Location of diodes in the Alternator Restart system?
 

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.

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

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Then each line goes through its respective diode (F4 in the parts manual, superseded at Textron to 1N2070);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

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.

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 ...

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

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

Rick

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).

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!

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.

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

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

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.

There is more info on this topic in other threads, and even more in the old CPA forums, but most of it boils down to this: Tom Carr recommended removing the alt restart battery pack and placard in the system inop with a logbook entry. His theory is that pressing that button releases unregulated current from the alternators into the system and if a diode is failed, could start an electrical fire or explode the batteries. My airplane was missing the battery pack on delivery, and I’ve chosen to just placard it inop and move on.

I’ve run the overvolt test and it went fine, and twice I’ve turned off an ALT and back on with no issues (all as ground tests). I mitigate the risk of not having the battery pack by limiting my flight in Hard IFR.

-LJ

Hi Ed, I have had a couple of High Volt events also, the checklist restored alternator power … for a while. The fix was two new voltage regulators and replacing all the diodes on the electrical panel circuit board on the front firewall.

I took me probably six to eight months for my name to come up on the wait list for a hangar. Since I needed one of the 48 ft. hangars, it took a little longer for one of those to come open. I expect you’re in that same boat. Once they let you know your name has come up and you get moved, it’ll be great having you here!

Rick

Tim, I’m a nerd and do all the tests, I can’t do the ALT restart as the system was removed at some point in the past and was not documented. The button is on the panel and with the panel upgrade this coming year, the restart kit is going back in. Years ago, while flying patrol in the 337, I actually used the button to restart the electrical system where other attempts failed.

I do the electrical system ops test on every flight after both engines are started, before the radio master switch is turned on. It looks at some very important stuff and takes seconds once you have done it a couple of times. The test actually caught one of the warning lights being out, one flight it was good, the next not. Individual lights are not replaced, the unit is pulled out and a new one is put in.

Thanks for all the info Rick...

Alot of interesting information here, have been reading and absorbing very interesting stuff.

I did find in the maintenance manual that the purpose of the restart switch is for when the alternators drop out due to high electrical load such as use of flaps motor (see attached photo of page out of maintenance manual for my E model). Apparently the older systems (voltage regulators) would drop offline with heavy load. Maybe we don't experience/expect that as much now with more modern electricals, lower draw avionics, better charging gear.

I can not find in my manual any specs on maintenance of the system.

Now I am probably going to do something I will regret as I am sure there is someone more knowledgeable than me. But I will stick my neck out here.

Thinking about the original problem, there is a low voltage at the field connection of the alternator when the restart button is pushed, 0.3v and 0.6V. Something in the back of my head, that I may have read a long time ago, says it doesn't take much juice to restart an automotive style alternator field. Witness the battery pack of 2 D cells (3V) or 4 C cells (6V). That said: thinking back to college physics, decades ago (gulp), V (volts)=I(current) x R(resistance). So we have a circuit with 4 cells and a resistor with 12 ohms. Best I can find, looks like healthy "C" cells are capable of 1 amp a piece unbridled. So that means when the button is pressed 4 amps max can come out of the box. Suppose the cells are at least a little dated. Maybe assume 0.5 amps max output a piece, or 2 amps when the button is pushed with a somewhat dated contents in the battery box. The voltage drop across a resistor will be subject to the V=IR rule. So assuming 2 amps of current and 12 ohms of resistor, 2 x 12 = 24 volts drop across the resistor. Subtract 24 volts resistor drop from the 4 "C" cells voltage of 4x1.5 = 6 volts and an output of 0.3 to 0.6 volts at the alternator field starts to sound pretty good. This gets to MSHAC's question of what should the voltage at the alternator field terminal be when that switch is pushed. And maybe the answer is pretty low.

Also, the issue to excite the alternator may not be so much voltage from the restart circuit, but current. Current, movement of electrons, creates the field that the armature spins in to generate electricity. When the button is pushed, it's not so much how much voltage is in the field but how much current is going through it. Again, V=IR or V/R=I. We have 6 volts of "C" cells and 12 ohms of resistor: 6 divided by 12 = 0.5 or 1/2 amp. My reading, 1/10 of an amp is enough to excite an automotive style alternator.

I will not be offended if I get quickly dispatched on my discussion by anyone more knowledgeable. Just sticking it out there. Maybe the voltage stats detected are the norm.

Perhaps the next test is not a voltage test of various wiring junctures but an ammeter test of the alternator restart circuit. See how much or if any current flowing through when button pressed. Also, another next step might be a test of the system switches per the procedure outlined by others here to see how the system works. Or was dysfunction on a test what started the dive into probing the circuit?

see attached

wslade2, thank you for lending some mental energy to this circuit. Your thought process seems sound.

1. Monday, I'll check the amperage load while the circuit is energized. Maybe it's not volts that are needed to restart an alternator, ... maybe it's amps?

2. The switch in the restart system is good. To eliminate the switch as a culprit, I have even bypassed the switch during testing, but to no avail.

The two diodes in the system, as well as the resistor, all check within specifications. The alternator switches are good. The voltage regulators are good. The entire electrical system works great ... except for this Alternator Restart circuit.

Is it to be expected that voltage should drop to zero when the dry-cell batteries are connected to the alternator's field terminal? Or, is there something wrong inside the seemingly good alternator/s?

3. Yes, all this started when the checklist specified test failed to restart the alternator/s. The checklist specifies in the event the alternators do not restart, reduce the electrical load, and try again. I attempted that, without success.

Thanks again,

I'll report back Monday evening ...

Rick

Also, be sure to check the ground. Will have to have a good ground to make a complete circuit especially if it’s low amps. I expect the negative side of the battery box to be secured to a ground somewhere. Also attention to the ground for the engine (which becomes the ground for the alternator).

Will do, thanks again.

So, today I tried to do an amperage test to see what the current draw is when the dry cell batteries are supplying power to one alternator. I know I'm not proficient in the use of a multimeter ... I had no success. I even resorted to YouTube to see how to measure an amperage load, and I followed those instructions, but to no avail. I'll attempt that again tomorrow.

I checked the grounding circuit and there was no resistance at all between the ring terminal on the negative battery cable and the airframe anywhere I measured it, even at the alternators.

Tomorrow is another day...

Rick

Ammeter should be hooked in “series”. one probe of the meter hooked to one end of wire and other probe hooked to the other end. Just think of it like cutting the wire and putting the meter in the line, like two leads of a light socket. If you’re able to select, start high on the amperage and work down. I don’t buy very expensive meters and most of those have a 10 amp capacity. It’ll say on the meter. They usually use a fuse and if apply too much current they’ll blow the fuse and end reading zero. If zero reading, check fuse in meter in case it’s blown. You should get some kind of reading. If not back to testing every connection and solder joint

wslade2, thank you again. That is how I thought the multi-meter should be hooked up to read amperage. That's how I thought I'd done it successfully in the past, and YouTube verified that. What I didn't check is the fuse in the multi-meter. Perhaps I can get to that tomorrow, as I'd still like to know.

Well, today I had some level of success. After doing three different iterations of an under-load test at each junction in the restart system, (for some unknown reason), the system started showing voltage at the field terminals. I don't know why, which is frustrating, because the problem could return anytime.

The decisive test will be to run the engines and perform the Alternator Restart Test to see if the 1.75-ish volts at each alternator are enough to excite the fields and restore power to a cold electrical system.

Rick

With wiring issues, sometimes you just have to keep going over it. A little detail can be the problem.

Today there was a measure of success.

We ran the engines and performed a pseudo test of the Alternator Restart system. It worked!

The entire system has not been reinstalled yet from the testing, so I just used dry cell batteries to excite the rear alternator and it fired off with 1.5v. Once the rear alternator restarted, the front came up as well. The next step is to reinstall the system and do another test. Since testing has shown I will have 1.75-ish volts at each alternator's field terminal when the system is installed, I'm hoping I'll be good to go.

I did see it would NOT restart with ANY load on the system though. Since an alternator's field has about a 3.5 amp load, I think it would be advisable to only try to restart one alternator, then turn on the other alternator.

There will be more testing to follow.

So, I have:

• Changed from the original two D-Cell batteries to the currently specified four C-Cell batteries,
• Replaced the Alternator Restart switch,
• Replaced the resistor,
• Replaced the two diodes,
• Reinstalled, and tested the system.

I now have 2.2 volts on the field terminals of the voltage regulator's Molex connectors.

Either alternator will now consistently restart when:
1. The electrical system is unloaded, "and"
2. Only one alternator switch is turned on.

My experience has yielded the Alternator Restart system will not restart both alternators at the same time, nor will it restart if there is anything but the most minimal load on the bus when the restart system is activated. I was able to restart an alternator, with one led taxi light switch turned on, but with nothing additional.

Both the "Alternator Restart Test" checklist, as well as the "Total Loss of Electrical Power" checklist, indicate the electrical load should be reduced, and specify both alternator switches should be turned ON when trying to restore power. So, if you really must use this system, contemplate turning only one alternator ON, as you follow the checklist.

Once one alternator is restarted, and power is being resupplied to the General Bus, there will be power to the remaining alternator's field terminal, so when the remaining alternator's switch is turned on, it will come online also.

Has anyone been able to restart more than one alternator at a time, and/or restart with any kind of a load on the electrical system?

Can anyone explain what the purpose of the resistor is in the line from the dry-cell batteries to the Alternator Restart switch? It sure seems counter-productive as it lowers the voltage at the field terminals of the voltage regulator's Molex connectors.

Rick

Rick, as I study the system, it makes no sense to me why the resistor is in the circuit. If its voltage that makes the alternator restart, and more voltage is better (at least up to the alternator's rated voltage), why would we want to reduce the voltage in the circuit with a resistor???

It would be like putting a resistor in a circuit with a light bulb - the light bulb would be dimmer due to lower voltage.

A resistor may be used to prevent voltage spikes, but again, in a DC circuit with small batteries like this one, I just don't see the need.

You could simply put an alligator clip test lead on either side of the resistor to bypass it, and then test your voltage at the field when pressing the restart button. You should see an increase.

If you do this test, please let us know your findings.

Here’s some interesting reading:

http://www.electronicsplanet.ch/en/e...-in-series.php

In short, don’t bypass the resistor, it protects the diode

wslade2,

Thank you for posting the article.

Jumping around the resistor was one of my options if I couldn't get the system to work, but thankfully it now works as designed. It's actually a pretty cool system. Hopefully, it will never be needed.

Rick

My airplane has the restart switch installed, but nothing is coming off it. A previous owner must have removed everything. I have the updated battery pack Kit, but I cannot find a wiring diagram with components. Can anybody help me figure out what is needed to get this completed?

I spoke with Don Neiser, he has the switches, but he hasn't gotten back to me with a diagram or material list.

I discovered my pack was missing the first time I did the 25 hour checks of the electrical system. I assumed the batteries in the pack needed replacing, which went several months up to the annual. Three annuals later, I'm still trying to get this done. No one wants to touch it. A metal panel is being installed in this third annual, and it needs to be completed.

I have used the restart in another Skymaster I flew on pipeline patrol. I consider it an airworthiness item based on that experience, and that there is both the pilot performed interval test in the POH and emergency checklist. Honestly, had this been caught on the pre-buy, I would not have bought the airplane.

PatrolPilot: The Alternator Restart System is pretty simple. I rebuilt mine from the ground up.

I've attached a highlighted shot of the schematic showing the system. This schematic shot is from plate E22.

If I'm remembering correctly, you have a G model, you still have the installed switch, and you already have the Service Kit that gives you the 4 C-cell battery pack. You just need the:

• Resistor, (#9) and the
• Two diodes (#12)

Plate E19 shows the:
Resistor to be a 12 ohm, 1 watt resistor, and
Diodes to be an F4, which was superseded to 1N2070. Both are readily available from eBay and Amazon.

Getting to the parts on my P model was a PITA though. I don't recall if you have a P though. The stuff lives above the pressurization panel behind the behind the switch.

Rick

Thanks, Rick, I owe you. My mechanic and avionics guy gave a sigh of relief as well.

I'm glad I could help.

Rick