MACS Service Reports - 2016 - MAR2

3). With the engine running and the control panel set to
MAX A/C, jumpering across these contacts would give a
technician that confirming Click! and resulting compres-

Steve Schaeber

At this point you think back to what you were taught
in school about how to test an A/C system, and you put
those lessons to work. You remove one of the service port
caps and use your fingernail or a small screwdriver to
momentarily press in the Schrader valve, hoping to get
that quick puff of R-12. If you do, it means that at least
there is some refrigerant in the system, and that it has not
yet all leaked out.
Next you'll want to see if the compressor is seized up
or not, so you try to turn the clutch over by hand and
are able to rotate the input shaft a few times. That's good
news, because if it wouldn't turn you'd be looking at rebuilding or replacing that compressor pretty much right
up front.
Now it's time to see if the clutch is still good, and the
first option that comes to mind is to feed power directly
to the clutch from the battery's positive terminal. This is a
quick test which lets you know right away if the electromagnet works or not, but before doing this you recall another cheat where you can test the clutch and its control
circuits all at the same time.

Figure 3: U-Shaped paper clips could be found in many an A/C technician's
tool box once upon a time, but don't use the small ones! Those compressor
clutches draw enough current to heat up a thin paper clip rather quickly.

sor operation (barring any other existing problems) that
would let you know the control system was capable of
operating properly, except of course for the low charge.
Removing the jumper would remove power to the compressor clutch, turning it back off again. Usually technicians would apply and remove power a few times while
observing clutch operation and listening to the sound of
the compressor as it ran. See Figure 4.
So when you break it all down, it was a pretty quick
test, and within just a few minutes you could tell if the
system needed a more in-depth diagnosis, or if a simple
recharge was in order to get the system blowing cold
once again.
Guys who have been around for a while know this
scenario all too well, as it was a commonly used service
procedure for many years on several makes and models.
However, control systems have evolved over the years,
and these simple on/off pressure switches are simply not
employed as often as they once were.
Modern A/C systems have for many years now most
commonly used three wire pressure transducers to report
high side pressure to the PCM (powertrain control module) as a primary input for compressor control (Figure 5).
Like other three wire sensors, these transducers convert
the pressure measurement into a voltage signal that is
monitored by the PCM. These three wires consist of power, signal and ground. Jumpering any of the two wires
together is not going to result in the compressor clutch
engaging, and in fact could cause damage to the PCM
itself, possibly resulting in a costly repair for the shop.
Sometimes these old habits are hard to break, and there
are still technicians out there who think they can use that
old paperclip trick to get the compressor running. More
often than not however, this results in unnecessary component replacement due to a faulty diagnosis more than
anything else. On MACS' A/C Techline, we often receive

Jumpering the Switch

Steve Schaeber

The low side pressure switch used to be the only device
that controlled the compressor clutch operation on many
of these CCOT systems. It's a simple OFF-ON pressure
switch that operated the compressor in a somewhat narrow range (Figure 2). The compressor would cycle on at
between 41 and 51 PSI, then cycle off again at between 20
and 28 PSI. In other words, as long as system pressure is
over about 25 psi, the compressor should run. In many

Figure 2: This low side pressure cycling switch is mounted on the accumulator. When refrigerant pressure is sufficient, the switch closes, allowing the
compressor clutch to engage. When pressure falls below a minimum threshold,
the switch opens, turning off the compressor.

cases however, refrigerant would leak out over a period
of time, and system pressure would drop to the point
where the compressor would no longer engage, even on
a hot day.
Before going through all the motions involved with
testing or recharging the system, an old trick was to
disconnect and jumper across the two terminals of this
low side pressure switch connector. Many A/C technicians at the time kept a heavy, U-shaped paperclip in the
top drawer of their tool box for just this purpose (Figure

March 2016

MACS Service Reports

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