N1277D wrote:It is also my understanding that an amp meter is not necessary, but it is wise to have one. The 170 is configured so that the amp meter measures the charging rate of the battery and not the output of the generator. Failures in either the generator or regulator can result in an overcharged battery that a volt meter would be slow to detect, but would be easly identified by an amp meter. I would recommend that you keep the amp meter and add a volt meter if space allows. Some but not all amp meters use a shunt, I believe the orginal 170 amp meter does not use a shunt. (This was the case in my 170A).
If you have a high electrical load, dual amp meters would be useful too, one to measure the charging rate of the battery, the other measures the generator/alternator output current.
We also have a stinson 108 (1946) in which the amp meter only measures the output from the generator; with all the electrical stuff turned on (landing and nav lights, nav/com, transponder, strobes) the battery will dischage even though the amp meter is showing a positive output.
The orginal 170 ammeter does indeed have an internal shunt. All ammeters are designed for either an internal, or external shunts. An ammeter without a shunt, or with an incorrectly/inadequately sized shunt will display the undesireable results such as a too-sensitive or too insensitive needle. One incorrectly installed may display like the mentioned Stinson, but more likely that battery has an internal short due to old age or rough handling, or it's regulator is maladjusted.
An ammeter will also show "no charge" if it's insensitive, or oversized for the circuit, such as a 60 amp meter in a 20 amp generator circuit. Your ammeter should be matched as nearly as possible to your maximum potential electrical load rather than your gen/alt capacity.
A voltmeter will show, say 16 volts, when your battery is boiling over from excess current due to a faulty regulator, while an ammeter may indicate only half-a-needle's width on the plus side. Not much warning from an ammeter. The most likely sympotom will be a "sulphur" smell like rotten eggs, consitently low battery water, or dripping acid from the battery overflow vent. (You DO check that yours is clear and free of insect nests dont' you? Pour some water into the battery box to check it. Better yet, pour some water into which a teaspoon of baking soda has been dissolved.)