G1000 power up – what is that clunk I’m hearing in the back?

[Subsonicflight]

‘06 Cessna turbo 182T with G1000 avionics: I’m hearing a clunk at some point in the avionics initialization process in the tail cone area every time I power up my avionics in the hangar.

More details:
I have a Schauer power supply model JAC2024H that plugs into the left side GPU three prong port. It’s rated at 20 amp so I’m supposed to be able to plug it in, power up the master and avionics switch and then turn on the power supply so that I can fiddle with the avionics buttons. Something’s not working because my MFD display indicates a 10 amp discharge, and I’m watching the voltage drop from 24.3 down to 23.8 VDC in about six or seven minutes. I never get the amber light indicator on the power supply equalizer, that is supposed to come on.

I called Schauer, and talked with Scott for several minutes as we went through troubleshooting on the power supply and it appears to be working like a good one.

The clunk I’m hearing in the back as the avionics initializes sounds like a door flopping closed on a big panel. It’s kind of a dull clunk. It happens every time. I’m wondering if it has anything to do with what appears to be malfunctioning airplane solenoid circuitry. I think the plane is supposed to recognize the external power and do something with solenoid switching. It seems as though my plane is just staying on internal power, and not accepting charge current through the external GPU three prong port. Not sure about all that.

Anybody have any clues or suggestions?

 

[Hang 4]

Is there an autopilot servo back there?

 

[Ryan Klems]

The G1000 182s will indicate a discharge on a GPU (where the main bus current shunt is in the system can't differentiate... the input from GPU plug flows to the GPU contactor and then on to the battery contactor and then on through the current shunt, it'll only read positive when current is flowing from the alternator side through the shunt), but your voltage is indicating that it's either not providing power, or not providing enough such that it is still drawing down from the battery.

The clunk is likely as @Hang 4 noted, the autopilot doing its self tests and engaging/disengaging the clutch in one or more of the servos.

 

[tree9er]

I have the same unit, my discharge never was as high as that though. All lights off?

Never heard the noise you are describing but I’m guessing the year you might not have the GFC700 that I had. Pull the AP breaker and turn on to verify what was said above.

 

[Subsonicflight]

I have the same unit, my discharge never was as high as that though. All lights off?

Never heard the noise you are describing but I’m guessing the year you might not have the GFC700 that I had. Pull the AP breaker and turn on to verify what was said above.

Yes, all lights are off.

I have a KAP140 auto pilot.

The G1000 182s will indicate a discharge on a GPU (where the main bus current shunt is in the system can't differentiate... the input from GPU plug flows to the GPU contactor and then on to the battery contactor and then on through the current shunt, it'll only read positive when current is flowing from the alternator side through the shunt), but your voltage is indicating that it's either not providing power, or not providing enough such that it is still drawing down from the battery.

The clunk is likely as @Hang 4 noted, the autopilot doing its self tests and engaging/disengaging the clutch in one or more of the servos.

Scott @Schauer had me take the back off the charger and verify the fuses on the AC input, and the 30 amp fuse on the output side were both good then he had me verify 26 1/2 VDC output at the gray output plug that connects to the Cessna three prong extension. All good. I’m hooking up the battery charger, with it powered off. Thenturning on my master switch, and then avionics one and two switch prior, to powering on the battery charger power supply, in that order, as specified by the power supply manufacturer. I’ve repeated that process several times. No joy. Battery voltage is dropping.

Re: the clunk. I think you are right about the auto pilot initialization cycling relays. It’s very consistent and the auto pilot gives me its ear-splitting tone shortly thereafter.

 

[Subsonicflight]

The G1000 182s will indicate a discharge on a GPU (where the main bus current shunt is in the system can't differentiate... the input from GPU plug flows to the GPU contactor and then on to the battery contactor and then on through the current shunt, it'll only read positive when current is flowing from the alternator side through the shunt), but your voltage is indicating that it's either not providing power, or not providing enough such that it is still drawing down from the battery.

The clunk is likely as @Hang 4 noted, the autopilot doing its self tests and engaging/disengaging the clutch in one or more of the servos.

Great information, thank you. I don’t know where the problem is at this point. Scott at Schauer said something about the intermediate output voltage being 26.5 V, as if there is some sensing required by the charger to see a significant current demand before it switches to outputting 28.8 V when set in AGM mode. In the flooded battery mode, it puts out something over 29 V, don’t remember exactly what he said.

 

[WDD]

Pull the breakers on the auto pilot and trim motors and then power up to see if you get clunking.

 

[Jon Wilder]

Autopilot servo clutch solenoids cycling during the autopilot pre-flight test.

Since this is an '06, I assume the autopilot is a Bendix/King KAP 140? Those servo clutch solenoids are quite loud when they cycle.

 

[Kitch]

06 206 G1000 KAP 140 the clunk is normal its the auto pilot doing its thing.

 

[Subsonicflight]

Confirmed. I pulled the autopilot breaker as recommended, and the clunk is no longer there. Discharge current also dropped from 10 A to 7 A. Thank you all.

 

[MauleSkinner]

It’s the avionics realizing it picked the wrong beverage…

 

[Hang 4]

If it's still drawing power after the clunk, something might need to be checked in the AP. Not familiar with that particular unit, but the servo still pushing against the limit of travel doesn't seem right.

 

[Palmpilot]

06 206 G1000 KAP 140 the clunk is normal its the auto pilot doing its thing.

I remember the aft clunk being a normal occurrence when I was flying G1000 182s.

 

[Subsonicflight]

Confirmed. I pulled the autopilot breaker as recommended, and the clunk is no longer there. Discharge current also dropped from 10 A to 7 A. Thank you all.

I need to amend this just a bit. I bought a new external power supply - Start Pac 25 A unit from Pilot John international. After a 90 minute IFR training flight today, up north to Deland, and back to Merritt Island, I was able to plug it in and found out that the old power supply was the problem. Very glad it wasn’t some solenoid switching thing on the plane. I’m still showing a 6 1/2 amp negative (draw) with the auto pilot circuit breaker pulled out. When the circuit breaker is pushed in, and the auto pilot has finished initializing, the draw is about 7 A continuously. That means the auto pilot is only taking about a half an amp when it’s sitting there powered up. Another thing worth mentioning is that I was showing a 10 amp draw when I had the alternator half of the master switch also on. I didn’t realize an alternator pulled so much power. Makes me wonder why I am turning on both sides of the master switch to start the engine… Seems to me that starting would be easier if the alternator side of the master was not on. I guess they don’t want you to forget that half of the switch, and fly off with your alternator not working. The very good news is that the Main bus and Essential bus are both staying at 28.3 V continuously - for two hours. So the new Start Pac power supply is working like it should. I can now fiddle with my G1000 flight plans and learn how to push all the buttons. Thanks again for your help.

 

[Dan Thomas]

The clunk is likely as @Hang 4 noted, the autopilot doing its self tests and engaging/disengaging the clutch in one or more of the servos.

Yup.

Another thing worth mentioning is that I was showing a 10 amp draw when I had the alternator half of the master switch also on. I didn’t realize an alternator pulled so much power. Makes me wonder why I am turning on both sides of the master switch to start the engine… Seems to me that starting would be easier if the alternator side of the master was not on. I guess they don’t want you to forget that half of the switch, and fly off with your alternator not working.

The alternator's field pulls less than half that. There are a couple of solenoids in that circuit that also draw some current. Once the engine starts spinning the alternator, the voltage regulator lowers the field current to keep the output voltage in check.

 

[Jon Wilder]

Yup.

The alternator's field pulls less than half that. There are a couple of solenoids in that circuit that also draw some current. Once the engine starts spinning the alternator, the voltage regulator lowers the field current to keep the output voltage in check.

The voltage regulator modulates the field current based on the output voltage feedback to maintain a steady output voltage regardless of current demand.

 

[Dan Thomas]

The voltage regulator modulates the field current based on the output voltage feedback to maintain a steady output voltage regardless of current demand.

I know that. But before startup, with Master and Alt on, the regulator sends full field current to the alternator, because it senses that the system voltage is well below the charging voltage. That's what the OP is seeing.

One the engine starts, the regulator starts throttling the current to get the system voltage where it needs to be. As loads change, or as engine RPM changes, the regulator is busy adjusting field current.

 

[AV8R_87]

But before startup, with Master and Alt on, the regulator sends full field current to the alternator, because it senses that the system voltage is well below the charging voltage.

Thread drift, but that's why I start with the Alt switch in the Off position, and only turn it on after engine start. No need to cook that field winding for a few minutes and waste battery power.

 

[Dan Thomas]

Thread drift, but that's why I start with the Alt switch in the Off position, and only turn it on after engine start. No need to cook that field winding for a few minutes and waste battery power.

We once had an alternator that would not charge after startup. I took it off and opened it up, and found the rotor (field) winding cooked. The insulating varnish was black and falling off and the windings had shorted and burnt out. The only cause I could think of was that some student had left the master on after shutdown. After that, we told the students to leave the beacon switch on when shutting down. It gets your attention.

It would take some time for the rotor to get hot enough to cook those windings. I never found any other cooked rotors, and I had those alternators off every 500 hours for the brush checks. Did that for maybe 30,000 hours, or 60 brush checks. We started the engines with the alternator switched on.

The alternator has some resistance to turning when the field is on, but it's not that great. Easily turned by hand. But once it spins up and the stator voltage overcomes the diodes' forward resistance, it will consume some horsepower. Even then, a 60-amp alternator at 14 volts draws a theoretical 1.1 HP. and if that alternator is only 70% efficient, it will need 1.6 HP. That's at full load, which you never have at startup. A starter that draws 200 amps at 12 volts is generating around 3 HP.

 

[Subsonicflight]

You guys rock. I’m a fan of this friendly discussion. There is so much to learn from you folks. Appreciate your comments.