MACS Monthly Newsletter - 2017 - AUG5

THOSE EXTERNAL SOLENOID VALVES
By this time, you should be familiar with the fact
that belt-driven piston compressors are increasingly being equipped with externally-mounted solenoids to control the compressor output. And in this
issue, we will cover a new, low-cost aftermarket
tester for them. But first, let's cover a subject that
hopefully will improve your understanding of the
solenoid and its part in compressor operation. As
noted, you'll only see these solenoids in belt-driven
systems, as the electric-drive compressor is typically a scroll type that varies output by increasing the
current feed, and therefore the speed of the unit.
In a previous issue of MACS Service Reports (August 2015), Dave Hobbs covered the troubleshooting of the solenoid compressors, and in other issues
we've shown new testers for them. However, we
haven't gone into the very basics of solenoid valve
circuit operation, so we'll do that in this issue. Because we also have a new tester to talk about.
The solenoid is mounted in the back end of the
variable-displacement compressor (Figure 9), a pis-

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overcome inertia to start tilting the lug plate. See
Figures 10 & 11.

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Figures 10: In this illustration, the solenoid valve is activated, and crank
chamber pressure drops, tilting the lug plate, which effectively increases
piston stroke and displacement.

Figures 11: Solenoid valve is deactivated and with spring pressure assist,
the lug plate straightens to reduce piston stroke and therefore displacement

Figures 9: Cutaway of Denso piston compressor shows location of solenoid, which controls a valve that regulates internal pressures to tilt the
"lug plate."

In operation, of course, the solenoid produces a
duty cycle so the "lug" plate tilt in light cooling
demand will be partial. The duty cycle can be anything from 0% (although as noted previously, with
the spring assist it will be just above zero) up to
100%. Each percentage should change the operating
pressures of the compressor and the performance.
Pressure gauge readings should reflect the changes
in duty cycle.
There are software functions to check duty cycle
in some scan tools, but the simplest tests are with
one of the aftermarket testers available, and a new,
very inexpensive one has just been introduced by
AirSept, the EVC-1. It differs somewhat functionally from the two already on the market.
The model currently available uses universal pin
connections. But by later this year, you will be able
to order a set of the most common harness adapters

ton type with a tiltable "lug" plate mounted on the
crankshaft. The solenoid control valve gets its signals from the HVAC control module ("amplifier" on
Japanese cars). When the solenoid is activated, this
closes its control valve. Low-side pressure (suction)
causes the crank chamber pressure to drop, and
the pressure on the back side of the pistons therefore is greater than on the forward side, which tilts
the lug plate against spring pressure, and results
in longer piston strokes, increasing displacement.
When the solenoid valve opens (de-energizing it),
the pressure on both sides of the lug plate equalizes, the spring provides the mechanical assist and
the lug plate straightens out. Actually, it just virtually straightens, so when the electronics call for
increased displacement, the system doesn't have to

August 2017

MACS Service Reports

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Table of Contents for the Digital Edition of MACS Monthly Newsletter - 2017