MACS Monthly Newsletter - 2020 - JAN4

Choosing the correct valve core

Some OEs are charging systems now through the low
side port, so they want to get the greatest amount of flow
area possible, including for evacuation. Dimensions of
the ports themselves (and the shape of the coupler mating surfaces) come from the SAE Standard J639, but the
insides of the ports can be modified as necessary.
*
For switch ports, the valve core is open all the
time, so it just sits there and doesn't have to open and
close. The port's design hasn't changed in decades (they
still use 3/8"-24 threads) along with standard valve cores,
which work well in this application. When the switch
or sensor is installed, there is a small depressor feature
molded into the fitting which depresses the valve core's
pin, opening the valve. If it needs to be removed for service, the valve core will close so the refrigerant charge is
not lost.

Should you decide to replace a valve core, it's important that you choose the correct part for the system you're
working on. We've all seen "that guy" who can't find
the right part in the service kit, and rather than order it,
heads over to the tire machine to dig through used valve
stem cores.

The Smallest Component
Valve cores are the smallest component in an A/C
system and there are four valves in most every system.
Two for the high and low side service ports and one or
two, depending on how many sensors or switches may
be used.
For example, a 2012 Chevy Malibu has only 3 valve
cores, two for the high and low side ports and one for
the high side transducer. Older systems like the Jeep
Cherokee XJ has 4 cores however, since it uses a low side
cycling switch along with a high-pressure cut-oﬀ switch.
These valve cores don't really work with the system
per se, but rather they are simply there to service the refrigerant. There's lots of misunderstanding about them,
and many new valves have come out in the recent 15 or
so years. There's also been lots of changes to them regarding elastomers (seal materials), so let's bring clarity
to the choices that exist in the field.

Lots of different cores
Back in the day, you only had one type of valve which
fit in all the diﬀerent ports (Figure 8). If for some reason

Selection of valves for MAC systems

January 2020

Steve Schaeber

This topic is best suited for those who spec out A/C
systems, rather than for service. That's because we don't
really have the option of changing out the valve core
based on size and shape; we're pretty much stuck with
what the system designer chose (that is, unless you want
to modify the line, add a new service port location, or
change the line out completely, but that would mostly
be impractical). Still, it's important for technicians to understand why certain valve cores are used over other designs that are available.
There are three diﬀerent kinds of service port applications under the hood: The factory fill (usually the high
side), the service fill (sometimes high and low side), and
the switch ports. The latter has pretty much been the same
forever, they often just use a simple "standard core".
*
On the high side, there's typically going to be a
large bore core, because that's the service port used by the
factory to evacuate and fill the system during assembly.
They need the largest port size they can get to speed up
the process. After all, most vehicles only hang around the
A/C charging station for about 90 seconds! They have to
evacuate (about 30 seconds), leak test (about 10 seconds),
fill the system with refrigerant (about 5 seconds), and
still give the operator time to connect and disconnect the
coupler. Sounds impossible, but they can do this for two
main reasons: the system has never been charged before
and they're using a positive displacement pump.
*
Next is the low side, which uses a medium flow
core that's sized to fit inside the smaller shaped port.

Figure 8: 1980s-era GM A/C systems used the same size
service port and standard valve core for both the high and
low side fittings. Inexperienced technicians would sometimes mistakenly connect their hoses to the fuel rail, which
used the same fitting configuration!

MACS Service Reports

MACS Monthly Newsletter - 2020

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