Auto A/C Diagnosis, Pressure Gauge Readings and Temperature Tips & Tricks - Be sure you take pressure gauge readings properly!

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Auto A/C Technical Tips and Tricks the pros use every day!
Auto A/C Pressure Gauge Readings
Is there a difference between taking
gauge readings with the hood up or down?

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Manifold Pressure Gauges Readings with the Hood Up or Down?
When you're diagnosing auto A/C system, there really is a difference!:

When you're performance testing an auto A/C system, should you take pressure gauge readings and duct outlet temperatures with the hood up or down? When you think about it for a minute, it may appear pretty difficult (not to mention tricky) to get pressure gauge readings when the hood is down, but there is a good reason why you may want to.

Understand that the air inlet for the A/C system is right at the base of the windshield. Typically, the air inlet will be covered with a screen to prevent debris and other foreign material from falling in. You should also notice that between the air inlet and the engine compartment, there is a long rubber strip that runs the entire length of the firewall. That's a really important piece of rubber when it comes to keeping you cool!

What's a 'hood seal'?
That long strip of rubber on top of the firewall is the hood seal. With the hood open, you should be able to see where that rubber contacts the hood from one side to the other. When the hood on the vehicle is closed, that rubber strip or hood seal prevents hot under hood air from being drawn into the air inlet of your car's air conditioning system. When the hood is up, instead of drawing outside ambient air temperature of 80° F or 90° F into the air inlet, you're pulling hot under hood air that could be 200° F or more!

What's it really mean?
For the most part, when diagnosing and testing auto A/C systems, you should be all right doing that with the hood up. You should be able to take pressure gauge readings and perform your tests without problem. However, understand that in cases where there is poor air circulation (typically inside service shops) and the vehicle is being operated for extended periods of time, there will be a continued increase in A/C pressures and duct outlet temperatures.

In order to check or verify the A/C system, lower the hood. Use caution not to get the manifold gauge hoses caught or damaged, but with the hood lowered, you will start to see an improvement in the gauge readings.

On sever applications or problem jobs, a good tech tip is to set up the gauges in a way that you can actually close the hood. That will provide you the most accurate pressure gauge readings and duct outlet temperatures for the A/C system. There is a difference and the pros will use this trick to assure themselves that the A/C is really working properly.

Here's a look at how the hood seal works to help prevent hot under hood air from entering the auto A/C system's air inlet:

When you're taking pressure gauge readings and duct outlet temperatures, it's a good idea to consider doing that with the hood lowered. That way, you will prevent hot under hood air from entering the A/C air inlet and you will get a more accurate pressure gauge and temperature reading!

This particular illustration does not show the hood. However, the hood seal is highlighted in 'red' and marked. That hood seal is a strip of rubber that seals against the hood when it's closed. That rubber prevents hot under hood air from entering the A/C system's air inlet, typically located at the base of the windshield. A good idea is to clean that hood seal and spay it with a silicone spay to help keep it soft and maintain it's seal!

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