Automotive Air Conditioning Information Forum (Archives)

20mercury on Fri April 03, 2015 5:17 PM

Year: 2003
Make: GMC
Model: Sierra
Engine Size: 5.3
Refrigerant Type: 134a
Ambient Temp: 85
Pressure Low: 45-47
Pressure High: ???
Country of Origin: United States

I have a 2003 GMC Sierra truck with a/c that was not cooling very well.

So, I have done some searching and found the low side and high side connection separated by what I think is an orifice fitting. I can check the pressure on the low side, but can not get a fitting on the high side and so can not get a reading on the high side. Also could not find any other high side tap anywhere else.

I used the low side tap and added some 134a and got the following results:
56F at the inside vent
45 to 47 psig at the low side tap
85F ambient with 70% humidity

When I turn the a/c off with the engine still running, I have about 80 to 85psig at the low side tap.

What numbers should I be getting for a correct charge and how to read the high side?

Much Thanks!

HECAT on Fri April 03, 2015 9:33 PM

Even though you can buy a can with a low side gauge at Wal-Mart; you will not get your answers this way.

Correct charge? You will need to recover, vacuum, and recharge the refrigerant by weight according to the spec for your vehicle.

How to read high side? You need a proper gauge set with the high and low side connectors.

Only with charge verified, can pressures and vent temps be evaluated.



-------------------------

20mercury on Sat April 04, 2015 12:52 AM

Thanks for the response.

I have an a/c manifold set and a vacuum pump and have done a couple of a/c systems, but not an expert by any means.

I suppose I could suck the whole system down, clean out and charge by weight but still would be nice to know what design pressure are supposed to be. My high side tap will not fit on the upstream orifice tap and it looks like it might be sealed so I am puzzled as to how to check high side pressure. Any online resources out there for 2003 GM a/c systems?

wptski on Sat April 04, 2015 7:53 AM

I think all R134A service ports are of the Quick Disconnect type but the low side is larger so one can't attach one of those DIY charge kits to the high side. There are some variations is sizes between different OEM manufacturers. Not sure why you can't attach your gauge set.

mk378 on Sat April 04, 2015 9:48 AM

All factory 134a ports are quick connect with standard sizes. Low side is smaller (13 mm) and high side is larger (16 mm). Having them both next to each other on the same line from the condenser to evaporator with the orifice in between is common for GM.

HECAT on Sat April 04, 2015 10:20 AM

There are no "perfect" design pressures. Automotive pressures vary and are drastically effected by RPM, ambient temp, heat load, and heat rejection. Low side would typically be somewhere around 30 and the High side around 2.5 times ambient. Since a low charge or an overcharge can produce near normal readings or some way off base, it is recommended to first verify correct charge by weight. This prevents one from chasing tail over pressure readings. Knowing proper charge is in there allows the pressure readings to be used to diagnose compressor performance, possible restrictions, heat rejection, etc.

-------------------------

20mercury on Sat April 04, 2015 11:00 AM

Ok, and thanks for all of the responses! This is all very useful information, 134a port connect info, ball park numbers and a good plan of attack. Happy Easter to all!