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Maybe not correct but I get the impression that the battery voltage immediately or in a short time frame rises to 18 volts when the tach wire is connected.
If unregulated, the alternator could only put out near its max rated amps. Also don't know details but assuming the alternator is under 100 amps, it seems pretty unlikely that this could cause the battery voltage to jump to 18 volts (unless your doing this at VERY cold temperatures - like considerably below freezing).. My charging system is much lower so am guessing but would expect that you would see no more than a volt or two rise in healthy battery voltage for max current out of the alternator in the short time frame.
If you are seeing 18 volts, I would suspect something other than the alternator/regulator - like maybe a bad ground between the battery and the alternator?? Have no idea how the tach wire plays into this however..
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