It goes without saying that no one putting together a street rod intentionally screws something up, but as veterans of a long list of building blunders, we know stuff happens.
Case in point, we were on the Road Tour with our buddy Rick Love of Vintage Air not long ago. As we wandered around one of the stops looking at cars, Rick glanced toward a very nice coupe that had pulled up and parked, and said, "I'll bet the heater valve in that car is installed backward." Sure enough, a subsequent conversation with the car's owner revealed the air conditioning system was not performing as it should and a quick inspection revealed the heater valve was indeed installed incorrectly.
About the time we decided to ask Rick to use his new psychic powers to give us a winning lottery number, he made a confession. "Did you see the big puddle of water under that car?" he asked. "When that much water comes out of the A/C drain on a day that's not at all humid, it's a sure sign something's wrong. Usually it's the way the heater valve is installed-we see that all the time."
At this point, we were struck by two things: First, here was a really nice car that appeared to be well built, yet a simple mistake was made during its construction; secondly, Rick's ability to spot and diagnosis a common problem so rapidly got us wondering how many other mistakes are easily made when installing A/C.
So, with help from Rick and Vintage Air, we've put together the following list of simple ways an air conditioning installation can be screwed up (as well as ways to avoid it). Just think of this as a how-not-to-do-it.
Don't Charge The System Properly
The easiest way to make an air conditioning system not work properly is to incorrectly charge the system; and the simplest way to do that is to treat a contemporary system as if it were the old R12 type. Dump in a couple cans of refrigerant when charging is necessary and watch the sight glass in the dryer for bubbles.
Unlike the old days with R12 refrigerant, the "bubbles" in the sight glass you see with 134A are actually small oil droplets, not air bubbles. Today's systems must be charged by weight with a good scale or preferably a charging station. Vintage Air's standard charge recommendation is 1.8 lbs (28 oz) of 134A refrigerant. This charge will be correct for most standard installations (dash-mounted systems), but some additional refrigerant will be required for a trunk-mounted system with a liquid line of more than 12 ft.
Don't Use A/C Gauges
A truly gifted method of screwing up an A/C system is the "by guess and by golly system." The guess part is assuming the system needs more refrigerant; the golly part is finding out later it really didn't. Dedicated, good-quality gauges are required to accurately determine if an A/C system is charged properly. Most street rod systems use an expansion valve and a fixed displacement compressor (CCEV system). The low side pressure should be 7-12 lbs when correctly charged under the following test conditions: 1,200-1,500 engine rpm; doors and windows closed; and blower on medium with fan in front of the radiator for air flow across the condenser and radiator. An overcharged system (too much refrigerant or oil) results in higher system-operating pressures and poor performance. An undercharged system will show lower operating pressures and poor performance as well.
Just Add Oil
New compressors sometimes come with extra oil. So, despite the fact that all new Sanden compressors come filled with the correct type and the proper amount of lubricant for efficient system operation, add some more if you want to screw things up. That will result in increased system pressures and poor performance.
The smart move here is to save any extra oil for use if and when the system needs to be re-charged (unless you're like us and can't find it when you need it, but that's another story).
Don't Evacuate The System
Leaving air and water in the system is a guaranteed method to reduce the efficiency of an A/C system.
Evacuating the system for a minimum of 30 minutes at 75 degrees Fahrenheit with a vacuum pump accomplishes two things-it removes air from the system, while the vacuum lowers the boiling point of water, so any moisture in the system boils away. However, there is still an opportunity to mess up the process if it's cold; all components must be at 75 degrees Fahrenheit for proper evacuation. If the components are colder than that, run the engine or heat them with a hair dryer to get them up to the correct temperature.
Don't Insulate Or Seal The Vehicle
This sounds silly, but think of cold as the absence of heat. Now think of an air conditioner as something that doesn't make the air cold, but removes heat from it. If you can come to terms with those statements, it stands to reason that providing more heat for an A/C system to remove will make its job that much more difficult. This can be done by not insulating or sealing the firewall, floor, doors, and all the other sources of heat that enter the passenger compartment.
An A/C system works by taking in air from a confined area and pushing it across a coil in the underdash evaporator where the heat is absorbed and the humidity converted to droplets that drain to the outside. Cooled air is then pushed out through the vents onto the occupants while the heat is carried to the condenser in front of the radiator.
Simply put, if you don't keep the hot air out of the vehicle, no A/C system will ever be able to cool off your vehicle. It's basically like running the A/C system in your home with the windows open.
Install The Heater Control Valve Incorrectly
A very effective way to make the A/C less efficient is to install the heater control valve backward or in the wrong hose. This allows hot water to circulate through the heater core, warming the air the A/C is trying to cool.
Most heater control valves are directional and must be correctly oriented in the high-pressure heater hose.
Most water pump heater hose outlets are on the low-pressure (suction) side of the pump. The intake manifold outlet is the high-pressure side of the system. As water pressure in the hose helps the valve seal, they will not shut off completely and some hot water will continue to flow if they are installed backward or in the wrong hose.