Piper Seneca Alternator Question

Hi All,

Quick Question…

In looking at my Alternator I found only wired 2 connections on the back.

I thought we would have had 3

  • a Battery Connection
  • a Field Connection
  • and a Ground Connection

So, I’m looking all around thinking I’m missing something and another pilot told me about a “Alternator Bonding Strap”.

I can see the Battery and Field Connection Posts and their wires but no where in sight is anything that resembles a Braided Bonding Strap or Wire?

He mentioned over the phone that it could be anywhere on the Engine Case…

So, I’m lost, no clue…

What am I looking for and where would I find it on a Seneca III engine??

I’m getting an intermittent “ALT INOP” Annunciator Light and resultant Ammeter going to Zero and I’d like to check the Ground Connection.






  • IIRC your Seneca II has the TCM TSIO360 engine similar to my Turbo Arrow. On mine, the gear driven alternator (ALX-9525B) grounds to the engine via the three attach bolts and the flange. My 'grounding strap' is a 1/2 inch or so diameter insulated white wire bolted to the engine case, routed through the firewall and bolted to the airframe. YMMV.

    As to the ALT INOP light, I'd double check the integrity of the field wire crimp connection for the 'zero reading' alternator.


  • Hi George,

    are you current on the 500 hour clutch release torque inspection?


  • Hi DJ,

    Thanks You!

    Took a look for that Grounding Strap and sure enough it was there. On this TSIO 360 KB the Grounding Strap is as you described, the gauge is about the same as a battery to starter cable and runs from the engine case, near the starter, to the firewall.

    I did take the Alternator Field Lead off and check the wire, it looks ok… Next, it’s a look at the Voltage Regulator connection's in the Foward Cargo Compartment area.

    Thanks again for the help!


  • Hi Bob,

    Thank You for asking!

    Yes, I have the updated Stamped Trunnions.

    Next up is Piper SB 1337 on the MLG to Wing Spar Inspection. It visually looks good but I know we have to NDI the area with Dry Penetrant to be Safe and Legal.

    Thanks again!


  • Hi George,

    most of the times I’ve troubleshot an alternator intermittent on the tsio360/520 engines, the root cause turned out to be a failed clutch. On the twins the symptoms can be particularly puzzling, so for example when you shut off field current to the LEFT alternator, the RIGHT alternator will fail in a few seconds as it attempts to pick up the entire load, and transmit double the torque through a failing/slipping rubber clutch.

    heres an older copy of the SB

    the other two things I usually find problematic when there’s an intermittent are a failed voltage adjustment potentiometer (who puts a 30 cent consumer pot in a $600 voltage regulator!? ), and the field current enable ganged to poles 3 & 4 of the MASTER switch.

    good luck,


  • Hi Bob,

    Thank You so much Sir!

    Ha! Am I the dummy, I thought you may have been referring to the Landing Gear!

    Thank You for the SB, I’ll research the Logbooks and see, I’ll also discuss it with my IA.

    I’ll be looking at the Voltage Regulators today.

    Thanks again!


  • $695 for the coupler + $300 core.

    The wiring for the voltage regulators and over voltage relays are in the nose just aft of the nose wheel in the retract position. If you are in a state that has lax child labor laws they are easy to get to. ;)

    Eric Panning
    1981 Seneca III
    Hillsboro, OR (KHIO)

  • Guys -

    I think my non-standard language was part of the confusion here.

    Installation and periodic inspection of the "Alternator Drive Hub" is covered extensively in Continental M0, sections 6-4.22 and 10-4.2.

    If you do need to replace the hub then I'll channel Carl & Bill Ross: Follow the M0 installation instructions to the letter. Continental has been emphasizing the procedure at their engine workshops after several emergencies where the hub departed the alternator shaft in-flight, with less than 10 hours after overhaul at big-name engine shops. In one case there was no cotter pin found during teardown; in the other case they found the cotter pin in the oil pan, but it had been inserted incorrectly.

    fly safe,


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