Maintenance Corner

So, you’ve run the alternator charging/ load test from last month, and your rpm is over 1,000 , or the ammeter needle is ‘wiggling’ , indicating unsteady charging, or the voltage doesn’t come up to 14.0 14.5 volts. Now what to do ?

  1. Troubleshooting the alternator charging system. – From the battery, to the alternator and the main buss, through switches and breakers, and back to the battery, there can be 26 to 51 connections, any one of which can cause a charging problem or a voltage issue on the main buss. See the sketch of a sample experimental electrical design. Each of the ‘black dots’ represents one of the 26 connections. Any one of these can cause a failure. The red colored wires source power from the battery, while the green colored wires supply alternator power to the electrical system and recharge the battery. The arrows show current flow.

    wiring1

    The Cessna 172R electrical schematic has 51 connections, so has twice as many points of failure.

    wiring2

    – The systems with the fewest number of connections and components will have the highest reliability.

  2. Tests:
    – voltage checks at each of the components. Referring to the sample wiring diagram above, check all the wire connections for tightness. Check the wire crimps – that the wires are tightly crimped to the terminals.

    1. With master and alternator ‘ON’, [ engine ‘off’], using your digital multi-meter on 20 volts, DC, there should be battery voltage at the ‘battery’ screw/nut, [ not the terminal] on the alternator, and within 1 volt of battery voltage at the ‘field’ terminal on the alt. – If both voltages are there, the alternator is defective. – if there is battery voltage at the ‘battery’ terminal of the alt., but no, or low voltage, [ more than 4 volts less than battery volts, ], at the ‘field’ terminal, there is a problem in the wiring and connections to the voltage regulator.
    2. Check the voltage at the ‘field terminals of the voltage regulator: – if no or low volts, continue checking for voltage at the input and ‘sense’ wires. – if there is voltage on the input, but not the output, check the other connections to the regulator, including the ground wire. If they’re ok, the regulator is defective.
    3. if still no or low volts, check the voltage at the alternator switch/ breaker or field breaker [ it depends on the parts installed], and connection to the buss.
      # Also measure the voltage across each of the terminals of the switch and breaker. There should be no more than 0.5 volts between the terminals. If there is, briskly ‘snap’ the switch off and on a few times, or pull and reset the breaker, to see if the voltage drops to a low value. The contacts inside switches, breakers , contactors and switch/breakers will corrode and wear over time. The useful life is about 10 years for these parts that are operated on each flight. Breakers will trip at lower currents than rated if they are continuously overstressed and made to trip. # Switches can sometimes be made to work for a while by quickly ‘snapping’ the switch on and off a number of times, with power applied. This can mechanically remove the corrosion from the contacts. As a general rule, switches and breakers should be actuated with a brisk movement, not a slow movement of the lever