[DrDave]

Greetings:

I am just finishing a wiring upgrade on a 172. Everything was "working fine." I found 426mV of voltage drop in the field circuit. High voltage drops/resistence in the field circuit cause the voltage regulator to be unstable, aka nervous needle. This does not include the 0.8-1.0 volt drop across the voltage regulator. I replaced the 1968 circuit breaker and replaced the regulator with a new Zeftronics unit. The old VR charged at 27.7V. The Zeftronics unit will charge at 28.8V. I replaced all the wiring in the field circuit. This includes the breaker to master switch, master switch to OVR, OVR to VR, VR to alternator. I found high resistance/voltage drop across the master switch. The total system voltage drop after the repairs is 125mV.

I then moved on to the alternator output circuit. I found a couple of questionable connections. I replaced the alternator to alternator breaker wire. The alternator breaker had a higher than expected voltage drop. It was also a 1968 breaker. After the new breaker was installed I chased out the charge wire that goes off the bus to the amp gauge and back to the battery.

Surprise time! The photo below shows the terminal connection at the amp gauge. There were no symptoms of failure evident prior to this discovery. I built a new cable to replace the cooked cable and terminal. The last wire to be replaced runs from the amp gauge to the line side of the starter solenoid. This wire completes the charge circuit back to the battery.

It is important to consider that the field circuit has nine or more connections. The alternator output can have ten connections before it gets back to the battery.

A good diagnostic procedure is to draw the system out in a block diagram. The field excitation/starter/charging system is not magic nor is it complicated. There is a series of connections that have to be clean and tight. You can have component failure. A little understanding of the system and a thorough inspection will reveal most of the common culprits. Having a meter to do voltage measurements and voltage drop tests is essential.

Dave