●●●●
T AW
ENUO A
TTDD
HO-O
UMN G
SOAU
IATGEIS
SVFO
TER THE
BY ROBERT M. VOSS, W2HTN
This article describes two add-on
automotive devices. One of these is not
found in production cars (to the best of
my knowledge), but will be of interest to
anyone who seriously wants to know what
goes on under the hood. The second
device seems to be standard equipment
on more and more cars, and — in my home
state of New York, at least — insurance
companies will give a discount for its use.
Both of these circuits can easily and safely
be installed in any car without modification to the existing wiring.
Today’s cars seem to have fewer gauges than those of
a generation ago, relying more on idiot lights to alert
drivers when something bad is about to happen. However, I
believe there are still drivers who would like to know things
like: how high or low their engine’s oil pressure is, not just
that it is about to seize; what is happening electrically, not
just that the engine is about to die; and what is happening
46
December 2008
with the cooling system, not just that boil-over is imminent.
Figure 1 shows some of the electrical components of
a typical 21st century cooling and air conditioning system.
When the engine is started cold without air conditioning,
relays A and D are open, so no power goes to either fan.
When air conditioning is required, and engine load — measured
usually by vacuum — permits (relay D closes) and/or coolant
temperature increases, relay
A closes, putting 12 volts
(nominal) across the two
fans in series. Assuming
equal resistance in both
(true in both of my cars),
each receives six volts.
When cooling
demands increase further,
relays B and C close, feeding
the full 12 volts to the fans
in parallel. Knowing that
the hot side of the radiator
fan (point X) will measure
■ FIGURE 1 either 0, 6, or 12 volts —
depending upon cooling
