[DMCForum] Re: A/C Question

["checksix3" <jetjock11@xxxxxxxx>]

Your charge looks correct, as do your pressures. It appears your 
system is ok so the low pressure cutout just needs to be set. (It 
being new is also a tip off). If you replaced any components in the 
system you should've added oil as required. Also, I assume you purged 
your gages and hoses prior to hookup.

Walt is pretty much dead on (good job Walt). Forget the orifice tube, 
if it wasn't working your low side wouldn't be correct and you 
wouldn't have cold air. 

What you're basically looking for is the low side switch to open when 
the evap freezes. You can either measure the evap directly, the vent 
outlet temp closest to it, or set the switch by using your low side 
gage. The lowest temp the evap will reach is dependent (assuming the 
system is propely charged and no other problems exist) on the heat 
load being removed. That in turn depends on the ambient temp and fan 
speed.

A simple and effective way to test performance is to insert a 
thermometer into the vent closest to the evap, car windows closed, 
high idle, fan on low speed. The lowest fan speed will keep the air 
passing thru the evap in contact with it longer. (To cool any car 
quickly use high speed, to get it as cold as possible use low speed). 

Let the system stabilize. Since there will always be about a 10 
degree loss across the evap, if you're getting 40-42 out of the 
closest vent on a hot day the evap is near freezing.  (Anything less 
than 40 degrees at the vent is frosting on the cake). Set the switch 
to cycle at that point. If the compressor cuts off too early (not 
your problem) set the switch to keep it on until the vent temp 
stabilizes then set it to cut out at around the specified temp. (If 
you can't get 40-45F you have other problems). You can also measure 
the evap directly (if you can get at it) and set the switch to cut 
off around 32F.

Technically you want the switch to cut out for sure down around 28 
psi to protect your system from a low charge but setting it at the 
evap freeze point is really what it all comes down to. You don't want 
your evap freezing up for obvious reasons.

As Walt mentioned, it can be difficult to charge properly without a 
scale or charging cylinder (and I prefer the hot water bath myself)
but all you really need is to slowly charge until the low side 
pressure is correct and the evap is close to freezing. (The two are 
related of course). 

Charge specs are only guidelines, as long as you have lower than, 
say, 45 degrees out of the vent on low fan speed the system is 
working as best you'll ever get it. In fact I charge all systems this 
way rather than simply dumping in the specified amount. I charge to 
80% of spec, let the system stabalize at a fast idle and add a few 
ounces at a time until the vent outlet temp is correct. Add an once 
or two more and you're done. (The charge may come out close to specs 
but why waste R12?)

Of course if you find the switch opens and the compressor still 
doesn't cycle off you'll need to find the problem in the electrical 
system.




--- In DMCForum@xxxxxxxxxxxxxxx, "Dave Stragand" <dave.stragand@xxxx> 
wrote:
> Hi All,
>  
> I hooked up the A/C today, evacuated it, and charged it up.<
>  
>

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