Any idea what could have caused a dual alternator failure in a Seneca III? (schematic attached for reference)

As I was flying I did notice that the right alternator was producing about 10 more amps than the left alternator, but they were producing power during during the flight.

After I landed all of the avionics started to flicker and go off while I was taxiing. And when I went to restart the plane the battery was dead.

Any thoughts on what kind of failure would take out both alternators? Not sure if it's relevant, but both regulators were replaced about 18 months ago (not sure why, I've only owned the plane for 9 months)

Frank MacKenzie

At this point you do not know either alternator is bad. The airplane was running in the alternators until you shut it off. Most likely the battery has completely failed.

I’m with Don. Sounds like a bad battery or a loose battery cable. If everything worked fine until you landed, it isn’t likely an alternator or voltage regulator issue.

That was my first thought, too.

But I have some updated information. They did a CAP check on the battery and it passed. So they charged the battery and started the plane. Neither alternator is producing current.

Frank MacKenzie

What are the chances one alternator has been failed for a while?

I've seen this a lot with dual inverters and dual transponders. The customer always thought they failed at the same time but of course they never did

I know alternators are not the same since you should have an indication, but if there are issues with the fail light or ammeter it might not have been obvious

Interesting topic. I had a III and also had a dual alternator failure. Though after a few minutes, an alternator would give out something, only to crap out again. Cycling it off, seemed to help it come back to life.

In my case it was the seal (has a name, but I forget it now) on the alternator shafts. Both were leaking engine oil into the alternators. When they were pulled, they each had almost a cup of engine oil in them. If you can, run your fingers along the bottom of the alternators, check for any oil. It wont be much at all, because it has been windblown away. But it is a start.

Overhauled them both, new seals, all fixed.

And wouldn't you just guess, that was the first and last time I did a low visibility take off out of Florida. And everything was fogged in, until North Carolina. Go figure .

But it was a lesson and a half. And one thing that always stuck with me from early training, was to always know where the VFR weather is. That was a help as well as my (early days) GPS 195 with a battery pack ready to go. Didn't need it though, in my case I generally could keep one alternator functioning, they were both intermittent. That was a long time ago, but time has not diminished this or many other life lessons.

Hope this helps.

Garlock seal ?

Garlock seal ?

Your two alternator systems are completely independent up to the main bus so other than a fire there's really nothing that could cause both to die completely (vs an overvoltage trip which is resetable). That said, if the two diodes that isolate the alternator outputs from each other are co-located it could be possible for one to overheat and damage both but I think that's very unlikely.

Thus I agree the most likely scenario is that one failed before the other. This could have happened on your last flight since when one fails the other must handle the entire load (twice what it normally does) and the extra load could have hastened the demise of the last one to fail.

You didn't mention any "low volt" indications. One thing I don't like about your charging system is that the low voltage light will NOT come on if the alternator is putting out ANY current, even if it's a minuscule fraction of the total load. That means you wouldn't get any low voltage indication on one alternator unless there was a total failure. For instance if the brushes exploded or the regulator died on one side the residual magnetism in the rotor could easily allow that alternator to produce several milliamps which is enough to keep the low volt light off.

IOW the only way to tell that one has failed is a big discrepancy between the two loadmeters when the engine are running at the same RPM.

Interesting point. I did notice a big discrepancy between the two loadmeters.

Frank MacKenzie

They are a pain to set up to take the load evenly. Adjustment is in the nose compartment I believe.

In the past I would do an alternator full load check on my runup. Checking to see if either one could accommodate the load with the other switched off. Sort of like a mag check.

In my case it was something that just crept up on me. I think I did have one or two times an alternator would annunciate a failure but cycling the switch fixed it. Then poof, both had the same problem. Bad coincidence for me.

Yes, I do know.

Owned a Seneca II for a decade and know the systems very well.

The one common element for both alternators is the master switch. The masterswitch isn't just one single switch, it is a 4-way ganged switch and a whole section of it is not visible to you until you remove it from the airplane.

The hidden part of the switch is in series with the field coils and guess what, that part fails! (twice in my case, both the 40 year old one and the brand new replacement switch after less than a year in service!)

The system also features a diode bridge that sums the output of the two alternators together and feeds that to the system bus and battery. It is very unlikely to fail, but if one of the diodes shorted, it will likely take both alternators offline. It's on a big heat sink behind the panel and the diodes are large, "puck" style diodes, about as big around a 50-cent piece screwed into the heat sink with a stud where the wire goes.

And if you go looking behind the panel, you'll also likely find all kinds of FOD hiding back there that might short things out.

That seems odd... alternators are inherently diode-output devices. What's the value add from that secondary puck diode?

Paul

My only experience with a Senaca was hand propping a SII due to no electrical. This after that rocker switch hung in the "start" position causing the battery cables/terminals to melt.

I'm guessing the starter became a generator once the engine was running, causing mayhem and overloading the electrical.

Lesson learned. Pay extra special attention to the rocker switch in a Senaca.

In the III's that had the optional priming system (cold weather start kit was it?) , those push buttons, some of them had large washers in front and worse, behind the panel*. And some could drag against the rocker switch and prevent it from reflexing back into the center off position. Leaving the starter engaged. So I quickly developed the habit of keeping my fingers on both sides of the switch so that I was feeling for it to come back.

*How that got into being, is another story from the brothers Grimm.