Electrical Troubleshooting?

[T0NY74]

Hello all, and Merry Christmas!!

I am the proud new owner of N3225A, s/n 25869 located in Cedar Rapids, Iowa. What an outstanding airplane!

Since buying the plane in October, I’ve done some minor upgrades & cleanup, but I’m struggling with an electrical gremlin. I have a Stratus Power Pro charge port that was installed as a part of a fairly substantial avionics upgrade in 2021. It has not worked since I bought it and the previous owner said he never used it. I’m not sure it has ever worked.

I pulled the glovebox yesterday and it appears the Power Pro is wired into the same circuit as the turn & bank indicator, which has also not worked in my few months of ownership. The hot wire is tied right to the push/pull switch labeled “turn & bank”, and the neutral is grounded to a screw that holds the copilot yoke ball in place. Nothing looks amiss with the wiring of the port itself, and it appears as if there is nothing else on this circuit. I pulled the turn & bank fuse, a 15 amp, and it was blown. Replaced with another 15 amp, and it blew the second I turned the switch on.

Any recommendations on how to track down the issue without totally disassembling my panel? Does anyone know what size fuse should be in this circuit? I’m tempted to just size up slightly and see if it works, figuring maybe the fuse was never replaced to accommodate the additional draw of the Power Pro, but am nervous about frying something accidentally.

Any help is greatly appreciated!

Thanks,

Tony Santacroce
1953 170B, N3225A / 25869

[n2582d]

Merry Christmas -- and welcome -- Tony!

Your electrical system has been modified from the original. Originally, -- on the 1953 C-170B -- the T&B had no fuse or switch. It, together with the stall warning, were protected by a automatic resetting two amp circuit breaker. When the master switch was turned on, the T&B was on.

'53 Electrical Schematic

The '53 Owners Manual says, "The turn and bank indicator and stall warning indicator are protected with an automatically resetting circuit breaker which provides intermittent emergency operation of these devices in case of a faulty circuit" (pg. 7). There are several threads on this Klixon circuit breaker. Here's one. While I have no experience with it, I have heard that the Schwien T&B "had a bad habit of burning up, which is why most were replaced." My suggestion would be to disconnect the hot wire (#15) at the T&B and see if the fuse still blows. If so not, have your A&P replace the T&B and restore the T&B and stall warning circuit to their original configuration. Also move the Stratus Power Pro charge port to the cigarette lighter circuit or the S-W heater switch circuit if that's available.

'53 Electrical Switch Panel

If you don't already have them, copies of the 1953 Owners Manual and Illustrated Parts Catalog are available for members in the "Mx Library" section here.

[GAHorn]

Hi, Tony!
Gary’s is “right on” with his comments/advice. I’d add that the yoke-ball mount is a lousy source for an electrical “ground” as well. The instrument panel SHOULD be well-grounded…but may not be as grounded as one might think at the control yoke mount area.

Also…. repeatedly re-setting a fuse OR a Circuit Breaker is a hazardous procedure ….as each time you do so you run the risk of over-temping the wire which the fuse/cb was supposed to protect. (A fuse/CB protects the WIRE….not the device.)

Also, I may have misread Gary’s advice….but the way I read it was that if the “hot wire” at the T&B is disconnected and the fuse STILL blows…. his sentence has you replace the T&B….(which in the case of being disconnected is actually un-powered…therefore untested…..and replacing the T&B would serve no purpose. If that situation actually exists…the fault lies elsewhere in that circuit….not likely the T&B.)

I would recommend you disconnect the T&B AND any other device on that circuit….. and re-wire the devices as they are Supposed to be installed per original instructions/design and provide a better “ground”.

The Mx Library is available to all Members of the 170 Assoc’n and not to people who are only “participants” in these public (free) areas of these Forums. I urge you to Join at our Home Page if you haven’t already. We also offer a simple Electrical Systems Manual free to Members which you may find useful. Hope this helps.

[cessna170bdriver]

That airplane belonged to Ed Booth for somewhere in the neighborhood of 50 years or more, and knowing Ed it didn’t leave him with that issue. I’d go back in the logbooks and see what’s been installed since he sold it (spring of 2021 IIRC).

The yoke ball cup is attached to the fixed part of the panel which is a structural part of the airplane, not the “floating” part of the instrument panel. However if the terminal is between the cup and attachment screw, and the cup and/or panel is painted, it won’t provide a reliable ground. The terminal should be attached to an unpainted point on a permanent metal part of the aircraft structure.

That said, if the fuse blew instantly on power up, a bad ground isn’t the issue. Something in that circuit is shorted to ground, either the T&B or the power port. I’d suspect the power port, as it’s likely it was the most recently item installed. Try disconnecting it and measure continuity between the power and ground leads. That, or just disconnect it and see whether the T&B works without blowing a fuse.

[n2582d]

Thanks for the correction George. Must have been too much eggnog. I’ve edited my comment.

A fuse/CB protects the WIRE….not the device.

This is good point that I overlooked in my initial response. If one replaces a 2 amp fuse with a 15 amp fuse it’s possible — if there is a short — that wires could start melting before the fuse blows. In this case, the wires in that 2 amp circuit were 18 gauge. Originally, circuits with 15 amp fuses have 14 gauge wire on your 170. (The smaller the wire gauge number, the larger diameter the wire).

[cessnut]

15 amps seems way too much for that circuit. I agree with previous comments. The turn and bank should be wired as it was originally. The power port should be wired to the bus with it's own separate circuit protection per the mfrs. instructions, probably nowhere near 15 amps. A couple minutes with an ohmmeter will identify the short circuit.