Stupid Electrical Question re Power/Ground connection

[hish747]

I recently bought a Cessna 172H with a Narco MK12D w/Glideslope that appears totally inop. I swapped it out with another same unit and also shows no life. I located the attached wiring diagram and discovered that power appears to be provided via connector P101, pins A, K and 9. Ground is provided by pins L and 10. With avionics power switched on, these contacts show somewhere between 12 and 13 volts.

What I thought was strange was when I tested using the the multimeter ohm/sound/continuity function and touched one probe to the power pins A,K or 9 and the other to the ground/radio tray they all beeped showing continuity between power and ground. Can this be normal? Seems like there's a fault somewhere in the system.

 

[Dan Thomas]

Some other stuff connected to the bus will provide paths to ground. The electric turn coordinator is one. Fuel gauges are another. So are panel lights. Anything that comes alive when you turn the master on will do it. They're effectively all wired in parallel, reducing the resistance. Depending on the sensitivity of your multimeter's continuity function, these things can fool you into thinking there's a short somewhere.

Narco MK12Ds were terrible radios. We had a few of them, and the constant repairs soon added up to more than their purchase price.

 

[Der Fliegermeister]

It's ringing through the breaker pull it and try again

 

[Bell206]

Can this be normal? Seems like there's a fault somewhere in the system.

FYI: Another method if you're trying to check just the Narco system is to physically isolate it. Determine the most accessible place and disconnect the +/- wires there. With the wires disconnected then perform your continuity checks on those wires. Or perhaps use a test light to check power at the connector. Sometimes you can have power and continuity but no amperage. The test light will provide some load.

 

[Dan Thomas]

Sometimes you can have power and continuity but no amperage. The test light will provide some load.

That there. Ohms Law says that Voltage = Amperes times Ohms. Or voltage divided by ohms = amps, or voltage divided by amps = ohms.

Now, if that Narco MK12D+ draws one amp in receive mode (probably less, but lets just stick with simple numbers) and the radio circuit breaker has oxidized contacts and has 10 ohms resistance through it, We now have a ten-volt drop. 1 amp x 10 ohms = ten volts. That 13-volt battery is losing ten of its volts though the breaker and the radio won't wake up on the remaining three volts.

Now, if we pull that radio out of its tray and take voltage measurements, we will see 13 volts. Everything looks good. But the multimeter draws only a few microamps to drive its measuring circuitry; let's say 50 microamps. 10 ohms times 50 microamps (.00005 amps) =.0005 volts. That voltmeter isn't going to show a voltage drop of .0005 volts; it will show 13 volts.

So Bell's assertion that the circuit needs to be under some load to reveal the resistance lurking somewhere is valid. A lot of components have been replaced because they didn't work and the power supply to them looked OK on the voltmeter. The new components don't work, either. Sometimes we learn this stuff the hard way.

The same phenomenon causes the owner to spill lots of money on new starters and/or batteries because the starter won't crank properly. The real problem is often in the starter contactor and/or master contactor. Because of the huge amperage drawn by the starter's low resistance, any resistance at all in one contactor causes a big voltage drop that makes the starter lazy. A tiny fraction of an ohm will do it. Ohm's Law. A $30 contactor instead of a $200 battery or $500 starter can fix it.

 

[hish747]

Thank you everyone for all the great info!