So here I was, bored at work, wasting time on AVCANADA and such; when a local pilot comes in with a problem on his Seneca. He was IFR, in cloud, enroute from Seattle to Invermere, and he had experienced a double alternator failure.
Right alternator and regulator replaced only three months ago, left one only two years old and about a couple hundred hours on it.
Down in the US, the A & P who installed the R/H alternator could only get 12.5V by setting the regulator to its maximum setting. Hmm….
The owner was flabbergasted. He is absolutely convinced that the system wasn’t adjusted properly and wasn’t paralleling and he wants me to set it up properly so he can feel safe amongst the clouds again. But he does not want me to condemn any components because they are all pretty much brand new. Sure…
So I get the manual and find the procedure; pretty simple really. As the regulators are accessed from the front baggage door you obviously can’t run the left engine. So you take the paralleling wire out and run the right individually; setting it up to 13.5V. You then compare the field voltages of the left and right regulators and set the left so the voltage is zero compared to the right. Put it all back together, bing bang boom you’re done.
Well… I could only get 11.5V out of the right with the engine running and a 30AMP load on the alternator—the regulator was still pegged at the top. Hmm…interesting.
We closed the baggage door and ran the left engine with the regulator pegged… still 11.5V.
We switched regulators and tried again… still nothing. Nothing common in both systems was at fault either (master switch, master contactor, battery, etc).
I traced every wire in both systems and found nothing wrong. I tested the field coils of the alternators and they were still giving continuity and good resistance. Diodes were still good as well.
Finally, unsure of my test equipment and at the end of my rope, the owner relented and I pulled both alternators and regulators to be bench checked.
Just packaging them up, I noticed that the drive gears on each seemed to operate freely of the rotor through the rubber drive coupling. Looking at them, they looked perfect. I’ve seen rubber couplings fail before. It’s usually followed by removing every plug and screen in the engine to get that non-magnetic shit out; and praying to God that none of it has found its way into an oil gallery.
Two alternator couplings failing at the same time—what are the odds?
The owner went to Calgary and picked up a couple new ones--$1200 a piece! I guess they are so expensive they don’t get replaced at engine overhaul or alternator replacement. The reason why the right alternator couldn’t achieve voltage when it was installed was because the coupling was slipping all along. You'd think it would have been checked though...
There is no non-destructive way to test these couplings. Aside from the slipping action, they looked perfectly serviceable.
So watch out. The odds are slim, but they are there...
