■ Note the LEDs are
lit to show the compressor is on and the
fan is at low speed.
requirements — and
that the hot sides of
the condenser fan
and compressor
clutch (points Y and
Z) will be at either 0
or 12 volts — depending upon cooling and air conditioning
demands — allows us to monitor the electrical end of
cooling and air conditioner operation.
The first device we’ll build is a compressor/fan
monitor. Its circuit is shown in Figure 2. I chose resistance
values which would allow the LEDs (T1 3/4, in this application) to be visible in daylight but not distracting at night.
For the same reason, I chose a blank panel fairly low on
the dash for the display unit. When wiring, connect the
fuses as close as possible to the relays and compressor
clutch. Voltage divider R2 and R3 provide about nine volts
at their junction when the fans are on, so the bi-color LED
glows green when it receives six volts from the radiator
fan (low speed) and red ( 12 volts) at high speed. The blue
LED will be either on or off. If you hook everything up
properly and get a compressor (blue) indicator light but
no fan indication (red or green), stop and find out why.
I have installed this device in two cars and glance at
them occasionally while driving to make sure things are as
they should be. At high speed, the bi-color LEDs may faintly
glow either red or green, depending upon the position of
the fans in the engine compartment and their relative ability
to act as generators. If the glow is very bright, that may
indicate you need to slow down. In a smaller vehicle, you
can use two T1 LEDs, double the values of the voltage divider
resistors, and put a 15K resistor in series with the blue LED.
When I bought a 2006 model vehicle, I didn’t check
the specifications to see whether it came with daytime
running lights (DRL) since I assumed this had become
standard by that year. It turned out I was wrong. I ordered
an add-on module from an
auto parts catalog which
failed after a couple of
weeks of use. I decided to
design and build my own.
Figure 3 shows the second
device: a DRL module.
When the ignition is turned
on, the low beam headlights
are fed somewhat less than full
voltage. This is typical, unless
the car is not in gear (the
automatic transmission switch
grounds relay 2 in park or
neutral) or the parking brake ■ FIGURE 3
is on (relay 2 is grounded
■ FIGURE 2
■ The unused panel from the
lower dash of my ‘06 Honda
Odyssey; drilled to epoxy in
two T1 3/4 LEDs. The wiring
was braided and fed through
the firewall to hook it up in the
engine compartment.
through the parking brake
switch). My car has separate
headlights for high and low
beams, so turning them on
together is not a problem. If you
have dual-filament bulbs, then
the black wire shown grounded
should go to the high beam
circuit instead so that you will
not have daytime running lights
and high beams simultaneously.
D4 is necessary so that — should
you flash the high beams with
the ignition off (not possible in
all cars) — the low beams
won’t also come on because
of reverse activation of relay 1.
While I was building this
December 2008 47