MACS Monthly Newsletter - 2020 - JAN1

Total Vehicle Climate and Thermal ManagementTM

January 2020

By Steve Schaeber, MACS Technical Editor

Valve Core & Service Port Testing
A while back I wrote a short story for my "Service Port" column in ACTION™ Magazine called "Leaks Under Vacuum" in
which I described a phenomenon I came across while helping a
friend service the A/C system in a 2000 Ford F-650 bucket truck.
Several readers found the story fascinating, as most had never
heard of such a test being done before.
But there was one misconception in the article needs to be
further defined and explained so that technicians understand
the right way to check for valve core leaks, and how to properly
"service the port" for the best possible results. But first, let's begin with a recap of what took place, along with a few pictures to
help illustrate the job.

The procedure is quick and easy: Following recovery, connect
a vacuum pump with a set of manifold gauges (or the high and
low side hoses of an R/R/R machine) to both service ports. Engage the pump, and if the system is tight, you should be able
to pull a descent vacuum within a minute or two (more if it's
a larger system). About 25 in Hg should be enough, but 28 or
29 will give better results. Then, one at a time, close oﬀ the service valve and disconnect it from the service port (for example,
pull vacuum on the high side port while checking the low side
valve core, and then switch to pulling vacuum on the low side
while checking the high side core). See Figure 1. If the valve core
is working properly, it should be able to hold a seal and not allow air to enter during evacuation. But if you hear that dreaded
hissing sound, you know there's a leak and a new valve core is
in order.

Leaks under vacuum

Steve Schaeber

Recovery showed the system had only 4 ounces of refrigerant,
which on a 42-ounce system is only about 10%. As little as that is,
it's still enough to build 49 psi and allow compressor operation
(albeit only for very short periods of time). This made the compressor "short-cycle", a common indicator of low charge.
After recovery came evacuation, and it was during this time
that I noticed something unexpected, and to be fair it was dumb
luck that I came across the problem in the first place.
During evacuation I wanted to tee-in a micron gauge to see
just how deep of a vacuum the R/R/R machine was pulling,
and likewise to see how tight the system was. Leaving the vacuum pump running, I closed the low side service coupling and
disconnected it from the service port (so the pump was still pulling vacuum on the high side). I immediately began hearing a
hissing noise and placing a finger over the hole told me it was
the valve core.
This is something that technicians rarely encounter, and the
reason is quite simple. How frequently do you disconnect your
service couplings during evacuation? Probably not too often,
right? But really, that's the best time to find out if the pin spring
is strong enough to keep the valve core closed and sealed under
vacuum.

Steve Schaeber

Figure 1: While evacuating through both sides, remove one
of the couplers and listen to hear if the valve core leaks while
under a vacuum.

Figure 2: Compare the "spring stiﬀness" between the old and new
valve cores (it's pretty easy to tell), and you should find the new
core can do a better job of sealing the service port.

ALSO INSIDE THIS ISSUE:
Off-highway use of R-1234yf .......................................................................................................................................... 8
MACS Service Reports is the official technical publication of the Mobile Air Conditioning Society Worldwide, Inc., P.O. Box 88, Lansdale, PA 19446. The material published in MACS Service Reports expresses the views of the contributors
and not necessarily that of MACS. Every attempt has been made to ensure

January 2020

the accuracy of the content of MACS Service Reports. MACS, however, will
not be responsible for the accuracy of the information published nor will MACS
Worldwide be liable in any way for injury, labor, parts or other expenses resulting
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