The International Cessna^® 170 Association

So this is one of those deals where I'm sure this isn't right, but there's just a grain of doubt in my mind.

The other day I shut off the radio while the motor was running, and for some reason pulled my headset off one ear. I've had this habit of switching off the master right after pulling the mixture then leaving my hand poised for a couple seconds till the motor quits to shut off the mags. This day with my ear out of the headset I could hear my poor old rotating beacon making it's grinding noise over the sound of the motor. I shut the master off, pulled the mixture off and then noticed I could still hear the beacon grinding away. So I put the mixture back in and listened for a sec. The long and short of it is my main power bus is hot as long as the motor is running, yet nothing is hot nor can you make it so when the motor is not running.

So I'm thinking this is yet another nugget of joy left for me by the idiot mechanic that muddled around with my engine conversion a few years ago, the same guy who didn't wire the voltage regulator up with a proper ground. Am I missing something here? Somehow the alternator output is holding the master battery solenoid closed when it should be open as soon as I switch the master switch to off. Am I not seeing the forest through the trees here?

Something isn't right! the power should go down when the master switch is shut. I would never have seen that with my shut down sequence......I use SLIM....Switches, Lean, Ignition, Master ...........Brad

If there is no buss power until the engine is running, how in hell are you getting starter power? Sounds to me like you better take your airplane to the electric shop. I think you need the assistance of a qualified electrical repairman. There are a s---pot full of A&P mechanics that don't know squat about electric systems.
When a charged battery is installed, the battery master solenoid should "Make" by simply grounding the small terminal. This should energise the buss. It should not be able to "Make" unless there is voltage on the battery side large terminal. For example, if your battery is completely dead , you can start the airplane by propping it. However, if it is equipped with an alternator you must have an electrical power source to energise the field to get the alternator going. This can even be done with a flashlight battery to get the alternator on the line and energise the buss. If you're equipped with a generator, the risidual voltage will bring the generator on the line and the buss will be energised. However, in neither case will the battery be able to be charged until the master solenoid terminals have been jumpered across to get voltage to the battery side in order to energise the solenoid coil closing the circuit to the battery.

Try shutting OFF the Master and leavng the engine running above 1200 RPM ....and let us know what happens. (Question: Does the electrical system remain engergized?)

Jr.
When your airplane was converted to an alternator it is possible that the Alternator output was wired through the Bus Bar, this is a common though not original on a 170 (it has some advantages). This wiring technique presents the possibility of mistakenly wiring the Alternator Field lead to the electrical system at a place that is un-switched, very dangerous as you have no way of isolating the electrical system in case of fire. If you have a split Master Switch you should be able to see this condition on your Amp meter by running your engine and turning off the Alternator half of the master switch if there is no change in the Amp meter your Field lead is not wired through your Master Switch. I personally would not fly this airplane until fixing this problem.

gahorn wrote:Try shutting OFF the Master and leavng the engine running above 1200 RPM ....and let us know what happens. (Question: Does the electrical system remain engergized?)

The way I was reading it, that is what I thought he said was happening.....Brad

gahorn wrote:Try shutting OFF the Master and leavng the engine running above 1200 RPM ....and let us know what happens. (Question: Does the electrical system remain engergized?)

Yep that's exactly what happens.

I may have to take a day off work so I can spend some quality time rooting around in the wiring. Thanks for the tips guys, the more I think about this the more it gives me the heeby jeebies.

I haven't had time this weekend to look at it, but I have spent some time in the past looking at what has been done behind the panel and I am pretty sure that I remember the alternator output does come straight in to the master switch. Actually I think it goes first to the breaker then to the master switch. I hope it's as simple as the field wire being on the wrong pole of the switch.

If the dynamo output is coming to the master switch, you must be runnng a split switch with it being hooked up on the field wire side. The other side of the master switch would haveto go to ground in order to operate the solenoid.

It's not a split switch. It's a single toggle.

I'm not familiar with the conversion, but depending on the solenoid used, and alt/reg used, wiring etc., you could have a missing or failed diode at the switch pole of the solenoid

wingnut wrote:I'm not familiar with the conversion, but depending on the solenoid used, and alt/reg used, wiring etc., you could have a missing or failed diode at the switch pole of the solenoid

Del,
Wouldn't the bus be hot all the time?

johneeb wrote:

wingnut wrote:I'm not familiar with the conversion, but depending on the solenoid used, and alt/reg used, wiring etc., you could have a missing or failed diode at the switch pole of the solenoid

Del,
Wouldn't the bus be hot all the time?

Well, there are some possibilities, but without a wiring diagram of this particular installation or the aircraft, it's difficult to troubleshoot. There is probably a diode and jumper wire at the master switch or solenoid. the alternator probably supplies power to to the buss thru a circuit breaker. If the diode, which allows voltage to flow one direction to the regulator (controlling field voltage) was installed backwards, or failed for any other reason, it would allow the regulator to continue supplying field current to the alternator, which will continue to supply voltage to the buss until shutdown. Just feeding itself.

Again, I'm not sure about this system, so this is only one possiblity