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How to interpret the various dials on a guage set

vettman on Tue January 06, 2009 12:18 PM User is offline

Year: 1976
Make: Ford
Model: F350
Engine Size: 460
Refrigerant Type: R-134A

I have a couple of auto AC-related instruction manuals, but neither has a discussion of how to read/interpret the various colored dials on a manifold gauge set; when to use the long or short end of the pointer, etc. Can someone point me to a source that will explain all this.

What should the low-side dial read when not connected?

I have an old but quality R-12 gauge set. I'm considering purchasing 134A adapters for it, but was wondering if/how I should flush it to insure I would not "contaminate" my new 134A system.

This is an invaluable forum; thanks for comments/recommendations

mk378 on Tue January 06, 2009 2:17 PM User is offline

Use the long end of the pointer for all readings. All references are based on the psi numbers around the outside edge of the dial. These are gauge pounds per square inch, the same units used in the USA to inflate tires, etc.

You can ignore the other scales on the inside. (They are the corresponding saturation temperature for different refrigerants.)

The low side should read zero (about 1/8 of the way around the scale) with the hose disconnected and vented. The numbers below zero measure vacuum. When the low side is under vacuum, the pointer will go below zero. Some hoses have self-sealing connectors on the car end, which will hold pressure in the hose when it is not connected. If the pointer doesn't quite come to zero, most gauges can be re-zeroed by taking the dial lens off and turning the screw slot in the dial.

Converting one time from R-12 to R-134a is not a big deal. Flush out the lines with solvent if you want to, though it's not essential. If you're going to be working with R-12 any, you should have dedicated gauge sets for each.

Make sure that you have the adapter for the yellow hose so you can hook up can tappers or cylinders of R-134a to it. They have different threads. Cylinders of R-134a with an R-12 style outlet do exist, but they are not sold in auto parts stores.

Chick on Tue January 06, 2009 3:06 PM User is offlineView users profile

Just to add the explaination you got, it would be good to have the Mastercool ac repair manual as it's loaded with information. Newer automotice guage sets don't have all the differnt numbers. colors etc, just the PSI which is used in automotive AC work..(Unless you're a pro, and can use the temp/pressures) R12 and R134a are simple if you just use the PSI readings.. has the adaptors you need to switch your set to R134a, just e-mail the guys at with your concerns, they are very helpful.. Also check out the mastercool R134a guages, they are very resonable.. Hope this helps..

Email: Chick


Freedoms just another word for nothing left to lose

vettman on Tue January 06, 2009 7:03 PM User is offline

This is exactly what I was looking for--thanks so much for the quick and thorough response.

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