Comanche Flyers--

About a week ago, I fired up N7406P and during the preflight I noted a significant negative draw on the ammeter (-20) during preflight. Everything had been fine on the previous flight, but obviously something wasn't right, so I sent her into the shop. The shop noted that the field & ground wires on the Motorola alternator were in bad shape and were replaced. The airplane was ground & flight tested with no issues and returned to service. With a clean bill of health, I flew the next 3 flights/6 hours issue free. However, in the crummy weather this morning on the east coast the issue came back with a vengeance. After take off, all was well through the climb until about 5k feet (thankfully on top of the worst of the muck). First, COM#1 went dead with no warning signs. While odd, it is a 54 year old airplane/30 year old radio, so it wasn't completely unreasonable. I established comms on COM#2 and flew on. However, 2 mins further into the flight I started getting low voltage indications on the EDM. Upon immediate inspection the ammeter was oscillating wildly between charging and discharging. Realizing I had a sick airplane in IFR conditions I focused on getting her home safely, and did so with minimal fanfare (though the prospects of dwindling electricity and a 300 ft overcast layer had the pressure ramped up for sure). With that behind me, I'm now trying to figure out what the heck happened and what to do about it. Any advice would be greatly appreciated:
- My A&P/IA thinks the Motorola alt might be internally failing and doesn't seem to really care for them.
- He is suggesting that I replace the existing STC with the Plane Power kit.
- I am more focused on getting a firm confirmation of exactly what the source of the problem is before sinking significant dollars into swapping something out that might not even be the true problem.

My specific questions:
1. How should I approach definitely identifying the problem without running further risk to the electronics?
2. I've read many posts that seem to describe the voltage regulators as a part that either "works or it doesn't" is this this case?
3. Is my A&P's judgement of the Motorola alternator fair? Is the plane power kit a significant safety upgrade that is worth the expense?
4. Are there other considerations/options that I'm completely missing?
5. While probably coincidental, I just had the battery replaced, could there be any correlation?

Much appreciated,

-Quint

Hi Quint,

I really don't have any great suggestions. Randomly occurring electrical issues are some of the worst issues an owner can have. Your mechanic's suggestion to just start replacing things and "hope for the best" though understandable, and the easy thing to do since he's not paying for it, is in my opinion not the correct strategy. I prefer to find the "smoking gun" so we know for certain the problem is solved. You may eventually need to adopt your mechanic's strategy but at this early point, I would find someone who actually has some ideas for troubleshooting and a thorough understanding of how the electrical systems function in our airplanes. It has been my experience that very few mechanics understand electricity. I know I don't, but the key is to find someone who does. You will feel much better actually finding the problem and resolving it then guessing and then wondering if it is really resolved or simply not occurring at the moment.

I really feel your pain!

Good Luck,

Ed Asmus

Most often random or intermittent electrical issues may be due to a loose or bad connector, solder joint, splice, bad wire, or bad ground. Before throwing expensive parts into the plane (unless there is strong belief that the alternator is the problem) I'd suggest getting an Ohm meter and checking the resistance across all the relevant wires. Be sure to check grounds as well. You can also do the "jiggle test". While Ohming out a set of wires, jiggle the harness to see if the ohm meter shows any variation which may be a bad connection or wire. It may only show up when there is vibration.

If your alternator is suspect, surely your A&P can pull it and have it bench tested to determine if it is good or bad vs just assuming it's bad and throwing parts at the problem.

Hope this helps,

- Charles

It's curious that you just replaced the battery. It makes me wonder if something is loose.

Bench test the alternator -- easy to rule it in or out.

Bench test your new battery -- it's the only new variable to the mix so figure out if it's the culprit, check connections, etc.

Voltage regulators do normally fail completely when they fail but it's not always the case. I have one (not in a Comanche) that would take the alternator offline randomly. Sometimes 10 min into a flight, sometimes an hour. Alternator tested good, voltage regulator tested good via test equipment. I spent a month tracking that down (replacing ground, field, etc wires, you name it.) Switched to a newer design voltage regulator and haven't had a problem with that aircraft since.

Check the connectors at the ends of your wires. We had one intermittent failure that was traced to the wire being loose inside the crimp of the ring terminal used to attach it to the rear of the alternator. That was also covered in heat shrink and wasn't until I actually went to remove the alternator that the wire pulled further loose relative to the ring terminal and it became obvious. New terminal crimped in place and that resolved the issue.

Electrical gremlins suck, but unless your mechanic has serviceable spares lying around to do this swap and test troubleshooting methodology it will be very expensive for you.

Good luck.

Tim

Thank you all kindly for your replies & sympathy - you've confirmed my gut - that we need to positively confirm the issue rather than playing expensive part swapping games. I will post back what I find.

-Quint

Quint,

Compliments to your decision-making and ditto to all those who said elec problems are no fun.

Quint, Did you change to an RG 35 battery from a flooded cell G35? There is a known issue with the zenier diode controller in the Interav voltage regulator that causes under charging while running with the RG battery due to the battery accepting a much higher charge rate. If your alternator is at fault, it will blow the regulator or the field breaker. Either way the system quits charging all together. Plane power makes a retrofit voltage regulator for the interav system that is adjustable and does not float as much based on charge resistance. If you want to check the alternator, take it to any autoparts store and tell them it is from a 1972 AMC Gremlin. That will set their test up properly.

Check the grounding. Random weird electric things in old airplanes, in my experience, are usually grounding related. Dirty or corroded contacts, cracked insulation, broken wires, and loose crimp connections can all cause bad grounds which can lead to the sorts of random intermittent electrical failures that slowly get worse and worse.

Your alternator needs some field current to generate electricity. If that wiring path has a bad connection that occasionally makes or breaks then you will get occasional episodes of the alternator simply not working. I had a Cherokee 180 do that to me once. The alternator would work, the field wiring would heat up, the connection would break, the alternator would stop working, the wiring would cool off, the circuit would make, and the alternator would start working again. Drove me nuts in flight - VFR day thank goodness - and once on the ground the wiring problem was obvious. Nut had worked loose and the field coil lug ring was loose under the washer.

Also, everything basically affects everything else in an alternator setup. Easiest and cheapest thing to do is carefully inspect each connection and test the continuity of each wire. Also, carefully write down and take photos of the condition of each thing before you have anything done to it. Helps with troubleshooting later.

Was there any serious static on your radio while this was going on?

 

If you start to see a 12.5 or so bus voltage, consider changing to the fixed voltage regulator with the RG battery. Im glad you found a hard fault. That makes things much easier.

It sure is nice knowing why!

Awesome, I'll definitely keep an eye on that - thanks again to all who lent an opinion!

Glad it was a relatively easy find and fix. Good luck with the other repairs!

Tim