What Happens with External 3 Pin Power

[WDD]

What exactly happens when you plug in 3 pin external power?

The third pin is to energize a solenoid switch to allow power to flow.

But is it the same solenoid switch that your instrument panel “Master Battery” switch activates? Or is it a separate one?

When you plug in external power is it sending power to your battery, or is it just energizing the starter circuit?

 

[Klaus M]

Not all aircraft shore plug power are created equal. Look up the schematic in your Aircraft's Maintenance Manual, take a picture of it and post it here. Cessna, Beech and Piper did all combinations to different model years. Only the correct schematic analysis can say for sure.

 

[WDD]

So I found this in the service manual. Application of external power opens the relay supplying voltage to the electronic bus.

I think my solenoid might be stuck open BTW, needing replaced. Battery was dead after sitting for a few hours when I tried to do the return trip. When the master was off I was still spinning up the gyro when it was attached to the external power.

Then again sounds like the start cart by passed the master switch and made it unable to switch on or off.

Battery is showing 12 volts plus, and Battery Tender is showing it charged. Battery is 2 years old.

Something isn’t right in any case. Guys will be looking at it Monday. I’ll be paying them Tuesday……

 

[WDD]

 

[Bell206]

But is it the same solenoid switch that your instrument panel “Master Battery” switch activates? Or is it a separate one?

External power relay is a separate dedicated relay.

When you plug in external power is it sending power to your battery, or is it just energizing the starter circuit?

In general, external power is sent to the battery relay. Whether it has any affect on the battery itself depends on various factors. However, Cessna has several variations of this system even within specific model runs. So it depends aircraft S/N which exact routing you have and if it is still original.

I think my solenoid might be stuck open BTW, needing replaced

Which solenoid (relay)?

Battery was dead after sitting for a few hours

Battery is showing 12 volts plus,

??? Was this after you charged the battery?

When the master was off I was still spinning up the gyro when it was attached to the external power.

Then again sounds like the start cart by passed the master switch and made it unable to switch on or off.

This would be normal with external power applied in most cases.

How did you determine battery was dead prior to return flight?
Have you checked any voltages with a VOM?
Are the manual ref above applicable to your aircraft serial number/model?
Do you have a separate avionics switch?

 

[WDD]

How did you determine battery was dead prior to return flight?
Have you checked any voltages with a VOM?
Are the manual ref above applicable to your aircraft serial number/model?
Do you have a separate avionics switch?

- I tried to start engine and it wouldn’t turn over. Nor did it spin up gyro. BUT maybe battery wasn’t dead and the start cart just unstuck the solenoid ???

- Voltage on a multi tester was 12, before putting it on a Battey Tender.

- After flying for 1 hour and landing to let passenger out, plane again would not start. Put car jumper cables directly on battery and plane started.

- Yes, the manual pages shown above are for my specific SN

- Yes, I have a separate avionics switch and for the Garmin , Audio Panel, and Stratus Transponder stack.

 

[Bell206]

I tried to start engine and it wouldn’t turn over. Nor did it spin up gyro. BUT maybe battery wasn’t dead and the start cart just unstuck the solenoid ???

Possible. Or one of several other possible issues. Unfortunately, without more info especially from a voltage value or voltage drop perspective not much I can offer with any certainty.

Put car jumper cables directly on battery and plane started.

And it wouldn't start with power applied at the aircraft external power plug?

 

[WDD]

And it wouldn't start with power applied at the aircraft external power plug?

This airport didn’t have the right plug for the start cart, so he used car jumper cables.

 

[455 Bravo Uniform]

Check the functioning of this solenoid. It should click when ground power is attached. POH will advise whether master should be on/off or split. Check your logs to see if someone changed something (like when/if an avionics master was added). It took me a while to figure out my system a few years ago when I had similar problems with ground power and battery jumping needed. 1976 182P

 

[Bell206]

This airport didn’t have the right plug for the start cart, so he used car jumper cables.

FYI: Since you have an external power plug, you should consider buying or DIY-ing an adapter to use jumper cables like below. Jumping direct at the battery posts in that manner can lead to new problems especially if you don't know for certain what caused the original lack of power.

 

[WDD]

……when I had similar problems with ground power and battery jumping needed. 1976 182P

Did it turn out to be a bad solenoid in your situation? My plane BTW is a 1974 182P

you should consider buying or DIY-ing an adapter to use jumper cables like below.

Not a bad idea. I was looking yesterday at buying a female plug and a set of WalMart jumper cables, cutting one set of connectors off the cables and soldering them to the female 3 Pin connector.

 

[455 Bravo Uniform]

Did it turn out to be a bad solenoid in your situation? My plane BTW is a 1974 182P

Yes. No click, so I replaced it.

I also replaced my starter contactor (solenoid) based on voltage drop. I then replaced the short 9” cable between starter contactor and starter based on the other half of my drop (I mail ordered a custom cable from an airplane guy that someone on POA recommended; he even asked how I wanted my terminals clocked).

The whole thing started with slow then no cranking (clicks only) around 2017/2018. I first replaced the 7 year old Concorde battery, which probably didn’t need replacing other than age. Then 2 years later replaced it again (absolutely the wrong thing to do). In the meantime my ground power relay wasn’t working, so had to resort to jumper cables at the battery.

There are a few threads on here about chasing down the culprit with a multimeter. I learned to diagnose rather than replace parts that someone thinks I need to replace. It saved me money and increased my confidence in understanding my plane’s electrical system.

It’s a bit of work and time to learn and figure out, but I’d rather be doing that than working my job to make money to pay somebody else to figure out out that would’ve probably been more frustrating for me and for sure more expensive.

By the way, make sure you’re working with an A&P who can sign some of this stuff off and makes sure you don’t cause new issues. When I told one of my guys what I was going to replace and why, he was happy to oversee and approve (log) my installations.

 

[Bell206]

I was looking yesterday at buying a female plug and a set of WalMart jumper cables, cutting one set of connectors off the cables and soldering them to the female 3 Pin connector.

Another option: I have several old customers that carry an adapter like the one I posted above along with the portable vehicle start pack they carry in their vehicles. Haven't heard anything negative except to make sure the start pack is strong enough.

 

[WDD]

Another option: I have several old customers that carry an adapter like the one I posted above along with the portable vehicle start pack they carry in their vehicles. Haven't heard anything negative except to make sure the start pack is strong enough.

Nice adaptor - but for me, maybe a little too nice for the $.

Adding on to that is that with my luck, as soon as I buy it I'll never need it but something else will pop up LOL.

https://www.amazon.com/Aircraft-Tool-Supply-Plug-Cessna/dp/B005VR88K6/ref=sr_1_2?crid=1VG5UJC7AQ001&dib=eyJ2IjoiMSJ9.bTeja50FXaajiTpdpN0Vyeoe_Xz9m2jRXBBXjmzMlHyVXPRxXC7UXih-EKcp_tVEAJxxC3_6LfO0AlwujEtXUuKk3Ovl1uKrp9FoOnnLPIjwtNM_XbOFbX953yhM38Sfqj4e-JYg6kXKereFtDIPilj_PcsDgZHtFoDzfB2yphtgETYGa_1OqgAhQu0MIdjxQ3F2z8CfOJSfApkwygXKj2Hcp5P4VdGLeFoe-F5keR0.3cc5B73Yv0rquIiyAs9WMfMhDofWtLa45EgcKrjtHK0&dib_tag=se&keywords=cessna+power+plug&qid=1724615066&sprefix=cessna+power+plug,aps,92&sr=8-2

 

[Klaus M]

What exactly happens when you plug in 3 pin external power?

The third pin is to energize a solenoid switch to allow power to flow.

But is it the same solenoid switch that your instrument panel “Master Battery” switch activates? Or is it a separate one?

When you plug in external power is it sending power to your battery, or is it just energizing the starter circuit?

Found a 182P POH online. The instructions for 'Ground Service Plug Receptacle' is in the back Section 7. The only instruction is to have the master ON when plugging into the Receptacle. From the schematic the Plug Power would charge the battery when the Master is ON. The down side to charging the battery is the power would be distributed between the battery charging and the starter. If the power source is not enough to do both the starter may not turn the engine even if the starter solenoid clicks. After plugging the Ground Power in 'Wait" (maybe 20 minutes) for the battery to have enough charge to assist in starting.

The schematic shows that the instruments 'are powered up' without the master ON. Also the starter solenoid will engage the starter with the master switched OFF. Clear the prop area before plugging in. If the power source is great enough to start the engine don't turn ON the master switch until after the plug is disconnected.

 

[Bell206]

Nice adaptor - but for me, maybe a little too nice for the $.

I got the pic from Aircraft Spruce $89.99. But I would never buy any "aircraft" parts from amazon. However, if that is still too rich then try the $47.00 here and DIY some pigtails to clamp on Or if your start pack has interchangeable leads like my NOCO pack rig up a direct connect? Or maybe check eBay or some fly-in swap meets for a used AN2551 plug which is what the $47 plug above is. Years ago I picked up a pallet of new APU cables with AN2551 type plugs for $100 at a govt surplus sale and fabricated about 20 battery carts for a number of customers. As they say where theres a will theres a way.

 

[Plano Pilot]

Which gyro was spinning up? If it was the T&B it looks like it may be normal according to the 182P manual I found.

 

[WDD]

Which gyro was spinning up? If it was the T&B it looks like it may be normal according to the 182P manual I found.

It was the electric turn coordinator. The nav lights and beacon were also lit even with the master switch off. So yes it looks like the external power connector triggered it’s own solenoid to energize the circuits as well as the master switch solenoid was (and probably still is) jammed closed also allowing the circuits to be energized.

 

[Bell206]

as well as the master switch solenoid was (and probably still is) jammed closed

If you have external power connected it will power your main bus and all items connected without the master switch on. However, there is the possibility if you had a diode failure for the master relay it could be closed via the external power circuit. Be easy to check with a VOM. If your master relay was jammed closed you would need to disconnect the battery to shut things down.

 

[WDD]

If your master relay was jammed closed you would need to disconnect the battery to shut things down.

I did pull the battery and put in a charger. I’ll put it back in tomorrow morning and first see if I can hear a solenoid click on and off, or if the plane is powered with the master switch off indicating a stuck solenoid. We shall see.

 

[Dan Thomas]

Let's look at that wiring diagram.


#6, way over on the right, is the external power jack. The bottom pin is the small one, and it is positive. Power runs though the top diode at #10 to the coil on the external power contactor, #11, closing the contactor. Now the + pin on the jack, the middle one, is the pin that carries the external power to one of the contacts in the external power contactor, though the contactor and through wire D-PA13 to another contactor's pin and then through wire D-PA11 to D-PC6 that takes it through diode #13, resistor #15 and fuse #14 to the "Bat" terminal on the master contactor. This allows the master to have the power it needs through its coil to turn the master on IF the master switch is turned on. The master contactor closes, allowing the big current to run through wire D-PA11 through the master contactor to the battery, charging it and making starting possible.

Now. The diodes at #13 and #10, resistor #15 and fuse #14 are all suspicious characters here. Any one of them will disable the external power if they're defective. They are found on small circuit boards. The first one here has fuses #14 and the clock fuse, and diode #13 and resistor #15:



And diodes at #10 will be on one of these two:



An open diode #10 (top) or #13, resistor #15 or fuse #14 will prevent external power operation. A shorted diode #13 will drain the battery overnight by keeping the instrument cluster energized. I have seen it only once. I have seen burned-out resistors and fuses. The fuses are also subject to contact corrosion since they're close to the battery and its acidic fumes. That battery box is not airtight.

 

[WDD]

Appreciate the "translation" - thank you. It seems then that..........

1) When getting external power, if I want to charge the battery and start the plane I need the Master switch "on".

A shorted diode #13 will drain the battery overnight by keeping the instrument cluster energized. I have seen it only once.

2) It sounds like it might be possible cause, but unlikely. More than likely it is the Master Switch solenoid. I will know more tomorrow morning.

 

[WDD]

I will know more tomorrow morning.

I know nothing. Mechanic can’t duplicate the fault. Battery load tested perfectly, shows good volts. Solenoid works perfectly.

I’m ordering a solenoid and will have it put in next week for some peace of mind anyway.

 

[Dan Thomas]

I know nothing. Mechanic can’t duplicate the fault. Battery load tested perfectly, shows good volts. Solenoid works perfectly.

I’m ordering a solenoid and will have it put in next week for some peace of mind anyway.

Replace both the master and starter solenoids. They get old, and the starter solenoid actually suffers more due to the arcing when it opens and the inductive voltage spike from the starter jumps the contacts. If either the master or starter solenoids have any resistance, even 1/50th of one ohm, starting becomes slow and difficult. Small resistances times large current flows causes large voltage drops. Ohm's Law.