Ac discussion. Can someone explain in detail how the system runs?

[moidsm1]

I have an 88 suburban with rear air. 5.7 tbi motor. There are some coils on the engine compartment air box. What are they for? Does the ac system run through the engine computer or does it have its own control module? What signal should be at the compressor electrical connector? If anybody knows the ins and outs of the ac operation please explain. Thanks

 

[Big Ray]

I have an 88 suburban with rear air. 5.7 tbi motor. There are some coils on the engine compartment air box. What are they for? Does the ac system run through the engine computer or does it have its own control module? What signal should be at the compressor electrical connector? If anybody knows the ins and outs of the ac operation please explain. Thanks

A/c principles of operation...the system is a loop split into 2 halves, the "hi pressure side" and the "low pressure side", commonly called the high side and low side.

The high side contains several components, starting with the compressor, then the condenser, the receiver-dryer (the silver "can" on the air box), and the orifice tube (which is inside the tubing just before the evaporator).

At the orifice tube the hi pressure liquid Freon is converted to low pressure gas as it expands into the evaporator. as it expands, it's temperature drops. (physics, I forget which law) As it travels through the evaporator it picks up heat from the cabin area (heat travels from hot to cold, more physics. You can make heat, you can't make cold. Ironic..)

at this point the freon is a hot gas on it's way back to the compressor. At the compressor, it is compressed and sent to the condenser as hot, high pressure gas. The condenser cools it gas as it condenses back to liquid form.

It then goes back to the orifice tube and evaporator to be repeated.

As for the computer, the only function it does with a/c is to bump up the idle when the a/c is on. should be 12 volts at the compressor to engage the clutch.


Suburbans have the rear a/c also. You'll see 2 lines teeing off in the engine compartment going to the rear unit. The only real difference is it has an expansion valve instead of an orifice tube. They do they same thing.

 

[moidsm1]

That is some good info. I noticed on the evaporator box that there are some sensing coils before and after the evaporator. What do these do? And is the AC clutch cycling only controlled by the Pressure switch mounted to the receiver dryer?

 

[Big Ray]

ah, you mean the resistors for the fans...I'm not sure, I think they are located there to keep them cool...the inboard unit is for the rear fan...

and you are correct on the pressure switch. I just recently repaired and swapped my '90 over to r134, changed out everything but the evaporator...

 

[moidsm1]

Thanks for the replies.

I am currently working on my conversion as well. One thing I will be upgrading will be the Condenser core. I am going to use a Parallel flow design which is what is recommended for R-134 freon.

How is your system running? What are the outside temps and how cool are your vents? Any problems while Idling?

 

[Big Ray]

Thanks for the replies.

I am currently working on my conversion as well. One thing I will be upgrading will be the Condenser core. I am going to use a Parallel flow design which is what is recommended for R-134 freon.

How is your system running? What are the outside temps and how cool are your vents? Any problems while Idling?

I have the 6.2 diesel, so idling is a non issue...

my rig doesn't have rear a/c, keep that in mind.

it's running right at 60", that weather is 90"+ and about 90% humidity...

'burbs have a lot of glass, whenever it clouds over the temperature drop at the vents is noticeable...drops a few degrees. actually had to turn the fan down today.

I plan on darker/more reflective tint on my rear area glass, and by next year having the rear unit in...


it's not like an r12 rig, but it beats having no a/c like last year.


I have another plan, to put valves on my heater hoses so I can isolate the core. Keeps the plenum area under the dash from heating up.

when I install the rear unit I think I'm going to try this stuff...hear it works well...

 

[moidsm1]

Isolating the Heater core sounds like a good Idea. Honda's do that from the factory with an actuated valve.

You should be happy with the rear air. Although mine isn't blowing cold yet the amount of air it pushes is amazing.

 

[Big Ray]

Yeah, when my 86 was running I used the rear as the main cooling unit...

It blew right on the back of my head, kept me cool all over...