1965 C337 "Alternator not charging" light

[JAG]

Dave,
Just want to thank you for your expertise as well - I am reading your posts with great interest. You have a great way of explaining things.

One question I have is when you said "I like running a piece of #8 from the base of the rear starter to the base of the front starter.", did you mean exactly what is written - running once continuous wire from the front starter to the rear starter? I think I understand the concept - but could you help me understand the reason for this clearly?

Thanks again for spending the time in going through the older Skymaster electrical system. I am not having any problems, but since I am doing a major refurb on my aircraft right now and cleaning up wiring, any upgrades now is worth the time.

Thanks,
Jeff

[DrDave]

Hi Jeff:

I hope the info is of some benefit to you. Let me try to give you an illustrative way of visualizing the aux ground/bonding wire. Let's assume that the wiring from the positive terminal of the battery through the master solenoid and rear engine starter solenoid, back to the rear engine starter is all happy.

Now let's try and work our way back to the negative terminal of the battery. The stream of electrons leave the starter housing which is bolted to the case go through the ground strap from the motor mount into the engine frame and into the airframe. Now we have to get all the way back up to the battery through the heart of enemy territory. It's helpful to think of this part as fish going up a fish ladder at a dam. We need to go across all of the overlapping skin pieces that have been vibrating and moving from less than soft landings for the last fifty years. Once this stream of electrons get to the front firewall they have to travel into the battery tray and go through the little bolt, up the wire, and into the negative terminal of the battery. That was a long journey with numerous pathways and connections. We have no control over the quality of those airframe connections that were meant to be structural connections. We are now using them as electrical connections. That point is worth thinking about for a few minutes.

You can test the quality of this path by putting a volt meter between the mounting bolt of the starter and the negative terminal of the battery and cranking the engine. I've seen over 2.0 volt drops between these two points. That's 2 volts that's not going to the start circuit. To do this test you need some long test leads for your meter.

As an aside, the alternator has to use this same path back to the alternator housing and negative side of the battery. It's a little bit easier for the alternator since it is generally down in the low amperage range. But the starter is looking at about a 100 amp draw. That 100 amp draw on the positive wire is the same 100 amps on the ground circuit. I don't want to dismiss that this path for the alternator return path as insignificant. It can certainly make a difference in the charging system operation.

So let's look at the remedy. If we run a piece of #8 Tefzel covered wire from the rear engine starter mounting bolt to the front engine mounting bolt we've got a good connection to get the negative path back up front, to the point of the front alternator and starter. This provides an alternate path that parallels all of the overlapping skin pieces and funky ground straps. Since electricity takes the path of least resistance this wire could carry a majority of the ground circuit load. Now all you need to do is take a piece of #8 from the front starter mounting bolt to the point where the negative cable mounts to the battery box frame. This gives us a good ground path for the both front and rear starters and alternators.

I've spec'd this out as a piece of #8. On a 182 I saw this auxillary ground wire carry 70 amps during starting. On that install I wish I had used a piece of #6.

I hope this answers your question and gives you some background as to the rationale.

* I have heard of mounting a marine grade terminal block where the negative terminal attaches to the battery box frame. This would make a nice place to bring these wires together. Again, I've only heard of this being done.

Dave

[JAG]

Dr. Dave,
That info is immensely helpful and now I get it!!! Thank you for the taking the time on this electrical info - it is great to have this type of data to be proactive in eliminating potential electrical issues. I have never thought of the ground plain like you have explained, but that makes complete sense. I will be making some of these wiring upgrades to my bird for sure.
Jeff

[DrDave]

Thank you for the kind words Jeff. I fixed a couple of typos and omissions from my previous post. It is quite gratifying to see these very capable electrical systems work properly. Mine does a very happy 28.8 volts under all loads. My battery voltage drops less than .1 between no load and everything turned on. I'm happy with that.

Dave