Location of diodes in the Alternator Restart system?

[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.

[Learjetter]

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

[Rick Erwin]

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

[patrolpilot]

Originally Posted by Timcote1960

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

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.

[edasmus]

Thanks for all the info Rick...

[wslade2]

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

[Rick Erwin]

Originally Posted by wslade2

1. 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.

2. 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.

3. 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

[wslade2]

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).

[Rick Erwin]

Originally Posted by wslade2

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.