Backup battery switch?

by Claus Mortensen in 912iS Technical Questions 8 years ago

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Re: Backup battery switch?

by Claus Mortensen » 8 years ago

Claus Mortensen

11 27 6

I just read this in another thread. It did add a lot to this discussion.

[quote]-The Back-up switch is in case you need to quickly re-start the engine in the air.
• It is much like the start power switch as it provides a separate ground and 12 volt power that “re-energizes” the ECU and fuel pumps.
• Certification rules require the fuel pumps to stop if the engine stops. If the engine stops for any reason (ran out of fuel) the ECU and fuel pumps stop receiving power from the generators. By turning on the Back-up switch you can re-start the engine much faster.
• The Back-up switch also provides the redundancy for the relays between the two Generators.
• The Back-up switch must have red guard/cover over it to prevent inadvertent use and to signify it as emergency use only.
• Caution: Do not turn ON the Back-up switch during engine start as this will defeat some of the ECU self-checks.
-The Start-power switch is a momentary SPST(m) switch that provides 12 volts and a ground to the ECU so the engine can perform a normal start sequence. It needs to be a momentary type switch so it is OFF once the engine has started; then the ECU can perform its own self-checks. See more information regarding “start-ability” in the section “Rotary key Switches:”
-Difference between “Start-Power” and “Back-Up” power switches: Both excite the ECU allowing it, and the fuel pumps, to power up.
• The “Start-power” only powers up the ECU until the gen comes online.
• The “Back-up” also powers up the ECU but it is wired differently so that it bypasses the ECU self checks (so nothing interferes with a emergency re-start)[/quote]

As you can see I miss several things. The Start Power is not momentary. This means I have to change the switch to a momentary DPST type. I just ordered the other stuff, so this will be changed later. I ordered this switch guard: Red guard
But from the quote above, another thing is wrong. The original checklist actually defeat the ECU self test. And it also looks like you do not need to turn Start Power on when you have to restart if Backup Battery is on.

So plan right now is:

1. Make sure everything is connected/wired correctly
2. Reverse the backup battery switch
3. Add the red switch guard to the backup battery switch
4. Change the series resistor (or LEDs) for Lane A/B to make it visible in sunlight
5. Add parallel resistors to the LEDs to prevent them from lighting when off
6. Change the Start Power switch to a DPST momentary type instead
7. Connect Lane A/B to the Garmin G3X
8. Figure out what the green LED on the Backup Battery switch is indicating!
9. Update checklists

Did I forget something? ;)

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Re: Backup battery switch?

by Claus Mortensen » 8 years ago

Claus Mortensen

11 27 6

After a long evening working in the plane, this is the latest status.

- The three wires from 3-pole connector on the fusebox looks okay. I still need to figure out how pin 3 is connected to the 12V net.
- I CANNOT find fuses F1 and F2 anywhere! I have asked the supplier where to find them.
- The green LED on the backup battery switch was connected across + and - at the battery backup switch. I have removed that. It will not be visible anyway with the safety guard.
- The Lane A/B LEDs were weak. And did light when off. It turned out the 1kohm and 1.5kohm was mounted in the each leg of the LED. Not one in serial, and one in parallel as it should be.
- The shunt was not calibrated. It had a x1 multiplier. It should have been x0.5.
- The pin 2 at the backup battery switch is NOT going to the 12V bus. It is going directly to the battery. I know that because it will turn on even if the master switch is off. I still do not know the flow of the +12V wires. Before taking everything apart, I noticed the trim making noises when backup battery switch = on, and master switch = off. That means that the battery backup switch somehow lets power to the normal 12V net. How = unknown.
- I did find some fuses behind the instrument panels. One of the is named MAIN and is 25A. Others are for the Garmin products, and the shunt.

I will continue the work tonight. I'm not very happy about making changes to the 12V net, but it seems necessary.

Kind regards, Claus

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Re: Backup battery switch?

by Claus Mortensen » 8 years ago

Claus Mortensen

11 27 6

I give up!

It is so bad, that the supplier have to send a guy to fix this. It is simple too much for me to do.

I connected the two Lane A/B LEDs. Now they are bright when on, and off when off.

Then I started to check the rest. I had to remove the master switch and the start power switch to follow the wires.

I can measure that pin 2 in X3 is connected to the 12V at the battery when the master switch is on. I cannot figure out how that is. But I now know why I cannot find the F2 (30A) fuse. The wire is not mounted! So it looks like the ALT-B did not supply anything.
But when I followed the wire from the battery backup switch I got scared. It was melted several places! It is not connected through F1 to the 12V net. It is connected directly to the master switch INPUT side = directly from the battery.

It all makes more sense now. After we returned and landed, something smelled "burned". I could not locate it, but now I know. It is one of pins in the switch. It is all loose and combined with the melted wire, no wonder why it smelled!

I will let you know what the supplier is saying to all this tomorrow.

Kind regards, Claus

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Re: Backup battery switch?

by geoffrey klestadt » 8 years ago

geoffrey klestadt

27 94 94

Claus, There may be an error on the wireing diagram on page 33 of the 2014 installation manual which shows an installation with key switch.

In this diagram, the backup battery switch wire is connected direct to B+ .

On the installation with start and master switches (p32) , it is connected after the master switch.

The latest diagram (2015) also shows it connected after master switch meaning that in all cases the master must be on before the backup battery switch will work.

Could this diagram be part of the source of your problem? In addition, If pin 3 Is not connected, is alternator A doing all the work?

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Re: Backup battery switch?

by Claus Mortensen » 8 years ago

Claus Mortensen

11 27 6

Hi Geoffrey

I think I need to take a look at some older manuals.

I can feel I have to go the plane again for more documentation. I just seen that all switches turns the same way on a picture. It means that somehow it is connected so I need to have backup battery switch on all the time. I was sure that it was reversed. Tonight I will draw the diagram and then make a report to the supplier. I talked with them this morning.

I'm not aware if ALT-A is suppling all the load. But I did notice that when we took of (the second time), the battery was actually still be drained. Only when I turned off the aux fuel pump it went to a positive charge. I know that the change on the battery was low, but still.

Kind regards, Claus

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