alternator issues

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jesse

Touchdown! Greaser!
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Jesse
Kent (flyingcheesehead) and I went flying in a C172RG yesterday. During the climb out after departure I noticed the low voltage light and the gauge indicating a discharge. We recycled the alternator portion of the master switch and it started working properly again.

I didn't notice the issue during the run-up and am fairly confident I would have had it not been working since I touch every engine gauge and make sure it's displaying data that makes sense.

I'm not that familiar with how that's all wired up. Based on the above, what was most likely the problem?
 
You went flying in a high-wing. Duh! :D
 
dmccormack said:
I've seen that happen on a C172 with a battery that was dying.

Volt meter or +/- gauge?
Click to expand...
Honestly, I don't know what the hell it was.

Generally an ammeter will sit around the 0 if everything is working properly. This gauage was rectangular. There were two labels DISCHARING / CHARGING and a needle pointing to one or the other. When it was discharging it was deep into the discharging side. Once I resolved the issue it was deep into the charging side.

Since the labels on it weren't of any actual unit - I'm not quite sure. I suppose it could be an ammeter and it was charging well after I fixed the issue. Or I suppose it could be a voltage meter and the charging side is 24+ volts and the discharging is something less?

The entire flight it kept deep into the charging side once I fixed it...which makes me think..it's probably a voltage gauge.
 
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Shot in the dark - Could have been a flaking voltage regulator. By killing the power for a bit, you in essence 'tapped it with a screwdriver' to get it acting correctly again.

Bad part about a rental - you don't know how long this has been going on. If you had a historical perspective on it, you could decide if it was something wearing out or something just flat broke.
 
CJones said:
Shot in the dark - Could have been a flaking voltage regulator. By killing the power for a bit, you in essence 'tapped it with a screwdriver' to get it acting correctly again.
Click to expand...

I have had that problem intermittently on a rental 172 and they eventually traced it back to a bad ground on the regulator.
 
jesse said:
Kent (flyingcheesehead) and I went flying in a C172RG yesterday. During the climb out after departure I noticed the low voltage light and the gauge indicating a discharge. We recycled the alternator portion of the master switch and it started working properly again.

I didn't notice the issue during the run-up and am fairly confident I would have had it not been working since I touch every engine gauge and make sure it's displaying data that makes sense.

I'm not that familiar with how that's all wired up. Based on the above, what was most likely the problem?
Click to expand...

I had the same sort of problem from time to time on a C172RG. The alternator did not always come online after starting the engine. You can check the alternator during the run-up by turning on the landing light and verifying that you don't have a discharge. The RPM should be at least 1500 RPM during the check.

The most likely cause is a temporary over voltage being detected by the voltage regulator, usually right after startup. Once an over voltage is detected, the regulator takes the alternator off line. This will eventually show up as a low voltage indication as the battery drains down. To reset it, just cycle the alternator switch. If it doesn't repeat, then all is OK.
 
It isn't necessarily the alternator. It could be other issues or a combination of issues. Such as;

Alternator
Voltage Regulator
Grounding issues
Battery issues (intermittent short in battery - that was a fun one I had :rolleyes:)
Bad/old wiring (another fun one)
Gremlins
 
Happened to me last summer turned out to be a chafing wire(s)
FWIW Ive always know those Cessna gauges to be the ammeter type simply showing if the battery is receiving charge, breaking even or losing it.
Ive only seen the bus volts type indication on the g1000 versions.
Could be anything..or could be nothing.
 
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taters said:
That happened to me last summer , turned out to be a chafing wire
Click to expand...


Is that like a chafing dish?

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Hmmm I went flying in one of our archers the other weekend, and the alternator kept spiking. The Ammeter would constantly spike, and I could hear a pinging sound over the headset. This is the same Archer that stranded me at Gaston's a couple of years ago. Anyone got a suggestion what to check for? Bad ground? Arcing cable? We've had to replace the VR a few times, but noone ever seems to come up with a root cause.
 
IIRC the 79 Cutlass has a mechanical VR. Moisture in the armature contacts--->get described behavior.

The 80 cutlass has a solid state regulator. Look elsewhere for troubles. Half Master switch, etc....
 
jesse said:
Honestly, I don't know what the hell it was.

Generally an ammeter will sit around the 0 if everything is working properly. This gauage was rectangular. There were two labels DISCHARING / CHARGING and a needle pointing to one or the other. When it was discharging it was deep into the discharging side. Once I resolved the issue it was deep into the charging side.

Since the labels on it weren't of any actual unit - I'm not quite sure. I suppose it could be an ammeter and it was charging well after I fixed the issue. Or I suppose it could be a voltage meter and the charging side is 24+ volts and the discharging is something less?

The entire flight it kept deep into the charging side once I fixed it...which makes me think..it's probably a voltage gauge.
Click to expand...


It's an ammeter.

There are a few possibilities. If it has the old electromechanical regulator, the contacts get burned and then will sometimes weld together just a tiny bit and stick, and then the overvolt sensor drops the regulator off line, which kills the alternator. Another possibility is a loose ground wire on the overvolt sensor under the instrument panel. The sensor loses its ground reference and gets excited and shuts things off. A worn alternator side of the master switch, or a worn field breaker, can also contribute.
If it has the newer electronic regulator/overvolt sensor combination, those things are sensitive to RF leakage from the com antenna cable and will sense the EMF generated by the radio transmission and will shut the alternator down. Key the mike and see if it drops offline. A corroded antenna cable ground connection at the bayonet in the cabin roof is usually the culprit.

Dan
 
I'm scheduled to fly it tomorrow, rather "today." I will keep an eye on it and see if I see anything too.
 
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