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Engine grounding, crankcase position sensitivity?
Is there sensitivity to the location of crankcase grounding? I have two independent straps, both B&C supplied, one from the crankcase to the mount (although I don’t see the need, but all the cool kids have one) and one from the crankcase directly to the battery. The battery negative has a strap directly to the firewall/chassis ground block on the firewall. The two crankcase straps come off the same otherwise unused an4 bolt on the RHS (1-3) of the crankcase. Battery is a PC680, about 1.5 years old. Alternator puts out a robust 14.7 volts. It never spun with gusto, but started. I have changed to a lighter prop and ‘never with gusto’ has become ‘charge before trying if cold’. I’m wondering if any meaningful resistance exists between the left and right side of the crankcase? I have a hard time thinking there would be, but before I start digging into diagnostics, I thought it would be good to ask. As a side note, I have no alternator or ignition noise issues.
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2 straps
If you have 2 straps to the engine, I think I would use 2 different bolts. This may become more important as the plane ages. Also it might be wise to measure the ground strap resistance from bat to case every condition inspection.
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The engine mount should not be depended on for grounding the engine. It is
best to ground the engine directly from the battery negative to the crankcase. |
I always just use a length of cable from the firewall ground lug to the nice ground lug on the starter. I use the same size cable for this ground that provides power to the starter. Remember all the current you push through the starter has to also go to ground - make the starter your best ground connection.
No issues on any of the my three RVs. Carl |
Clarification and leanings
I clarified the orig post.
John, I don?t measure resistance, but do give them a tug to check the crimp and an eyeball for corrosion, so good there. Joe, I have parallel grounds, I am not depending on the mount to ground the engine to anything. I recognize it?s a ground loop potential, I was trying to not let it float. Carl, your comment gets to the root of my question...is grounding on a boss anywhere on the crankcase the same as to a bolt or stud used to mount the starter? I?ve got #2ga to the starter. My ground strap engine to battery is equiv to #0, it?s a fist full of nickel plated copper, so unless there is potential between the case halves, I should be in good shape? After doing a bunch of forum search-fu, I?m leaning toward a less than awesome PC680 life cycle, more than grounding. I do see voltages fall pretty quick at first attempt of start. This battery was only ever fed by an oddyessy brand charger with a max current rating of 4A, with maybe 60 flight hours on it, but two winters. It?s not ever been drained or on a traditional charger. |
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No...
Not yet. I’m on the research side / making a plan. I’m just coming off a satisfactory condition inspection where all of these items got at least a critical eyeball per my checklist, many a wrench, but nothing gets a proactive disassembly and cleaning like the battery or ground connections unless the inspection indicates so. I confirmed the in-service date of the battery of 10/2018.
I have not deep cycled the 680 per se, and since I was using a Oddyssey branded charger recommended for the 680, I did not consider the 4A limit a negative, I was always more interested in the appropriate maintenance charge support. Reflecting on what I see on the EFIS during The last start, yes, I get into the high 9 volts as it struggles through compression, and when it doesn’t pull through I saw mid-5 volts bouncing back to 11v. I don’t recall what the first static press start voltage was. That seems less strong than what I remember, but I can’t prove it. The new light prop changes the feel, or maybe it has nothing to do with the prop and All weakness of the battery showing on compression. Time for a cold beer or a nice whiskey, and reflection on a $34 Apex or a $450 earthx? |
I would suspect the battery. I have had 4 PC-680?s and all of them have been on the airplane for 7+ years. I changed them due to time mostly, but they were all getting a little ?tired? when cranking the engine. The important thing is the loaded power output. If the voltage drops dramatically under load (engine start), it?s time to replace the battery, no matter how old it is. That fat wire has to carry 4-600 amps from the battery to the starter, and if it?s only being pushed there by 9 volts, you will have issues with starting that high compression Lycoming. There are some variables here, like the starter contactor that draws some current and offers some resistance to flow, and the various connectors between the battery and the starter that have a voltage drop due to resistance, but the main culprit, if everything is solid, is the battery. If you can borrow somebody?s battery and just try it, then you?ll know. You can use jumper cables.
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Slow Starter Motor
I had exactly the same problem. Same Battery, I added the additional earth strap. I had the light weight starter motor and changed it over to an NL series starter motor. Same battery, same earth leads and now nothing will stop my starter motor from spring strongly. Just a thought.
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