Ammeter oscillation

bburnett

bburnett

Pre-takeoff checklist
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Bryan B PA30
I have a small oscillation in the ammeter on my Twin Comanche. It is worse on the left alternator than the right and it seems to get better as the electrical load gets higher.

Any ideas on troubleshooting?

Thanks,

Bryan
 
bburnett said:
I have a small oscillation in the ammeter on my Twin Comanche. It is worse on the left alternator than the right and it seems to get better as the electrical load gets higher.

Any ideas on troubleshooting?

Thanks,

Bryan
Click to expand...

Swap the voltage regulators and see if the problem moves with the regulator.
 
bburnett said:
I have a small oscillation in the ammeter on my Twin Comanche. It is worse on the left alternator than the right and it seems to get better as the electrical load gets higher.

Any ideas on troubleshooting?

Thanks,

Bryan
Click to expand...

Sounds like voltage regulators kicking the alternators on and off line as need requires. Remember, the battery feeds everything and the alternator replaces the electricity to the battery. Kinda like your thermostat in your house, there are separate 'on' and 'off' levels where the system floats in a satisfied range.
 
Henning said:
Remember, the battery feeds everything and the alternator replaces the electricity to the battery.
Click to expand...

Not quite. The battery powers everything when the alternator can't (engine off or alternator dead) but once the alternator wakes up, it takes all the load and recharges the battery as well. The battery's voltage is lower than the alternator's regulated voltage, by design, so that the alternator overrules the battery and also forces current backward through it to recharge it.

alter4.gif


Note that the alternator's output ("battery" terminal) goes to the bus, the same as the battery's positive, via the master contactor. Whichever source has the higher voltage is going to carry the loads.

Dan
 
Henning said:
Kinda like your thermostat in your house, there are separate 'on' and 'off' levels where the system floats in a satisfied range.
Click to expand...

What? alternator output varies - it is not at all like a thermostat's on and off. A useful display would be field potential but it is reflected in alternator output.
 
Clark1961 said:
What? alternator output varies - it is not at all like a thermostat's on and off. A useful display would be field potential but it is reflected in alternator output.
Click to expand...

The voltage regulator turns the alternator on and off under light load requirements when the voltage in the battery gets high enough, and you see the little draw on the ammeter. When the surface charge gets burned off by the transponder and radio the VR kicks the alternator back in to the system, the ammeter clicks up to positive as the battery absorbs the electrons back.
 
Henning said:
The voltage regulator turns the alternator on and off under light load requirements when the voltage in the battery gets high enough, and you see the little draw on the ammeter. When the surface charge gets burned off by the transponder and radio the VR kicks the alternator back in to the system, the ammeter clicks up to positive as the battery absorbs the electrons back.
Click to expand...

never have seen an alternator work that way in a car or an aircraft - the voltage regulator adjusts the field potential to maintain the spec'd voltage on the system - the spec'd voltage is above the battery potential so the alternator is always producing
 
Clark1961 said:
never have seen an alternator work that way in a car or an aircraft - the voltage regulator adjusts the field potential to maintain the spec'd voltage on the system - the spec'd voltage is above the battery potential so the alternator is always producing
Click to expand...

I didn't say the regulators were functioning correctly, they are most likely dying and the cold is showing it early, I was trying to explain the reading. If he still has the Prestolite alternator, he has the regulator out of a 70s vintage Chrysler, this is a failure mode for them. It is a possibility to consider that I throw out there, I need to have some instruments and the airplane to do better.
 
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Henning said:
I didn't say the regulators were functioning correctly, they are most likely dying and the cold is showing it early, I was trying to explain the reading. If he still has the Prestolite alternator, he has the regulator out of a 70s vintage Chrysler, this is a failure mode for them. It is a possibility to consider that I throw out there, I need to have some instruments and the airplane to do better.
Click to expand...

Agree that it may be a regulator problem which drove the suggestion to switch regulators to see if the behavior followed the regulator. I've seen enough crap from voltage regulators that I'd just change them out for modern tech any time there is a hint of a problem. The Zeftronics regulator has been flawless.

I also agree that decent instruments on the charging systems would tell us all we need to know. The sad part is that decent instruments would be simple to do with modern electronics but nobody does them. We still have idiot lights & gauges...
 
Clark1961 said:
Agree that it may be a regulator problem which drove the suggestion to switch regulators to see if the behavior followed the regulator. I've seen enough crap from voltage regulators that I'd just change them out for modern tech any time there is a hint of a problem. The Zeftronics regulator has been flawless.

I also agree that decent instruments on the charging systems would tell us all we need to know. The sad part is that decent instruments would be simple to do with modern electronics but nobody does them. We still have idiot lights & gauges...
Click to expand...

Old Chryslers you knew you were losing it when you hit the turn signal to change lanes on the highway and see the ammeter flicker.
 
bburnett said:
I have a small oscillation in the ammeter on my Twin Comanche. It is worse on the left alternator than the right and it seems to get better as the electrical load gets higher.

Any ideas on troubleshooting?

Thanks,

Bryan
Click to expand...

Mine has been doing it for a few years, oscillates in varying degrees. The 'average' still shows fairly accurately. The needle varies according to the electrical load. My charging and electrical system have been working fine.

For these reasons, I just let it be. At this point I see no need to spend $$ dealing with it. YMMV.
 
Henning said:
Electricity flows from negative to positive.
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Yes, it does, but the effect is still the same. The alternator carries the loads as long as it's running fast enough and isn't defective. The battery only provides stability in that situation; disconnect the battery when the engine is running and watch the voltage swing wildly.

When I taught this stuff in college I had a couple of boards with all the components on them, and the students wired them up to make everything work. It was easy to point out what component did what, what the voltages and amperage flows looked like under various loads, and what happened if the battery was disconnected during alternator operation.

Dan
 
I will try swapping VR this weekend and see if it follows. I do have a multimeter and an engineering degree (mechanical) so I can do some trouble shooting.

I will report back with results.

Thanks,

Bryan
 
bburnett said:
I will try swapping VR this weekend and see if it follows. I do have a multimeter and an engineering degree (mechanical) so I can do some trouble shooting.

I will report back with results.

Thanks,

Bryan
Click to expand...

It can be a pain in the rear to do but it can be worthwhile to see what the field potential is doing while the ammeter is bouncing. I had one voltage regulator that would run the field up to a particular value (and I forget now what that was) and then just drop to zero. On the ammeter I would see it start to charge and then drop. Since I had good continuity everywhere and could see the field drop I knew I had a bad regulator. I get a similar ammeter behavior when the vulcanized drive coupling fails and it's a major pain to change it so it's nice to know what the problem is before diving in.
 
Dan Thomas said:
Yes, it does, but the effect is still the same. The alternator carries the loads as long as it's running fast enough and isn't defective. The battery only provides stability in that situation; disconnect the battery when the engine is running and watch the voltage swing wildly.

When I taught this stuff in college I had a couple of boards with all the components on them, and the students wired them up to make everything work. It was easy to point out what component did what, what the voltages and amperage flows looked like under various loads, and what happened if the battery was disconnected during alternator operation.

Dan
Click to expand...

The thing is, it only works that way when the system works correctly. In most failure modes the battery becomes primary. That's why for diagnostics the other view has to be considered. The voltage regulator being the most common point of failure in a charging system, this becomes my default view of the system given no other information. Knowing his system with probably an 80% chance of accuracy and the failure indications that system produces, I gave my opinion as to what he was looking at and why I thought that. Looking at something on a bench in school is different from looking at a broken piece of equipment and being told "Fix it now, we're bleeding money." Or worse, "Fix it now or we are totally ****ed."
 
Henning said:
The thing is, it only works that way when the system works correctly. In most failure modes the battery becomes primary. That's why for diagnostics the other view has to be considered. The voltage regulator being the most common point of failure in a charging system, this becomes my default view of the system given no other information. Knowing his system with probably an 80% chance of accuracy and the failure indications that system produces, I gave my opinion as to what he was looking at and why I thought that. Looking at something on a bench in school is different from looking at a broken piece of equipment and being told "Fix it now, we're bleeding money." Or worse, "Fix it now or we are totally ****ed."
Click to expand...

The school stuff leads to a far better understanding of the system and enables much smarter troubleshooting.

Actually, I've had more failures attributable to worn field brushes than to regulators. When the brushes get really short they start cocking in the holders and sticking, and if the slips rings are out of round (they often are, by that time) the ammeter can jump around some.

If it's an old mechanical regulator, burned contact points on the regulator relay will stick and make the ammeter inconsistent. A bad regulator ground (loose hardware) will also do it. And loose field wire hardware or loose wire crimps, especially at the alternator end, can cause all sorts of ugly symptoms.

Dan
 
I solved an issue like this in a 152 by replacing the alternator on/off switch on the panel. Cleared it right up, bad switch.
 
FIRTFT(fix it right the first time).....powers and grounds first...then proceed.
 
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