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Thread: Starting/electrical problem

  1. #1
    DailTragger's Avatar
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    Starting/electrical problem

    I'll caveat this thread by saying I'm not mechanically/electrically inclined by any stretch of the imagination.

    I have a PA-12 with O-320 (150 hp). Recently replaced the starter with an overhauled Aerotech of Louisville MZ-4204 about 4 months ago after a remote shop said the previous starter was drawing too much power. Worked fine for a while but, suddenly, engaging the starter won't turn over the engine (it seems to be trying as the prop moves a little bit). Also hearing a loud aural "horn" sound any time the starter is engaged. Not sure it matters but the starter pinion is still engaged with the flywheel (my understanding is that it should disengage by itself after approx. 600 rmp).

    Battery appears to be in good health and fully charged. Volt/Amp gauge doesn't show anything unusual (000 amps & 12.3 volts), but it's installed in the alternator lead so it doesn't show the electrical load unless the engine is running.

    Any ideas what the issue could be? I certainly won't be attempting repairs myself, but I'd like to be able to have an intelligent conversation with my shop. (Sorry if this is a repeat thread. I tried searching the forum and couldn't find anything on point).

  2. #2
    wireweinie's Avatar
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    You've got a bad connection. By that, I mean a connection (connections?) has a high resistance to current flow (amps). First place to start looking on a small Piper is the ground path for the current flowing through the starter motor. That motor needs a clean, metal to metal contact with the crank case. The crank case, in turn, needs to be grounded to the airframe. Piper typically used a braided ground that was bolted to the accessory case on one end and soldered to the engine mount on the other. This was a poor design as a painted/powder coated engine mount can be very well insulated from the airframe. Also, corrosion can build up on the four mounting 'feet' that are bolted through the firewall making a high resistance current path. A better way is to use a braided ground strap bolted to the accessory case on one end and bolted directly to the airframe structure on the other end.

    Also check both battery terminals and clean the connection where the battery ground cable connects to airframe ground.

    After checking the above items, let us know the results.

    Web

    P.S. Next time buy a starter that automatically disengages the pinion from the flywheel. This allows for hand propping if you run the battery dead. With the pinion engaged (old style starter) you'll never be able to hand prop that thing (and I've tried).
    Life's tough . . . wear a cup.
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  3. #3
    DailTragger's Avatar
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    Thanks for the advice, Web. I'll check it out tonight and let you know.

    Didn't have the time to order in a different starter when I had the Aerotech installed, but a lightweight starter is on the list!

  4. #4
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    Update: A local pilot with experience rebuilding 12s stopped by while I was working on checking/cleaning the grounds. Helped with some trouble shooting and believes the issue could be the starter solenoid. Will need to finish checking grounds while waiting for a solenoid and continue to update.

  5. #5
    wireweinie's Avatar
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    Quote Originally Posted by DailTragger View Post
    Update: Helped with some trouble shooting and believes the issue could be the starter solenoid. Will need to finish checking grounds while waiting for a solenoid and continue to update.
    So, he didn't troubleshoot the relay. This is called 'shotgunning' or, as I like to refer to it, 'guessing'. I.e., replacing parts until the problem goes away. What made him think the relay was bad? He has you buying parts while still checking grounds, when you should be addressing one issue at a time.

    If you want to check the start relay for proper operation, all you need to do is connect the two large terminals together with a jumper cable. If the engine cranks good, the issue was with the relay or it's cable connections. If not, the issue is somewhere else.

    Web
    Life's tough . . . wear a cup.

  6. #6
    frequent_flyer's Avatar
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    Quote Originally Posted by wireweinie View Post
    If you want to check the start relay for proper operation, all you need to do is connect the two large terminals together with a jumper cable.
    Wouldn't it be easier, and perhaps safer, to measure the voltage drop across the starter solenoid/contactor/relay while cranking?

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