Location of diodes in the Alternator Restart system?

[Rick Erwin]

There are two diodes in the Alternator Restart system, one in each line just downstream of the Alternator Restart switch going to each alternator.

Does anyone know where these two diodes are located on a Pressurized G model?

Rick

[mshac]

Originally Posted by Rick Erwin

There are two diodes in the Alternator Restart system, one in each line just downstream of the Alternator Restart switch going to each alternator.

Does anyone know where these two diodes are located on a Pressurized G model?

Rick

Check around the restart battery box under the panel. Its behind and above the center pedestal. For future reference please report back when you locate them! The diodes allow the restart batteries to be in-circuit with the alternators, without taking a charge from the alternators.

[Rick Erwin]

Thanks for the reply, Mark.

The diodes are not anywhere near the batteries ... that I have found. The diodes are somewhere downstream of / between the Alternator Restart Switch and the respective alternator.

I think both my diodes might have failed closed as I have .6v at the rear alternator field terminal, and .3 volts at the front alternator field terminal. So, I need to find them and replace them.

Rick

[mshac]

The chances of both those diodes failing at the same time is infinitesimal.

I suspect you have another problem.

Can you explain in more detail what exactly occurred leading to you posting this question?

[Rick Erwin]

Originally Posted by mshac

The chances of both those diodes failing at the same time is infinitesimal.

I suspect you have another problem.

Can you explain in more detail what exactly occurred leading to you posting this question?

Hi Mark,

I doubt both diodes failed at the same time. I don't know for sure as I don't think the system has worked since I've owned the plane. The diodes are part of (and the resistor could also be part of) the problem.

The system is quite simple, here's the power flow:

Either, 2 D-Cell batteries (or 4 C-Cell batteries, if the SK337-46A Battery Pack Kit has been installed) supply the voltage ... I now have 4-Cell batteries installed);

The voltage from the batteries goes through a connector (which I haven't found, but must be good as I have voltage all the way to the alternators);

A resistor (12-ohm, 1-watt is located at the Alternator Restart Switch (the resistor is not open, because I have voltage at the alternators, ... and voltage is being reduced). I've not ohmed out the resistor yet as the cabin pressure instruments must be removed to check/replace it, so it "might" have the rated resistance value;

The power splits at the Alternator Restart Switch;

Then each line goes through its respective diode (F4 in the parts manual, superseded at Textron to 1N2070);

The power goes through another connector (which I haven't found, but must be good as I have voltage all the way to the alternators);

The power then reaches each alternator's field terminal.

I have about twice as much voltage at the rear alternator as I do at the front alternator (albeit both voltages must still be too low to excite the alternators). This voltage difference points to something downstream from the Alternator Restart Switch (ergo ... "The Diodes") as being, at least, part of the problem.

That's why I need to know where the diodes are located on the airframe.

[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 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!

[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 Timcote1960

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

Tim

Hi Tim,

In addition to your three questions to the group, I am also interested in knowing:

Has anyone here ever had the "need" to actually "use" the Alternator Restart system inflight? If so, what was your experience with the system?

*****

For reference, my airplane is a FT337GP. Same as a P337G.

There are two checks we should be performing.

1. The POH alludes to the need to do a "functional check" of the electrical system as part of the "Before Takeoff" check. While this functional check is not delineated in the "Before Takeoff" checklist (Page 4-8), it is delineated in the "Before Takeoff-Amplified Procedures" (Page 4-15). I suppose the obvious place for this functional check to occur would be at item (6).d. "Alternators -- CHECK", in the "Before Takeoff" checklist.

2. The POH, on Page 7-44, indicates that every 25 hours we should be accomplishing an operational check of the "Alternator Restart" system. This check is to be accomplished during Day-VFR-Cruise flight, with a heavy load on the electrical system.

During each of these two checks, the ENTIRE electrical system of the aircraft will be shutdown. Personally, I'd rather not do this in-flight, ... even if it is Day-VFR.

I am more inclined to start the engines, accomplish the functional check, and ... if due - load up the electrical system (without avionics), accomplish the "Alternator Restart" check, unload the electrical system ... then, turn on the avionics, and get going.

Does anybody here care to answer Tim's three, and my two, questions?

Rick

[edasmus]

My experience with this is limited but here it is. My plane has had overvoltage situations occur over the years and the system worked as designed in my case. I wrote a long post on this years ago and I'm sure it's somewhere on this site.

It has long since been resolved and the condition does not occur anymore but it took awhile to find the culprit of what was causing the overvoltage condition. It was a short in the circuit breaker box to the left of the pilot's knee. The main large thick wire that provides power to the the buss bar vibrated and wore a small hole through the plastic insulator box and allowed intermittent contact with the skin of the aircraft and caused arcing. It actually burned a small hole in the skin of the aircraft.

Anyway, voltage would exceed the limit (which I think is about 32.5 volts if memory serves) and the overvoltage sensor would trip the alternators offline. Turning everything off and then turning the master switch off for a few seconds and then turning the master back on would reset the overvoltage sensor. Turning the alternators back on and then the remainder of the electrical system and I'd be back in business until the next time it happened (which might be months later).

Anyway, occasionally turning the alternator switches back on after resetting the master switch would not bring the alternators back on line. A press of the alternator restart button would solve that.

Anyway, my alternator restart batteries get replaced at every annual inspection and realistically I test each system about once, maybe twice a year. I figure that's good enough for me. If my alternators quit for any reason, (which they never have once this aforementioned problem was resolved), I'll attempt to get them back online. If that fails, I'm unloading the electrical system, putting the gear down, and then deciding how long I care to fly without the electrical system. I fly day VFR so as long as the gear is down, not having the electrical system isn't a serious problem. You serious instrument pilot's obviously have more at stake.

It's kind of pain to test the overvoltage system which is why I do not do it all that often. If one is to test it, make it the very first thing you do after starting the engines and bringing the alternators on line. I would strongly suggest keeping all avionics out of the equation. They shouldn't "feel" anything which is the point of the entire system but no sense injuring your avionics while finding out the over voltage protection didn't work as designed. I share Rick's sentiment as I test this on the ground.

Further opinions appreciated. Interesting thread. Thanks for starting it Rick.

BTW Rick, I've been on the waiting list at Bowman KLOU for about 5 months now. Hoping for a hangar there by next year sometime.