MACS Monthly Newsletter - 2022 - JAN4

Figure 5: Expansion Valve System with a restricted condenser.
pressure
showing up as 130 and the low side at 50, someone
could easily diagnose that as a weak compressor. See
Figure 5.
Now let's take the expansion valve system illustration
with the multiple service ports and utilize it to illustrate
what the pressure readings would look like at other points
in that system with the restriction present. As you can see,
the high side pressure at the compressor is actually 350
psi. By comparing that reading to the one taken at the
original high side service port, we find a difference of 220
psi and that indicates a significant restriction. If we check
the pressure at the inlet and outlet of the condenser, we
see that it is 350 going in and 130 coming out, so that
would tell us for sure that the restriction is in the condenser.
With all of those ports, we could also do further testing
to verify that there weren't any other restrictions present.
See Figure 6.
It would be great if we had systems configured like this
with the multiple service ports, but we don't. However, we
can use temperature testing to find restrictions, unless
there is a total restriction, because pressure drops create
temperature drops. NOTE: Temperature drops don't occur
with a total restriction because there is no refrigerant flow.
In this case a 220 psi pressure difference between the
inlet and outlet of the condenser would result in about a
75° temperature difference and that is way more than the
typical 20° to 50° temperature difference we would expect
to see on a system with a fixed displacement compressor.
NOTE: Vehicles utilizing parallel flow condensers with
variable output compressors can experience temperature
differentials far above 50 degrees, so be extremely careful
when temperature testing them and condemning condensers.
If we were to further temperature test the lines and
January 2022
4
Figure 6: Expansion Valve System with restricted condenser
shown with pressure readings in multiple places.
drier, there should be almost no difference in the inlet and
outlet temperatures on those components. CAUTION: You
must use a contact type temperature reading tool to get
accurate readings. Infrared temperature guns will result in
inaccurate readings and an incorrect diagnosis. Infrared
guns have their place in the shop when it comes to taking
high temperature readings such as on an exhaust manifold
when trying to find a misfiring cylinder but are just too inaccurate
to use on lower temperature components such as
those in an A/C system (Figure 7).
Now let's take this information and apply it to a diagnostic
situation I encountered a number of years ago. This
involved an actual visit to a shop and not a tech call. The
vehicle was a late 90s or early 2000s Honda with a Conventional
valve system and a fixed output scroll compressor
and this shop had been working on it for over a week,
on and off. The warehouse had just sent out a second replacement
compressor and after installation the system
was still behaving the same way and the shop wanted to
replace it with a third unit. I happened to be visiting the
city where this warehouse was located, and the manager
had asked if I could pay the shop a visit because he felt it
was very unlikely that the two compressors that had already
been installed were both bad. So, he provided me
with all of the information that he had and I headed over
to the shop.
After walking in and doing a short introduction, I was
immediately met with a somewhat hostile, " How come
you guys have so many problems with your compressors? "
Then the conversation proceeded to go something
like this:
MACS Service Reports
Richard Hawkins
Richard Hawkins
MACS Monthly Newsletter - 2022

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