The International Cessna^® 170 Association

Hey all - have some time on my hands, so I am going to (under AP/AI supervision) rewire and replace fuses with circuitbreakers.

I had a few questions before I go to him so I am fully armed.

In checking the C170 parts manual, figure 23...

The master switch has two halves (confirmed looking under the panel). I had the original generator replaced with an alternator. I suspect one of the sides of the master switch is redundant, but I'm not sure. In a modern split-switch master one half is battery-ammeter/bus, and the other half is the field switch for the alternator. I know that my installation accomplishes this field switch with a dedicated circuitbreaker/switch.

The parts manual lists 23-6 as ""master switch to master solenoid" which is pretty straightforward. This would be connected to item 23-54 ("Master Switch to Ground") to close the circuit on the master solenoid and connect the battery to the bus.

Item 23-7 is "master switch to generator" and there is a figure to the right of that I cannot discern in my PDF (it looks like it might be "69" ("Voltage Control to Master Switch") which would make sense - acting as a control signal to enable the voltage regulator (similar to, but not the same as a field circuit for an alternator). If so, then I don't think this is required in this installation. I'd rather remove things not required, so going to a SPST switch is my choice.

Can anyone clear this up for me?

Thanks -

RT

Richard, the original cockpit Master switch had two circuits within it. One circuit, merely provides a path to "ground" for the battery relay (the little cannister on the battery box) so it could close and thereby pass battery power on to the starter and aircraft systems.
The other circuit simply completed the connection between the generator field and the voltage regulator "Field" terminal.

You didn't say, and I'm not necessarily familiar with, your particular alternator installation. But your alternator was probably installed in accordance with an STC or other approval basis, and you should not alter that situation. If your present alternator uses the cockpit Master switch, then you should not make any change in that wiring (except perhaps to change it over to new "tefzel" wire of the same or greater gage.) Don't make any unapproved "improvements".

I hope this helps.

George, thank you as always for the quick advice. The alternator is the Jasco/Skytronics, and I have studied their wiring diagram and seen that they show only a single connection on the master, using the CB as a switch for the field. I will trace out the wires on the other half of the master tomorrow - I think I have figure out what the two tie-wrapped & unconnected cable are for now...

And always under AP/AI guidance, of course.

There is a wiring diagram for the 50A Jasco alternator at http://www.skytronicsinc.com/wiring_diagram.htm . This diagram doesn't seem to be as detailed as the one that came with my Jasco alternator in 1990. I don't have it with me to be sure, but I seem to remember that my diagram depicted using the double-pole master switch as George describes, one side to energize the master contactor (the "disconnect switch" on the above referenced diagram that appears between the ammeter and battery), and the other side in the field circuit. The field circuit also has the reccommended circuit breaker so the alternator can be disabled separately from the battery, a la the newer split master switches.

I drew up a complete diagram of my installation showing ALL of the charging system components and wiring, including connectors, firewall passthroughs and wire gauges. If I can remember I'll scan and post it.

Miles