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Topic-icon Seneca III fluctuating voltage on left engine

  • Terry Reed
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4 months 3 weeks ago #1632

I recently purchase a 1984 Seneca III. While doing a prebuy flight at daytime , I noticed the alternator master warning light would occasionally wink on and off. The owner (who had owned the plane for 20 years said that he had recently replaced one of the voltage regulators and that they needed to be "synced up". I took this to mean that the left and right alternator systems were not adjusted to be producing the same voltage. I purchased the plane and began a 1200 mile flight home which caused me to be flying at night. Again, the alternator master warning light would occasionally "wink" on, but now I could definitely see there was a correlation to this "winking" and the cockpit/instrument lights varying in intensity. The single ammeter was rock steady. When landing for fuel, I called the seller to discuss this anolomy further. He now said that his mechanic had attempted to adjust the voltage regulators to create the same output voltage, which he had done, but that had not eliminated the problem. He also said that he had discovered by putting a high voltage draw on the system, hot props and/or the heater, that the system would "settle right down". I did just that on the remainder of my flight home and there appeared to be no additional problem. However, back in sunny California with no need to have high draw accessories on, this "winking" of the master alternator light became a nuisance so I further trouble shot the system. What I discovered was the voltage output on the left alternator was varying between 12.7 to 14.2 volts when under a light load (collision lights and avionics only). When applying a heavy load at cruise rpm, the voltage would stabilize at 13.7. The right side was rock steady at 13.8 volts regardless of light or heavy load. My mechanic then attempted to adjust both voltage regulators and discovered that the left system would occasionally drop in voltage and then vary back to high with a rather rhythmic pattern when not under a high load. He then swapped the two voltage regulators to see if the problem transferred to the right side. It did not. He there fore concluded that the problem must be in the left alternator (possibly a diode) or in the alternator drive coupling. He then removed the alternator, the drive coupling as well as the left side voltage regulator and had them sent out to a very reputable accessory testing/rebuild shop. They reported back that all three components tested fine and that they had adjusted the voltage regulator to 13.8 volts which is what they recommend for sealed batteries (which this plane has). They did however talk me into replacing the alternator since my unit had a SB that they highly recommended should be complied with. My mechanic received everything back and installed all the components. If anything, the fluctuating voltage and dimming and brightening of the cockpit lights now appear to be worse. So much so that I am not going to operate the plane until the problem is rectified. Any thoughts out there or similar experiences?

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4 months 2 weeks ago #1634

The electrical system in your Seneca is pretty simple. Since you have tested the big $$$ components and they are good, the fault must be in the remaining system components. I have seen fluctuating alternator outputs in Cessnas; the problem was usually traceable to resistance in the alternator on-off switch. I suggest you look at the connections of the switches and/or swap the switches to see if the fault follows the switch.

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  • Terry Reed
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4 months 2 weeks ago #1635

Thanks for the advice. I have now accumulated a list if items in the electrical system to either measure the resistance of, or to bypass. I am also going to jumper around the existing alternator field lead to see if there is a problem with the there. It is still strange that the left alternator system causes the dash and interior lights to flash from dim to bright while both engines are running and the right system is rock steady at 13.8 volts and the left system is not exceeding that value...in other words the left system is not exceeding 13.8 volts at the top end of its output. It seems to me that the right system would mask the left side fluctuation...but it does not. Keep in mind, that the left alternator is never completely dropping off line...and the "alt out" idiot light is never as bright as it is when you actually turn off an alternator. Anyway, thanks again for the advice.

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4 months 2 weeks ago #1636

Hi, It seems as though you and your mechanic have a good idea of steps to take to run this down. And since you've explained it further, I can see how it is confusing.
Obviously the light and the fluctuating lights are indicating a problem; why is that one alternator have control over the bus voltage when the other alternator is on line and steady??
The buss voltage is varying--more at slow rpms than at high rpms.
A healthy alternator bridge will result in what's called ripple DC --it's not without some variance (which can be seem with some expensive multimeters or an O scope) but good enough for aircraft systems. When a diode in the bridge fails, it's fairly easy to troubleshoot because it will result in a high pitched whine in the headset that varies with rpm. Turn off the right alternator and see if you hear the whine.
Please keep me in the loop on your progress and what you find.

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4 months 2 weeks ago #1641

STEVE,

Thank for your interest in my problem. So far I have now chased down any possible problem with the separate alternator switches as well as the master and the field wire itself. No Joy! You however bring up an interesting point in that there is noise over the headset when running the heater and hot props. My son has flown the plane more than I and I am in the process of getting him to weigh in on the high pitched whine issue. I will ley you know what he says. But you make a good point about one alternator with fluctuating voltage taking over when the other is steady and performing within the same upper limit. And the fluctuating one is never going off line. This weekend I am going to remove the Parallel connection between the two voltage regulators and try to determine if the balancing feature within the stable regulator is being high jacked by the other. I like your idea that the bridge is the problem. I don't know if this fits into our discussion, but the previous owner has recently volunteered that while on a check ride, the check airman had him shut down the left engine. Upon restart while in the air, his KNS 80 got fried. Those are his words. That means that the voltage spiked. He replaced the KNS 80 but the fluctuating voltage problem apparently showed up at the same time. It is amazing the information one gets after the purchase is made.

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4 months 2 weeks ago #1643

Balancing alternators is done by taking one off line, putting a meter on the buss and adjusting the one that's running to the desired voltage (14.2 or 28.4 for instance). Then turning the one just adjusted off and doing the same on the other one.
While your meter is on the buss bar, check the output voltage of each one at different RPMs--the regulators should keep outputs stable.
The best voltage regulators come from a company called Zeftronics and it's repair air Hazotronics. Very knowledgeable.

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