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.