up to the low side port to see what it looked like (so I'd have a base line idea of what the k3500 should look like)...
It built up to 45psi then the compressor kicked on and it immediately dropped to 25psi and as it ran the pressure dropped 3-4psi then it shut off... then it slowly built back up to 45psi then the compressor kicked on and it immediately dropped, etc. etc...

So I hooked it up to the k3500 and it does basically the same exact thing, just quicker. It builds up to 45psi then the compressor kicks on and it drops to 25psi and runs for 5 seconds or so then kicks off and it builds up to 45psi then kicks on and drops down to 25psi, etc. etc....


So iono... I think I've done some good maintenance with cleaning it out and replacing the blower motor so I'll just wait for a nice hot day to see how it does...

Thanks for the help guys.
 
Just an update... aparently it worked out because we've had a day or two that touched 90* and the A/C has worked out pretty well...
I'll be excited to see how it holds up when it's 100*+ in a month or so...

On a related note the heater core went out so I had to replace that tonight... it was dripping a few drops here and there but after I was in there working on A/C stuff I think I made it worse... so anyways we replaced the heater core tonight and I'm good to go on my HVAC now :)
 
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