I recently bought an MGB in above average condition. The father of the girl who owned the car did the selling (what little was needed). In the discussions he mentioned that they had recently put a new battery in the car because the old one wouldn't hold a charge. He also indicated that the replacement battery was a bit old, so initially I didn't think too much about any electrical problems.

The car ran fine with no problems for a while, then suddenly the starter began turning the engine slowly, and if I didn't drive the car every day (heresy) I really worried whether it would start the next day. Eventually it wouldn't.

With the help of a friend, we began disconnecting one thing at a time with a meter hooked to the battery. We finally discovered the thermostatic fan switch was shorting out when it was in the "off" position. The switch worked in that the fans ran and shut off presumably correctly. The problem showed up after I had topped off the radiator. Apparently, when the radiator was low there was no problem because the switch is mounted in rubber and there was no coolant to conduct through. When I filled the radiator it submerged the bulb in coolant. In the "off" position, the fan switch shorted through the coolant to the radiator, causing the battery to drain.

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