Location of diodes in the Alternator Restart system?

[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.

[Rick Erwin]

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

[wslade2]

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

[Rick Erwin]

Originally Posted by wslade2

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