Hello Listers,
I am the culprit who started the weeklong thread on electric fans a couple
of months ago. I certainly don't wish to get into all that again -- I
respect all of your opinions, and have mine as well.
After a number of off-line conversations with people who want to install
one, I thought this note and wiring tip to the list might be helpful.
I installed a fan with a separate adjustable thermostat. That way, the fan
runs only when it needs to run, after the water temperature exceeds the
setting of the thermostat set point. The thermostat also has some built-in
hysteresis, so that it keeps the fan running until the water is cooler than
the setpoint temperature. Otherwise, the thermostat contacts would
"chatter" when the setpoint temperature is reached.
Initially, I wired it to a "hot" circuit that was powered all the time.
(The fan has its own in-line fuse.) That way, I reasoned, the fan would
keep running after I shut the engine down, (like some German cars do) until
the engine was cool. That worked as planned, but it ran for such a long
time, without the generator charging the battery, that the battery was
always partially discharged. Not a good idea.
Therefore, I rewired it and connected the hot side to the "switched" side
of the ignition switch, so that the fan would not run after the car was
shut off. That introduces another possible problem. The fan motor is an
inductive load, not resistive. An inductive load, when switch off,
generates an inductive "kick" that tries to keep the current flowing in the
same direction as before. This inductive "kick" is a large voltage spike
that can pit the ignition switch contacts. The simplest way to eliminate
this problem is to wire, in series with the hot side, a silicon diode. It
is wired such that the diode is forward biased when the fan is on. (Anode
to power source.) When the large voltage spike occurs (when you turn off
the ignition switch) it reverse biases the diode, effectively disconnecting
the ignition switch from the fan. Problem solved.
I also wired in a dash-mounted switch to turn on the fan manually, if
needed, in case the thermostat ever failed.
This system has been in place for over a year with no problems, and I am
very satisfied with the results.
Sumner Weisman
62 TR-3B
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