It would be a good idea to check the resistance between each diode pin and a Vcc pin and a Ground pin. This might give some indication of how the damage occurred, and consequently the cause. Knowing whether the CPU otherwise works could also help in knowing what happened.
Have you checked all the relevant wall outlets with one of those plug in testers that tell you that the wiring is right? I'd check the surge suppressor outlets as well. Incorrect outlet wiring could be consistent with the power strip being off and the pump still conducting a voltage into the water.
Bill's guess, "measure the resistance between the coolant and the motor leads (AC wires) seems that something is shorted", seems the most plausible explanation to me at the moment. Some precautions I'd suggest:
1. Ezlid's suggestion, "Solder a ground wire to one or more of the brass barbs." is a good one. I would suggest connecting to a brass or copper fitting on each side of the pump. (intake and outlet) Tie those two fittings to the ground of the PC which has the temp diode in it. If the pump manages to electrify the water in the impeller housing, the grounded fittings will keep current from getting to the CPU.
2. Dedicate a surge supressor to the pump alone. Pumps can put out nasty spikes when they are powered off. A quality surge suppressor will be nearly as good at preventing outgoing spikes as it is at preventing incoming spikes. I had a Danner Mag5 that would cause my printer to reboot every time I unplugged it. That Little Giant may be much worse.
3. I'd guess that a non magdrive pump, with a seal between the water and circuitry that can fail, would be required by law to have a GFCI circuit built in, but I don't know this. Even if it does have one built in, it may not be working. Probably a good idea to add an external one.
The thought that initially occurred to me, was that that you most likely have a big loop in your setup that can easily generate signals in response to the pump's magnetic field. (This isn't consistent with the pump being plugged in but powered off, and still affecting the temp measurement. It could result in bad measurements and damaged components when the pump is running though.)
The loop is:
diode->
diode reader->
ground of PC connected to diode reader->
ground of PC containing 'metered' CPU->
'metered CPU'->
diode
The best way to 'collapse' this loop, is to cut the earth ground connection to the PC the diode reader is connected to. Then connect the chassis of the PC the diode reader is connected to, to the chassis of the PC the diode is in. This ground connection should be routed alongside the diode reader cabling. Ideally twisted with it. To be safe, both PC's should be plugged into the same surge suppressor. (If a picture would be helpful let me know.)
The issue here, is that a changing magnetic field can induce a voltage in that loop, and that voltage would be applied between the temperature measurement diode, and the rest of the CPU that the diode is in. I'd guess that it may not take much voltage between the diode and the rest of the CPU to do damage. Generally the diode reader would be on the same motherboard as the CPU and inducing a damaging voltage between diode and CPU would take an enormous magnetic pulse. A big loop like you probably have, could easily pick up a voltage from the magnetic field of a powerful pump.
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