This is a simple non regulated 3 wire rectifier ???
If you are using a digital multimeter to test it you should use one with a diode test range. The ordinary ohms resistance ranges do not usually work for testing diodes. Sometimes one renge will work but not others. Sometimes none will work. Hence the reason to use a meter with a diode test range.
Dont forget that in a three wire rectifier there are 4 diodes to test. Each one should be tested in each direction
Using a diode test range, in one direction you should get a reading, in volts or mV depending on the meter, of the forward volts drop in the other direction no reading. If you get no reading in either direction the diode is open circuited. If you get a reading of zero the diode is short circuited.
Old fashioned moving coil meters usually work Ok on the ohms ranges but will give a low or zero reading in the forward direction, not the forward volts drop that a digital meter will give.
Sorry cannot help with your tachometer problem
