Ammeter problem

[N9937c]

I have a 1981 m20k ser 25-0544 with a problem. Since the purchase of this plane 2 years ago the ammeter has not moved.

I know its charging by using the voltmeter and the battery always stays charged. Well I decided  to have a 231 that EVERYTHING worked and removed the glareshield. The shunt wires in and out of the shunt from the alternator to the 70 amp alternator breaker are 20 ga. wire instesd of 6 ga. Anybody else had this trouble? The wires are marked the same as the rest of the wires with the MOONEY codes.

Thanks for your time

larry

[Cruiser]

Not sure why wire gauge has anything to do with the ammeter not indicating. Did you check the fuses?

[N9937c]

The 6 ga wires carry the 60+ amps of alternator and the 20 ga wires carry the voltage to the ammeter.

the fuses are good.

It looks as if the shunt is in parallel with the alternator to 70 amp cb.

larry

[Cruiser]

An external shunt is basically a very low value resistor that is in series, and then the ammeter is in parallel to the shunt. Basically MOST of the current goes through the shunt and only a small percentage goes through the ammeter. By knowing the resistance of the shunt and the meter, the meter is able to calculate the total current by measuring only the small current that goes through the meter. This kind of setup allows small wires to be run to the ammeter in the cockpit, and the high current to remain in the engine compartment. The reason for the fuse on that small ammeter wire is if the shunt is damaged or a connector breaks, it would try to flow all the current through the remaining path which is the small ammeter wire and that would melt that wire.

Sounds like the shunt resistance is not matched to the ammeter