Wire to battery?

[dstates]

I’m about to install a GT-50 clock to replace my wind up clock which doesn’t always run. The installation instructions describe one power wire that needs to always be connected to the battery. This is a very low draw just to keep the clock setting. Here is the text...

“Red (Always on Power) Connect this wire directly to the battery (not the master switch) to power all functions
except the backlight. This wire must remain powered at all times including when the yellow wire is powered to keep the clock powered. If you cant connect the red wire directly to the battery, you can tie it together with the yellow wire, but the clock will have to be reset each time you fly.”

My questions are, where is the best place and the what is the best method for connecting a 22ga wire to the battery? I don’t have a picture of the battery are of my plane handy to brainstorm.

Thanks,
Doug

[daedaluscan]

Mine went to the positive terminal of the battery with an inline fuse right there. I am in the process of some rewiring and it will now go to the "Hot" side of the solenoid, simply because I want only one cable on the actual battery terminal.

[dstates]

Mine went to the positive terminal of the battery with an inline fuse right there. I am in the process of some rewiring and it will now go to the "Hot" side of the solenoid, simply because I want only one cable on the actual battery terminal.

I’m not sure why I didn’t think about the solenoid. I should be able to put a ring terminal on that.

Thanks!

[daedaluscan]

I will be making new wires to solenoid, to starter solenoid and to starter (0-360). The old wire to the solenoid had a small wire crimped in with the heavy gauge that went to a small post on the solenoid. I am thinking of connecting clock to this post. It’s the original solenoid with a Bakelite looking top. I took this cover off and the contacts look brand new.

[FredL]

You should have battery voltage to your master switch as well. It may less hassle to wire your clock from there but either would work, i guess.

[cessna170bdriver]

You should have battery voltage to your master switch as well. It may less hassle to wire your clock from there but either would work, i guess.

That wire on the master switch is on the opposite side of the solenoid coil from the battery, and gets grounded when the master switch is turned on.

[cessna170bdriver]

When I had an analog electric clock, I just ran a fused wire from the main bus, so it only ran when the master switch was on. I set the clock to 12:00 before the beginning of the first flight of the day, and whatever time it read at the end of the day went into my logbook. For time of day, I used my wristwatch.

[GAHorn]

To enlarge upon Miles’ comment: Connecting a clock to the cockpit master can have unintended consequences. 1-If the clock draws sufficient current it can unintentionally close the battery solenoid. 2- When the cockpit master switch is deliberately activated the clock may not realize sufficient current to operate properly (it will see a “ground” on both clock leads.) 3-When the cockpit switch is de-activated the clock may experienced unintended / damaging surge, and 4- the battery solenoid may not release with alacrity thereby causing component damage to other equipement.

I.E.- Run a clock “keep alive” wire directly to the battery-side terminal of the solenoid and protect the clock circuit with a properly sized fuse at the solenoid-end. IMO

[FredL]

I appreciate the feedback from Miles and George but I don't believe we are on the same page, or should I say "diagram".

[GAHorn]

Here are two screen-shots of typical schematics for a late B-model. (These are not significantly different in any earlier 170 except as to numbering.)
There are two circuits which connect to/pass thru the cockpit Master Switch.

One circuit is illustrated by wires 7 and 136 and neither of those carry current when the Master is “off”. Their only purpose is to connect the generator Field terminal to the Regulator.

The only “power” from the battery is conducted (by wire 125 and terminal 55 on the battery solenoid) to wire “127” and might be used to power something hot-wired to that Master-switch-terminal serving wire “127”. When the Switch is activated by the pilot to the closed-position, all that current in wire “127” is directed straight to Ground via wire “54”. If the clock were connected to the wire “127” terminal on the switch...the clock would then see a grounded-circuit and suffer loss of power.

The best place to “hot wire” a clock will be to terminal 55 on the battery solenoid. IMO

Overview of Aircraft Elect Schematic

Cockpit Master Circuitry

[cessna170bdriver]

You should have battery voltage to your master switch as well. It may less hassle to wire your clock from there but either would work, i guess.

I appreciate the feedback from Miles and George but I don't believe we are on the same page, or should I say "diagram".

Using the diagram George posted, the only “battery voltage” on the master switch is wire #127, but it comes through the master contractor coil. With a voltmeter, it will read battery voltage with the switch off, but gets connected to ground when the master switch is turned on.

The only way to get battery power into the cockpit with the master switch off is to run a new wire directly from the battery. The best place to connect to the battery would be where wire #55 connects to the solenoid coil. Of course, this wire would need to be fused according to the wire size, preferably forward of the firewall to mitigate accidental grounding where it passes through.

[n2582d]

Cessna 172B wiring diagram as shown in 1962 and Prior Service Manual pg. 17-39:

Click to Enlarge

[FredL]

Based on the schematic George supplied, I don't disagree. If it were me, I would stick with the wind up clock.