What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Engine grounding, crankcase position sensitivity?

rjcthree

Well Known Member
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.
 
Last edited:
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.
 
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.
 
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.

I think you may be right about the battery. My memory seems to think that the recommended minimum charge current was 6 amps? I presume you undid all the connections, including the battery ground, and checked for no corrosion?
 
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?
 
Last edited:
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.
 
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

Actually, the current flows out of the negative (grounded) terminal of the battery and return to the +side..... So, your grounds are very important.
 
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.
 
Strap and starter

Alan, if I understand your comments correctly, you added a strap without any improvement, then changed the starter and found happiness? I am using the lightweight plane power starter at the moment, so I find your experience interesting.
 
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.

If you have a lot of hours, look at the starter solenoid. They can wear internally and as they do, they can limit the current that can pass through them.

Larry
 
Measuring resistance via meter is really only going to catch a grossly inadequate or open circuit. You typically will have symptoms way before you see measurable resistance under no load. Slow cranking at the very least.
If you want to use a meter to check your grounding, measure voltage from engine case to battery ground. This must be done with a load applied, ie, while cranking.
Obviously, take extreme care. I am sure very few do this until they have a problem and I wouldn't recommend it as a pre-emptive practice.
 
Sigh...

I’m at the airport now. Three days ago I left with a fully charged battery ( having run for at least 20 minutes at 14.7 solid volts after being on the charger for a while. With just the master and the Dynon EFIS running (less than 2A) I see 12.3 volts from the battery. More damning is that it won’t charge above 13.1 volts on an approved AGM charger. Methinks the battery (PC680) is toast. It was installed October 2018, and has about 70 flight hours on it. It’s funny that I removed the previous one for time (7 years installed, through a good part of construction and 60 hours of flight) and it’s still starting my buddy’s ZTR mower. This battery was not abused. Master never left on. The only time it saw a charger until recently was when I was doing avionics work - I put the charger on during the work session. I never left the charger overnight, even though it’s an approved model to do so.

Apex? Earthx? Decision time. I like the idea of losing another ten pounds off the nose, W&B will allow it, but the BMC makes me flinch a bit.
 
Last edited:
SNIP

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?

This will start a flame war, but for me the answer is no. Why go through all the pain of case grounds, big braided straps and such and wonder if that is causing startling problems or not? I especially do not like the RV-14 ?ground the engine to the engine mount, not the firewall? instructions.

But shoot fire - what do I know other than I never had a engine cranking issue over the last couple of decades and three RVs.

For you, the first thing I?d do is swap out the battery. If you ever abused it (e.g. left the master on) and gave it CPR with a battery charger to bring it back to life, most cases you caused damage and it needs to be replaced. You can always use it in your lawn tractor. For me I replace one battery (two PC-625s) every three years. As neither battery is more than six years old I get some confidence of reserve battery capacity at my design margin.

If you find yourself chasing your tail, swap out the starter solenoid. Odds are this is not your issue, but if you have the same issue with a new battery this would be suspect.

Side note - if you routinely use a charger I find many people end up with short battery lives. I never use a charger, but I do use a regulated 30 amp power supply to bring up the panel in the hangar when needed. I adjust the output voltage to match the alternator. Watching the supply current drop down after connecting gives me good indication of battery health as well.

Carl
 
Just one...

Hmm...

I've always used a single, 2AWG wire from the engine case to the firewall as shown in DWG OP-31, Zones D-7,8,9 and OP-30, Zone B-10. The only caveat was to remove the paint on the engine case where the wire attached, and burnish the firewall at the location where the wire attaches.

I ran an Odyssey PC-680 for a number of years, and recently upgraded to the EarthX ETX-680. It spins the engine exuberantly...and the 10+ lbs wasn't missed :)

As always, your mileage may vary.
 
Hmm...

I've always used a single, 2AWG wire from the engine case to the firewall as shown in DWG OP-31, Zones D-7,8,9 and OP-30, Zone B-10. The only caveat was to remove the paint on the engine case where the wire attached, and burnish the firewall at the location where the wire attaches.

I ran an Odyssey PC-680 for a number of years, and recently upgraded to the EarthX ETX-680. It spins the engine exuberantly...and the 10+ lbs wasn't missed :)

As always, your mileage may vary.

I think most of us have a "per plans" set up and are swimming along just fine.
 
I think most of us have a "per plans" set up and are swimming along just fine.

I have used a 2 gauge strap from block to firewall on both planes and have no issues. I used the engine mount on the 10. Though I took care to remove paint from the f/w mounting points of the mount and used dielectric grease. no issues here.
 
What ever you do, DONT try to use a stainless braided hose as a ground strap.
Sound crazy? Yep was tried--inadvertently.

Yeah I know. nope wasnt me
Tom
 
Ugh. Back to grounding?

While I believe I may be suffering from battery weakness (not holding a charge, not a lot of depth to the charge, causing cranking voltages to fall into the 9V range If it doesn’t fire on the first or second blade) I’ve had two indications of possible grounding issues in the past two flights over two consecutive days. First is my CPI2 doesn’t want to shut down cleanly after its countdown, during yesterday’s and today’s flight. The second is jumpy oil temperature indications, which was new today. Oil temp goes from normal 180F to 120F, jumping back and forth for a few seconds, or it may hang on 120F for a couple minutes, but jumps back to 180F ( 180F previously determined plausible with a separate TC). It seems like all the other threads talk about oil temp going high with grounding issues, though...

There’s a bunch of thread history on Dynon sensor ground, not so much yet on the CPI2, and I started this whole thing with a grounding question. Frustrating. Yeah! I get to pull the cowling again!
 
Last edited:
I agree 100% with carl on this one, what is the biggest current device on the airplane? the starter, as carl stated all the current that goes through the starter needs to go back to ground. so since it is the biggest draw, why route it through bolts,castings,gaskets,ect to get to the back of the case to to go to the ground wire. just put the ground to the starter and eliminate a lot of possible resistance points.

bob burns
rv-4 n82rb
 
I agree 100% with carl on this one, what is the biggest current device on the airplane? the starter, as carl stated all the current that goes through the starter needs to go back to ground. so since it is the biggest draw, why route it through bolts,castings,gaskets,ect to get to the back of the case to to go to the ground wire. just put the ground to the starter and eliminate a lot of possible resistance points.

bob burns
rv-4 n82rb

Per plans has worked for thousands of RV?s. All of the airplanes I have owned, certified or otherwise, use case grounds. Must work.
I personally don?t want another wire in my already tight harness going forward nor the added bit of weight even though it?s probably less than a pound.
Like always, build it like you want, but I have never been hurt following the plans. It really isn?t that difficult to install a case ground properly.
I see a lot of posts about starter issues. They all start with the ?must be a bad ground? but most end up being a bad battery or one of the solenoids.
 
Per plans has worked for thousands of RV’s. All of the airplanes I have owned, certified or otherwise, use case grounds. Must work.
I personally don’t want another wire in my already tight harness going forward nor the added bit of weight even though it’s probably less than a pound.
Like always, build it like you want, but I have never been hurt following the plans. It really isn’t that difficult to install a case ground properly.
I see a lot of posts about starter issues. They all start with the “must be a bad ground” but most end up being a bad battery or one of the solenoids.

+1

This approach is well proven. 100's of millions of cars have been produced without a dedicated ground at the starter. Battery ground goes to engine block. If the approach were flawed, I think it would be obvious by now. Nothing wrong with doing it; But I fail to see the need. The fact that a case ground needs ongoing maintenance, does not necessarily mean it is a bad idea.

Larry
 
Last edited:
Grounding of sensors/equipment

The difference with us and two decades of automotive/heavy truck is almost all of the return paths tend to be back to the ECU for sensors (and most powered equipment). Where we get into trouble is case grounding of a sensor signal. Automotive started doing away with that in the 80?s, mostly. It essentially became required with OBDII (mid 90?s) not so much in writing, but in expectations of self-diagnosing faults, and further reliance on digital tools to rapidly diagnose issues in a warranty environment with the simultaneous gap of technician skill availability growing larger each year. It?s only gotten more important from there with respect to safety systems. Refer to ISO26262 in case you care....

You will still find more than one fat drivetrain to chassis ground on most ground vehicles.
 
The difference with us and two decades of automotive/heavy truck is almost all of the return paths tend to be back to the ECU for sensors (and most powered equipment). Where we get into trouble is case grounding of a sensor signal. Automotive started doing away with that in the 80’s, mostly. It essentially became required with OBDII (mid 90’s) not so much in writing, but in expectations of self-diagnosing faults, and further reliance on digital tools to rapidly diagnose issues in a warranty environment with the simultaneous gap of technician skill availability growing larger each year. It’s only gotten more important from there with respect to safety systems. Refer to ISO26262 in case you care....

You will still find more than one fat drivetrain to chassis ground on most ground vehicles.

I have seen all of that. My 10 has 100% dedicated sensor grounds; The block ground really only services the starter and alt, though it helps to tie the engine ground to airframe ground to deal with the nasty spikes floating around the block from the ignition (EI wasted spark) . I still don't see auto's using a dedicated ground to the starter, though I have seen them add small gauge grounds for the solenoid (likely for the reasons you mention). They reliably use the large gauge chassis to block grounds for the motor and it seems to work quite well.

If I were to run a dedicated ground, it would be to the alternator, as the safety of flight factor is much higher for me (elec dependent engine) than the starter.

I feel that a dedicated ground to the starter is just as likely to need maintenance as a std grounding strap between the airframe and the block. Corrossion between the starter and block is mostly a non-issue as both are aluminum, at least with contemporary starters.

Larry
 
Last edited:
I have seen all of that. My 10 has 100% dedicated sensor grounds; The block ground really only services the starter and alt, though it helps to tie the engine ground to airframe ground to deal with the nasty spikes floating around the block from the ignition (EI wasted spark) . I still don't see auto's using a dedicated ground to the starter, though I have seen them add small gauge grounds for the solenoid (likely for the reasons you mention). They reliably use the large gauge chassis to block grounds for the motor and it seems to work quite well. SNIP

Larry

One note of caution, if you have more than one firewall ground wire to the engine (or anything attached to the engine) it needs to be as big as the ground wire for the starter.

Consider a single ground wire for the engine - and that it fails. The starter would not work and you would go find the problem. Now consider the same scenario where you have engine sensors or such with separate grounds. These separate grounds will now try to carry the starter current - and will fail. If lucky you will not take out the panel but that has happened as well.

If your sensors are truly not grounded to the engine then this is not an issue. This would be the case for standard, modern EMS systems with three wire sensors (e.g. +5vdc reference, signal, ground).

For pMags it is important to ground the pMag to the engine case, not the firewall. This is for the above reason as well as you want the pMag to still work if you loose the firewall ground. The pMag internal generator wants engine ground for the plugs to work, just like a standard magneto.

On the wasted spark, the spark still happens just like it does for the power stroke. No need to do anything about it other than normal practice for spark plug leads.

Carl
 
Back
Top