PA-39 electrical problems

[Tom Straughan]

Since 01/27/12, I have had electrical problems with my twin comanche. Initially, the problem manifested itself through the illumination of the "discharge light" on my Electronic International V/A and the shutdown of both alternators. The 2 A&P's who annualed my plane in April were convinced that I needed a new main voltage regulator. The original voltage regulator was replaced by a Zeftronics unit. When this didn't solve the problem, an electronics specialist told me that my new voltage regulator was bad and after it was replaced, I still had the problem. Next, I replaced the original aux. voltage regulator with a new Zeftronics unit. When I still had the problem, the electronics specialist tested numerous electronic parts and we checked and filed down the battery grounding lug and both engine grounding lugs and all connectors on the attached wires. He became convinced that the main/aux regulator switch needed to be replaced along with both over-voltage relays and each 90-Amp circuit breaker/alternator switches. After purchasing and installing all these components, the problem became somewhat abated, but it seemed that the problem was confined to the right side and another A&P told me to replace the R. E. alternator. Until the new alternator was delivered, I flew 6 flights using only the left alternator and when the "discharge light" did illuminate, I was able to bring the alternators back on-line by switching to the aux voltage regulator. After the new Hartzell Technologies ALX8521 alternator was installed on the R.E., a different problem arose; during engine run-up and during flight with a significant electrical load, the R.E. alternator was supplying 4/5 of the total amp load. Then the new A&P recommended that we replace the L.E. alternator. Another new Hartzell Technologies ALX8521 alternator was installed on the L.E. on 10/19/12 and the problem reversed; that is that the L.E. alternator was supplying 3/5 of the total amp load during engine run-up. In summary, except for the two filters, every electrical/electronic component shown in the non-paralleling alternator wiring diagram has been replaced with new components in my twinco, and numerous old cracked alternator field wires have been replaced and 3 main grounds have been thoroughly cleaned and new Bogert battery cable systems installed and after all this, I now am unable to get the two alternators to equally share the amp load. I really need some help to make this electrical system perform properly!

[Kristin Winter]

I can't tell you how much I hate that system that Piper used with main and aux voltage regulators. They finally got rid of it for the last 6-8 PA-39's. I replaced the entire system in my PA-30 with PlanePower's system, but unfortunately, that doesn't work for a counter-rotating engine.

The field wiring is sort of bassackwards IIRC. My recollection is that the regulator controls the ground of the regulator circuit. Have you replaced all the field wiring? I would be interested to see if the voltage to the field terminal on the alternator is identical for both engines. If so, then I would be interested to compare the resistance to ground on both sides.

If both alternators are seeing the same field voltage, then the alternators should put out the same voltage. When the field voltage differs, then the alternator that is seeing the low voltage will not take up its share of the load. In a paralleling system, the paralleling circuitry will match the voltage output of the alternators. However, with his system, everything has to be right to to get both the carry the load. It may be that both alternators are just enough different that they will not synch up. If the test I suggest don't find a problem, I would call Hartzell and ask them if they can adjust these alternators to that they put out the same, or could they send you a matched pair.

That is the best I can come up with.

[Tom Straughan]

Thanks, Kristin, you have given me some troubleshooting leads to pursue!