[pauldan181]

The other day I turned on my landing lights descending to my local fuel stop and noticed my bus voltage had dropped to 13.2 volts. I switched the lights off and it came back up to 13.8v. My alternator has always put out 14.2v regardless of load or RPM. With no load I still get 14.2v and it begins to drop as each light or device is switched on. I confirmed this voltage drop with a multimeter attached to the alt. output post. With everything I have on (about 22 amps load) it gets as low as 13.0 volts

I replaced the alternator (40 amp Denso IR) and is behaves in the exact same way. The engines starts fine and the battery appears to charge normally. I replaced the battery anyway but no change. I've checked all applical circuit connections and all are secure. My electrical system is pretty simple and this should be an easy fix. I'm hoping somebody else has seen this and can point out what I'm missing.

 

[BobTurner]

I would look for a slightly poor connection somewhere in the electrical bus, between the alternator sense line and your voltmeter. It will look okay, you'll have to find it with a portable voltmeter and the system under load. Or randomly loosen then tighten connections.

 

[Walt]

Make sure the alternator itself has a good ground, and at low engine RPM the alt is not going to be able to carry a 22 amp load.

 

[turbo]

again get some corrosion x and spray all associated connections. start from each end of the system and see where the drop is. watch out for that spinning prop!

 

[pauldan181]

Fixed

I removed my alt. output circuit breaker located in the fwd. baggage compartment well and noticed some water stains on it. I bolted the two terminal lugs together and ran up the engine and with a full current load the voltage never dropped below 14.0v. I'm not sure how water got in there, I'm gonna blame Sandy and a leaky hangar roof.

Make sure the alternator itself has a good ground, and at low engine RPM the alt is not going to be able to carry a 22 amp load.

As it turned out I had misunderestimated the current load when I first noticed low voltage in flight. It was more like 30 amps confirmed with a clamp meter attached to the alternator lead. My panel ammeter is a + or - battery charge indicator which is pretty useless, I may wire it up to show alt. output in the future.

 

[az_gila]

I removed my alt. output circuit breaker located in the fwd. baggage compartment well and noticed some water stains on it. I bolted the two terminal lugs together and ran up the engine and with a full current load the voltage never dropped below 14.0v. I'm not sure how water got in there, I'm gonna blame Sandy and a leaky hangar roof.



As it turned out I had misunderestimated the current load when I first noticed low voltage in flight. It was more like 30 amps confirmed with a clamp meter attached to the alternator lead. My panel ammeter is a + or - battery charge indicator which is pretty useless, I may wire it up to show alt. output in the future.

It just goes to show that a voltmeter is just as good (better?) as an ammeter for showing a charging system problem.

 

[Ron Lee]

Clarify this for for a electron dummy. The output of your alternator goes through that water-stained circuit breaker. So there was apparently corrosion or similar that negatively impacted the alternator current flow?

Why would that show up as a decrease in alternator output voltage?

Is it related to V=IR and I am dense on what is happening?

 

[az_gila]

Clarify this for for a electron dummy. The output of your alternator goes through that water-stained circuit breaker. So there was apparently corrosion or similar that negatively impacted the alternator current flow?

Why would that show up as a decrease in alternator output voltage?

Is it related to V=IR and I am dense on what is happening?

It created a resistance in the output lines of the alternator (the "R") and a the current (the "I") went up there was a voltage loss (the "V").

It was all consistent with V=IR...

A good breaker would have R=0 so the voltage drop should be 0...

 

[pauldan181]

Because my alternator is internally regulated it's not going to compensate for a poor connection in the bus feed line. If I had an external regulator, it would increase the field voltage to compensate for a drop in bus voltage Is this a correct assumption Gil?

Also since the circuit breaker's resistance is no longer zero it probably was getting warm, making things worse.

 

[Walt]

The voltage ref for the internal regulator is the Alt 'B' terminal compared to the the alternator ground.

That's why internet troubleshooting is so difficult, you stated in your original post: "I confirmed this voltage drop with a multimeter attached to the alt. output post". We can now surmise this test was at idle with a heavy load, in which case a lower voltage would be normal.

 

[pauldan181]

That's why internet troubleshooting is so difficult, you stated in your original post: "I confirmed this voltage drop with a multimeter attached to the alt. output post". We can now surmise this test was at idle with a heavy load, in which case a lower voltage would be normal.

Yep, that and the fact my first load estimation was low. I also took your advice and cleaned up the alternator mount brackets, this probably helped some. Thnaks for your input Walt.