Location of diodes in the Alternator Restart system?

[mshac]

Rick, I hope you can forgive the erratic thought process here, I kept thinking of other thing to check as I wrote:

While repairing electronic circuit boards, I've replaced dozens of diodes. They most commonly fail "closed" whereby voltage is passing through, but now it can pass both directions, instead of only one direction, as designed. The other way they fail is "open" whereby no voltage can pass. 95% of the time, its the first case.

If you're inputting 6Vdc at restart battery box, but only seeing .3 & .6 Vdc at the fields, my primary suspect would be the resistor, because I've never seen a diode fail whereupon it acts like a resistor and lowers voltage.

Time to remove those pressure instruments I'm afraid. You need to check that resistor. Without seeing the diagram, its not clear to me why there would be a resistor in the circuit at all. What is the voltage supposed to be at the field???

Have you checked that you're getting 6 vdc out from the restart switch? I would test all the points of connection in the circuit prior to the switch, as this is where the power splits F/R, so the culprit is likely upstream from here.

Also, you say you have voltage "all the way to the alternators", but that voltage is miniscule. Why not find the connectors you refer to and test the voltage there? In this manner you can isolate the portion of the circuit where the voltage is dropping.

Another culprit could be corrosion at the connectors. Another reason to check them

I hope you figure this out, but honestly who has ever used the alternator restart system? I've flown many other types of twins with no such system, and lived to tell about it!

Seriously, if someone has had an occasion to use the restart system, please tell us about it.

[Rick Erwin]

Thanks for the thoughts, Mark.

I was thinking that tomorrow I'd dig into the airplane and start tracing wires till I find the two connectors and the two diodes.

I have new diodes coming, so that should be an easy replacement, and I can measure the resistance across the resistor to rule that out.

Should be fun ...

[Kim Geyer]

Look at the connector for the voltage regulator one of the pins has 2 wires on it. One of those is the alt restart. I’ve had that connection get janked up and cause problems

[Rick Erwin]

Thank you for the tip, Kim. I’ll check that tomorrow also.

Rick

[Learjetter]

Rick,
I found the "droids you were looking for!" :-)

Here's a photo of my ALT RESTART switch, as removed from my panel yesterday. A previous owner removed the ALT RESTART battery pack years ago, so during this recent annual, I opted to remove the switch, and will document the removal in the logbook.
Hope this helps!
-LJ

--Upon further reflection, the ALT RESTART switch is going back in the panel. Turns out, there is a no-kidding emergency checklist called TOTAL LOSS OF ELECTRICAL POWER that calls for pressing that button. I guess I'll add the battery pack back in too...but I'll be very reluctant to press that button unless the checklist calls for it (for all the reasons described in other threads).

[Rick Erwin]

Originally Posted by Learjetter

Rick,
I found the "droids you were looking for!" :-)

Here's a photo of my ALT RESTART switch, as removed from my panel yesterday. A previous owner removed the ALT RESTART battery pack years ago, so during this recent annual, I opted to remove the switch, and will document the removal in the logbook.
Hope this helps!
-LJ

Learjetter, thank you for posting the pictures.

I did get access to the resistor and the diodes and checked them; they are good.

Even after bypassing the resistor, the switch, and the diodes, (by running a test wire from the dry cell batteries directly to the field terminal on the alternator) the same problem exists. As soon as the wire from the batteries touches the alternator's field terminal, the voltage drops to zero. There must be something in the alternator that's causing this behavior. BTW, the alternators work fine.

I'll tackle this problem again on Monday, and report back when I have anything significant to report.

[Timcote1960]

Been reading this thread with interest. I myself have never tried the alternator restart system, and feel a bit guilty about it because the POH says to test it on a regular basis. I am wondering how other 337 pilots have managed this---is it all that important? What about the high voltage and the high voltage test system? I haven't run those either, but there they are recommended, mostly as a procedure to do while aloft.

When I was taught to fly the 337, the CFI told me "Just treat it like a really big, heavy 182." While I respect the aircraft's complexity, I guess I have glossed over some of the finer point. The electrical system has been one of them.

What say---do you run these checks as prescribed in the POH? Do they work? Is it worth it?

Tim

[Rick Erwin]

Originally Posted by Kim Geyer

Look at the connector for the voltage regulator one of the pins has 2 wires on it. One of those is the alt restart. I’ve had that connection get janked up and cause problems

Kim,

I looked at those Molex connectors, and one of the field connections did need some rehab, but unfortunately that wasn't the source of the problem. Thank you for leading me to look there though!