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Amp Meter Oscillation

Louavul

Well Known Member
Anyone have any thoughts on my amp meter oscillation? It's done this from day one and I really haven't considered it a discrepancy until flying with a previous co-worker who seemed to think it was concerning.

My alternator is from a older Honda, 35 amp and my regulator from an old Ford LTD.

https://www.youtube.com/watch?v=GtfhzPUMaX0

As always, Thanks for your thoughts,
 
I had that problem a few years back. Here's my note to Bob Nuckolls and his response:

My question:

I had the opportunity to do a 7+ hour X/C a week or two ago and it was driven into my head that I have a soft ~3 CPS noise in my headsets and the voltmeter and ammeter readings are fluctuating at the same time and frequency as the noise. If I turn off the alternator, the noise goes silent and the ammeter and voltmeter stabilize too.

The ammeter shunt is measuring battery charge or discharge and the voltmeter comes off of the buss downstream of the diode. Both gauges are analog, and the needles are bouncing up and down by a half volt (13.5-14V) and a half amp or thereabouts. The alternator is the B&C 40 amp unit and the regulator is the Ford unit recommended 15 years ago when the airplane was built. All of the components have been in service for 14 years and have approximately 950 hours of use on them.

Any suggestions on rectifying this condition?

Thanks,

Kyle Boatright​


Bob's response:

The voltage regulator sense (sensor?) voltage shares
a path with alternator field current in this
regulator. This condition is well known in
architectures of this type and is often called
the 'galloping ammeter'. In the older Cessnas,
I recommend replacing everything from the bus
bar to the regulator which would include breaker,
alternator switch and wires.

Doing any ONE thing might 'cure' the problem but
it's the sum-total of environmentally driven resistance
creep that finally stacks up to cause the instability.
You may cure it with one replacement but only by
replacing ALL will you get back to as-new condition.
 
Check the tightness of connections

My ammeter acted erratically at first. I traced it to a loose connection that made variable contact under vibration. Try checking connections to the ammeter and on the circuit that the ammeter measures the current on.

Jim Sharkey
RV6
 
A different jiggle on mine

I have the same Vans ammeter in my RV-4, and I mounted a Flightcom FL-760 radio in the hole next to it. When I key the PTT on my stick, the ammeter goes crazy, but settles still when I release the PTT. I figure one day, it may kill it, but I'm not sure how to stop it from happening. Is it possible you are getting a similar, but much less interference than what I experience from another source?
 
I have the same Vans ammeter in my RV-4, and I mounted a Flightcom FL-760 radio in the hole next to it. When I key the PTT on my stick, the ammeter goes crazy, but settles still when I release the PTT. I figure one day, it may kill it, but I'm not sure how to stop it from happening. Is it possible you are getting a similar, but much less interference than what I experience from another source?

I had similar. Vans amp meter maxed out whenever I had PTT depressed. Mine was due to the ammeter wire running along the PTT wire for a short section. I re-located the wires and all fine.
 
Several causes.

1) Shunt/wiring has issues giving inaccurate reading. Shunt produces very low voltage, so very sensitive to resistance and other wiring issues. The leads often have very small fuses that can cause issues.

2) voltage regulator is bad, creating the oscillations. Not a typical failure mode.

3) voltage sense input of VR (typically coming from Alt Fld CB and then through Fld switch) has something lose, creating infinitely variable resistance and therefore variable voltage to VR, which causes variable fld current trying to met a target voltage, causing variable current output on Alt. Typically in this case, the bus voltage will wander as well

4) Alt has problems.

5) loose battery connection or battery problem (intermittent open between cells, for example) that is requiring variable amounts of Alt output to maintain target voltage.

Larry
 
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Battery

Mine started doing it also but stayed in pace with my strobes. Strobes off meter steady.... Final fix was new battery.
 
If you have a Cessna-type split Master/Alt switch that is the first thing I would replace.
 
Kyle and Bob

I’m with Kyle and Bob on this one, based on some work my company did on early detection of alternator failures. Put a decent multimeter on that line and see if you can see the frequency varying with RPM. If so, alt is the path, and that may be unfun to rewire to a better place, but may avoid gremlins in the future.
 
Some good ideas here, I'll start my annual condition inspection the first of September so, guess what I'll be doing along with that.

Thanks,
 
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