Ooh, maybe I install the air thermostat in addition to the coolant one, with power always available. Either one could trigger the fan, with the air thermostat being primary for after shut-down.
Actually, Zener diodes are in inductive load circuits such as motors and electric clutches to kill the voltage pulse from the relay opening, causing the magnetic field to collapse (or a freewheeling fan). Protects against frying the semiconductors in the control units and exceeding the other diode breakdown voltages.
Voltage rating of the Zener would normally be a few volts above the expected circuit voltage, maybe 15v in this instance. Current rating a few amps.
Thanks, Robert! Good info… I think I need to find an Electronics 101 course.
A good example being the one in the AB14, which protects the output transistor in the GM HEI module.
But the ceramic ball diode shown is IMHO unlikely to be a Zener. All I have seen are conventional rectifiers, usually higher current than the those similar to the other two in the photo (4A, say, as opposed to 1A). Higher current rectifiers have thicker wire, too, as this ball diode does. The thick wire not only conducts better but also heat sinks the diode junction.
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Also the schematic symbol is different…
if you look at the schematic above it says terminal on suspect component “not used”.
If that #3 terminal was provided 12v it would activate the relay- is that the signal from the a/c clutch? The circuit shown shows the condenser fan latching function for afterrun fan.
You could use terminal 3 for your air temperature sensor input and do away with your extra relay…just another of those honker diodes…
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Great suggestion, Robert. Done and working! Zener or not, that last diode snapped like a baby charcoal twig.
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Gerald, great job! Probably not the greatest environment for diodes, might have to change them again in 30 years…
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