Well, it seems to be the season for alternator problems, as just this week I had to diagnose another interesting one. As with any problem or incident, there is sometimes a chain that leads up to the actual event. One of the practices we have at Base Leg Aviation during the condition inspection is to look at items that may not necessarily require attention immediately, but due to the customer’s annual flying hours, may need to be addressed during the year. Our intent is for our customers to have an uneventful year maintenance-wise, so we will discuss these findings with the customer and together decide on timing. Normally, these are wear items such as tires, brakes or spark plugs. Some can be more invasive, such as magneto SBs, alternators, etc.
This aircraft had over 600 hours on a particular alternator that we’ve noticed has a history of failures between 500–600 hours. The customer uses the airplane regularly and flies IFR as well. I mentioned that he should be prepared for an alternator failure in the coming year. With dual electronic ignitions, both of which require an external power source, a reliable source of power is needed. Since the aircraft had a backup alternator and a backup battery, a decision was made to replace the alternator if/when it failed.
Sure enough, about four months later I received a text from the customer claiming I am the “Zen” mechanic, as his alternator had failed. This owner does a lot of his own maintenance, so changing the alternator was not out of his purview. I did walk him through checking for battery voltage at the field connection on the alternator, and he assured me he measured 12 volts at that point. With 12 volts at the field, and no alternator output with the engine running, it is a pretty good sign that the alternator has failed.
So now the chain continues. Anyone who has changed alternators recently knows that many of the manufacturers have changed the field connector over the years. It’s usually no big deal to cut the wires and splice the new connector into the circuit. Many of the connectors have a wire jumpered across two pins on the connector. The new alternator had two threaded terminals, so it seemed logical for the owner to connect the field wire to both terminals. He called and said the alternator was still not working.
I looked at the texted picture of the new alternator and realized that one of the terminals needed to be tied to ground, not to the field wire. He did that, and it still didn’t work. He also assured me that there was power on the field wire. Hmm…it seemed as though he may have received a bad alternator, so he got another one and it still didn’t work. Now I needed to dig in a little closer, still all remotely, of course.
By looking more closely at the part number on the alternator, it was clear that this one required an external regulator, whereas the original one was internally regulated. I pointed that out and told him which regulator he needed and how to wire it into the aircraft. Guess what? The next text I received was that it still wasn’t working! Now it was time for it to come to the shop and let me figure out what was going on.
Looking for Clues
Alternators are very simple in that they need power applied to the field for them to generate power. So, the first thing I always measure is the field wire. In this case, he had installed the regulator right on the firewall, with easy access, and I measured the voltage at the Enable pin, which should be the wire coming from the alternator switch in the cockpit. The meter showed 3.05 volts. Well, it sure isn’t going to work with that voltage level, as normally on the ground you will have somewhere around battery voltage at the field wire, approximately 12 volts. In flight, once the battery is charged, the field voltage will taper off to some lower steady-state voltage. As a matter of fact, what had been bothering me during our back-and-forth troubleshooting was why the alternator had not “run away” prior to the regulator being installed.
Now I had a real strong inkling as to the source of the problem. I opened the fuse panel and there it was, staring me very brightly in the face and lighting up the whole fuse panel: a blown field fuse, one that has a light in it to show you it’s blown. I replaced the fuse and the alternator proceeded to deliver 60 amps on startup.
Here’s what I think happened, and the owner agrees. The first alternator did fail, as we thought it might. Prior to replacement, he did verify there was 12 volts at the field wire. When the new alternator was installed, erroneously connecting the field wire to both terminals on the new alternator, one of which is a ground, blew the fuse on the field wire. When the new alternator still didn’t work, he claims he checked the field wire voltage and saw something, but he didn’t really notice it was not the bus voltage. Field wire voltage was not checked, even after installation of the external regulator.
Another tidbit that made the troubleshooting quick and easy for me is that the standby alternator was working. Since they are both electrically connected on the firewall side of the aircraft, I was immediately convinced that everything on the output side of the alternator aft of the firewall was intact.
Some overall lessons here: Perhaps we should more firmly recommend replacement of failure-prone parts while they are in the shop, as certainly it could be less time-consuming for everyone during the year. Second, once parts are replaced, don’t assume that nothing else has changed. When troubleshooting remotely, it is imperative to ask detailed questions. In this case, I should have asked for an actual field voltage value each time. It would have gotten the fun factor back online a lot quicker for this customer.