Another MX Issue - Story of My Life....

ARFlyer

ARFlyer

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So I got the fuel sender issue worked out to only have another issue creep up!

I’ve been chasing a Gremlin today that I can’t seem to find the source of. So a few months back I noticed that the ammeter windshield wiped horribly. I asked the about it to the other partners who said it had been doing that for years. Anyway I convinced them to look into it and we discovered that both the Voltage Regulator and Alternator were bad. Those got replaced with a overhauled and new unit respectively. Fast forward to today and it’s been working wonderfully minus the ammeter still windshield wiping. It’s not as fast inflight as it was before. The mechanic said everything tested okay and said that it must be something in the avionics. The ammeter will stop flipping out and show a steady discharge needle if you turn the alternator master off.

Today the alternator started tripping offline in flight. The Aspen will alert to a low voltage situation and the ammeter will show a steady discharge. Recycling the master will bring it back online for a few minutes before it trips again. The mechanic and I tested everything but couldn’t find the issue. The voltage is holding a steady 27-28v with everything powered on. So we went our merry way and again the alternator started tripping offline in flight this evening,

I tested the over voltage light and found that it doesn’t come on whenever you turn the alternator master off and leave the battery on. I’m down to thinking it’s the over voltage sensor and that we aren’t noticing it because the light has been burned out of years, I didn’t know that the light was suppose to come on during start up as I’m new to Cessnas. But I asked one of the other guys who said that he hasn’t seen it light up in several years but never thought twice about it.

So anyway anyone have a good starting place or an idea of what’s causing the tripping issue?
 
The ammeter could also be correct and the alternator has a dead diode and isn’t putting out constant current. Up down up down.
 
The over voltage relay would be my first look, along with system grounds.
 
Ever since I brought a partner on board I've been plagued with squawks. No the partner's fault, just how it is. I've had more maintenance this year than the previous 5 combined.
 
Sounds like a bad master/alternator switch to me. Jumper the alt side of the switch or put a temporary switch on that wire.
Bad switch contacts (and CB contacts too! ) will cause the problems you describe as well as the annoying "ammeter dance" you spoke of...

Chrs
 
denverpilot said:
The ammeter could also be correct and the alternator has a dead diode and isn’t putting out constant current. Up down up down.
Click to expand...

a bad diode won't flicker the ammeter... it's a hundreds a times per second variation, too fast for the needle to see.
 
wsuffa said:
The over voltage relay would be my first look, along with system grounds.
Click to expand...

28 bolt system doesn't have a separate OV relay... it's in the alternator control unit, aka regulator. Though I'm wondering who sells overhauled ACUs like he said he installed?!
 
I think I would seriously have to train a partner in the nuances of legacy SEP airplanes.

My ammeter “windshield wipers” been doing it for 12 years and there ain’t a thing wrong with the airplane. It’s the stupid intermittent load of the beacon causing it...

Airplanes are an endless opportunity to throw money at issues that won’t improve safety, sure some things will and a lot won’t.
 
TLDR: Get a Plane-Power voltage regulator.

I had this problem in my C172N. You will probably find more tips if you google "nervous needle". I put a scope on my bus and could see the voltage spiking with the ammeter. Previous owner replaced the regulator. My A&P swapped out another regulator to test as well as another alternator. No change. Every cable and connection on the contactors were cleaned. Still no change. New shop performing maintenance on my aircraft recommended a Plane-Power regulator. I figured why not try it. Ammeter and voltage regulation is perfect now.
 
Make, model and year?
 
I am not trying to make any recommendation here, but just want to share my past experience. My mechanic and I had to do a witch hunt for a similar issue with the electrical system, but nothing worked. Then someone told me that an old battery can invite a Gremlin or two... Since the battery test showed that the unit was healthy, I was a bit skeptical, but we were running out of ideas. So, I tossed the old wet cell for a new gel battery (this good idea was from Doc Bruce). The Gremlin has never come back. I now replace the battery every four years or so. My 2 amp...
 
Welp it’s a bad new voltage regulator. We installed it about two months ago while chasing down the needle issue. The overvoltage light had burned out who knows how long ago. Mechanic said thats quite common or people just pull it out and splice the wires.

I can’t believe people will just ignore a warning light or remove it to save a few $$.

So anyway a new voltage regulator is being sent under a warranty exchange. The overvoltage light is getting replaced today if they can chase down the wiring as it’s a complete unit and wire swap.

That is my second trip to get changed to long distance driving....:(
 
Alternator tripping off in airplanes using an ACU can be caused by RF leaking from a badly-grounding antenna cable connector. We had a 185 doing it and we couldn't find anything. Read the maintenance manual, saw the tip, tried keying the transmitter, and the OV light came on. ACU had tripped. Cleaned the connectors at both ends of the COM cable, found corrosion at the one in the cabin ceiling under the antenna. Cleaning fixed the issue. ACUs use tiny amounts of current in their voltage-sensing circuitry, and stray RF generates volltage spikes in the airplane's wiring. The ACU picks it up and interprets as an overvolt condition. The old mechanical regulators use a lot more sense current and are immune to the RF. They just shunt the RF-generated stuff to ground.
 
Dan Thomas said:
Alternator tripping off in airplanes using an ACU can be caused by RF leaking from a badly-grounding antenna cable connector. We had a 185 doing it and we couldn't find anything. Read the maintenance manual, saw the tip, tried keying the transmitter, and the OV light came on. ACU had tripped. Cleaned the connectors at both ends of the COM cable, found corrosion at the one in the cabin ceiling under the antenna. Cleaning fixed the issue. ACUs use tiny amounts of current in their voltage-sensing circuitry, and stray RF generates volltage spikes in the airplane's wiring. The ACU picks it up and interprets as an overvolt condition. The old mechanical regulators use a lot more sense current and are immune to the RF. They just shunt the RF-generated stuff to ground.
Click to expand...

I’ll keep that in mind if it trips again. This round we caught the OV condition by shear luck while testing a circuit. We found that whenever I increase the power above idle and held it the voltage would slowly creep up then suddenly spike to around 40v before the alternator tripped.
 
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