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#11519 - 08/29/10 03:25 PM Amp meter oscillations with alternator?
vettdvr Offline
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Diamond Pilot

Registered: 06/24/10
Posts: 6654
Loc: Slidell La
172 N with alternator. The regulator has been replaced once for this issue.

The amp meter does not move to a charge position amps and stay as a car might. It is constantly fluctuating about 15% + or -
and always on the charge side. The battery stays charged and has stayed charged with both regulators having the same result. So I can't see changing the regulator again and don't understand how an alternator could do this even with 1 set of diodes failing because the frequency doesn't come anywhere close to matching the rpm of the alternator.

My thoughts are there is a shunt issue on / inside the gauge and no real issue exist.

So is this pulsation Normal? or is there a hidden issue. The belt is also good and no slippage is expected but can't prove as the sharp whirley thing is directly in front of the alternator.

So should I do something and if so what? I am at a loss on this one since it is in fact charging and has been for a while now.

Sorry this was included in another post and I realize it should have been stand alone and not detract from other question so I started a new thread.
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Vettdvr

Single/Multi/instrument/type/commercial But then I am still learning.

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#11534 - 08/29/10 08:34 PM Re: Amp meter oscillations with alternator? [Re: vettdvr]
Randy Crosby Online   content
Second in Command

Registered: 05/17/10
Posts: 137
Loc: Trapper Creek, Alaska
Does the "oscillation" happen with all the lights, radios, etc. turned off? Does it happen at all power settings? It could be a load on the system like a strobe or beacon that is causing it which may be a indicator of a problem with that unit or may be an indicator of a bad diode in the alternator.

Also I would check your battery connections and clean them real well as well as your all main main grounds.

I wouldn't suspect the regulator or ammeter as the problem but electricity does some strange things and you never know. It's easy enough to hook up a external meter and see what your alternator is doing.

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#11551 - 08/30/10 08:11 AM Re: Amp meter oscillations with alternator? [Re: Randy Crosby]
vettdvr Offline
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Registered: 06/24/10
Posts: 6654
Loc: Slidell La
the oscillation is consistant. But next time I will do a progressive load turn on to see and confirm. good suggestion also to volt check with external meter.
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Vettdvr

Single/Multi/instrument/type/commercial But then I am still learning.

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#12091 - 09/13/10 08:31 PM Re: Amp meter oscillations with alternator? [Re: vettdvr]
steve71e Offline
Second in Command

Registered: 02/22/10
Posts: 172
Loc: Atlanta, Ga
Check for high resistance in your field circuit or a bad load resistor for your beacon. (If you have a beacon, and only found one bad in 25 years) Unplug your beacon to see if the problem continues. If jumpy needle continues, I call this the "nervous needle" symptom. It could be any thing in the field circuit causing high resistance including bad connections at the the field post of the alternator, master switch, etc.... Quite often replacing the crimp connections on the wires at the master/alternator switch or alternator fix this and often I have had to replace the master/alternator switch entirely. What is happening is, the voltage regulator is trying to push a small amount of current through the field circuit to excite the alternator field as needed to keep voltage steady. It builds up at the high resistance point then bursts through and suddenly there is an excess of voltage going through the field of the alternator which results in a jump "up" on the ammeter. The sense wire to the regulator from the buss now sees the jump "up" in voltage then decreases the voltage to the field dramatically thus the ammeter dropping to discharge, the regulator now sees a drop in voltage on the buss and the cycle starts all over again. This is all happening many many times a second. See, it's simple !! Hope this helps !!
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#12108 - 09/14/10 07:11 AM Re: Amp meter oscillations with alternator? [Re: steve71e]
vettdvr Offline
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Diamond Pilot

Registered: 06/24/10
Posts: 6654
Loc: Slidell La
Thanks for the information. Will take a look at it. Plane is now getting looked at by A&P for popping strut.
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Vettdvr

Single/Multi/instrument/type/commercial But then I am still learning.

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#12365 - 09/20/10 07:51 AM Re: Amp meter oscillations with alternator? [Re: vettdvr]
vettdvr Offline
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Diamond Pilot

Registered: 06/24/10
Posts: 6654
Loc: Slidell La
Ok now I am confused. I have flown the plane about 6 hrs in the last 3 weeks. Yesterday after oil change and run to leak check the amp meter was showing (all avionics on) just over the 0 center indicating charge. Turning the alternator off it would show about mid range discharge, turning back on went to + charge above the 0. No fluctuation was seen in the needle and it is indicating a charge. The plane sat for about 3 months without use before I began flying it regularly. I suspect the battery is now back to a full charge after the frequent flying. That is the only thing I see changed now that the amp meter is stable.

There could of course have been high resistance as steve pointed out that with frequent flying might have reduced.

So I am open to thoughts as all now appears "normal"..
_________________________
Vettdvr

Single/Multi/instrument/type/commercial But then I am still learning.

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#12376 - 09/20/10 12:21 PM Re: Amp meter oscillations with alternator? [Re: vettdvr]
Randy Crosby Online   content
Second in Command

Registered: 05/17/10
Posts: 137
Loc: Trapper Creek, Alaska
The Cessna ammeter was never what I would consider a high quality instrument. What you are describing is what I would consider some what normal but you might get in another identical airplane and have slightly different results. I am not sure if when you say "avionics" you are referring to all electrical equipment or just radios and navigation equipment but the landing lights should, in most cases, cause some sort of charge indication when left on for a while.

I would try running the engine for about 5 minutes with the alternator turned off but all your lights turned on then turning on your alternator. Your alternator is obviously charging but if you have any concerns, it is not that much trouble to pull it off and have it bench checked under a known load.

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#12655 - 09/28/10 09:00 PM Re: Amp meter oscillations with alternator? [Re: Randy Crosby]
steve71e Offline
Second in Command

Registered: 02/22/10
Posts: 172
Loc: Atlanta, Ga
Well exercised planes always do better than ones that sit. Just exercising the master/alternator switch may have helped clean some resistance out. If the battery were weak or high resistance in the battery cables, you would notice it cranking sowly on start up then showing a constant high rate of charge even after running for a bit. If you can run a full electrical load and hold a steady voltage of 13.5 on 12 volt system or 27.5 on 24 volt system, from full power down to about 1000 RPM, you don't have a problem. Check the electrolyte level in the battery if you have a serviceable battery. It gets pretty hot here in Atlanta during the summer so I check mine (28 volt)two or three times during the summer. Sounds like you fixed it to me.
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#12782 - 10/03/10 09:42 AM Re: Amp meter oscillations with alternator? [Re: steve71e]
vettdvr Offline
Club Sponsor
Diamond Pilot

Registered: 06/24/10
Posts: 6654
Loc: Slidell La
The ampmeter now is showing charge just slightly above center with full load (all systems on inc pitot heat).

Master off is shows heavy discharge so it appears to be back to normal. My guess is steve71e comment on using the master switch. The plane had not been flown in the previous 3 months and now I fly it on a regular basis. Thanks for the info.
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Vettdvr

Single/Multi/instrument/type/commercial But then I am still learning.

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#12865 - 10/06/10 06:48 PM Re: Amp meter oscillations with alternator? [Re: vettdvr]
steve71e Offline
Second in Command

Registered: 02/22/10
Posts: 172
Loc: Atlanta, Ga
Sounds like it is working properly. Cheap fixes are always good !!!
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