Whine from alternator

[Indiana_Pilot]

My Cherokee 180 has developed a whine that I only hear when talking on my intercom or radio. I do not hear it other than when the intercom is activated.

I pull the breaker for the alternator field wire and it stops. Whine does change with RPM and output is 13.8V at full RPM.

I have went over all the connections and test okay. I am leaning towards a failed diode in the alternator. I replaced the Alternator and VR with a solid state one back in 2017 and probably have 800 flying hours on it.

Any other suggestions to look for? This is a Chrysler Aerotech alternator.

Thanks!

 

[chemgeek]

One common cause is indeed a failed diode in the alternator. Once one of them goes, the electrical system can develop a whine that can show up in your audio as you describe.

 

[Indiana_Pilot]

Can anyone recommend a shop that has a quick turn around for the alternator repair? I need it back before Osh Kosh!


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[Dan Thomas]

Whine can be caused by a bad ground somewhere. A "ground loop" is formed, and it has nothing to do with taildraggers. If, for instance, the alternator ground cable, between the alternator and firewall, becomes corroded or loose, the alternator's ground current finds other paths such as through engine controls, which carry it directly to the instrument panel, where it can cause noise by entering the headphone jacks, which should be insulated from the panel and a ground wire from them directly back to the radio stack. That alternator ground current can also make oil temperature gauges read high. It runs though the engine case into the temp sensor, and from there to the gauge and then to the bus. That little extra bit of current therefore pushes the sensor flow higher and makes the gauge read high.

Some alternators have capacitors (condensers) on the alternator case, and connected to the output stud or field stud, and in some cases there will be condensers on both. If those are loose or shot you get noise. If there's a filter on the firewall that the output cable goes to, that thing might be loose on the firewall, or corrosion has isolated it from the firewall.

Lots of things. I remember chasing a whine for a day or two. I finally rolled a table saw up to the front of the airplane and ran a V-belt from the saw's motor to the alternator pulley, and let the motor drive the alternator for some time while I probed around under the panel and along the firewall, all while wearing a headset. I found that the firewall was made of two sheets, and the riveted seam had oxidized and made a bit of resistance that shunted some of the alternator ground flow to some less desirable path. I made and mounted a short ground cable across the seam and got rid of some of the noise. The rest of it was coming through the portable intercom, which someone had helpfully mounted by running screws through its metal enclosure and into the pedestal structure. Ground current was getting into the system via the now-grounded headset jacks. I removed the screws and mounted the thing with Velcro and stopped the whine.

I have a small oscilloscope I occasionally used to see the alternator's waveform. A defective diode would be instantly visible. Never found one.

 

[Indiana_Pilot]

Whine can be caused by a bad ground somewhere. A "ground loop" is formed, and it has nothing to do with taildraggers. If, for instance, the alternator ground cable, between the alternator and firewall, becomes corroded or loose, the alternator's ground current finds other paths such as through engine controls, which carry it directly to the instrument panel, where it can cause noise by entering the headphone jacks, which should be insulated from the panel and a ground wire from them directly back to the radio stack. That alternator ground current can also make oil temperature gauges read high. It runs though the engine case into the temp sensor, and from there to the gauge and then to the bus. That little extra bit of current therefore pushes the sensor flow higher and makes the gauge read high.

Some alternators have capacitors (condensers) on the alternator case, and connected to the output stud or field stud, and in some cases there will be condensers on both. If those are loose or shot you get noise. If there's a filter on the firewall that the output cable goes to, that thing might be loose on the firewall, or corrosion has isolated it from the firewall.

Lots of things. I remember chasing a whine for a day or two. I finally rolled a table saw up to the front of the airplane and ran a V-belt from the saw's motor to the alternator pulley, and let the motor drive the alternator for some time while I probed around under the panel and along the firewall, all while wearing a headset. I found that the firewall was made of two sheets, and the riveted seam had oxidized and made a bit of resistance that shunted some of the alternator ground flow to some less desirable path. I made and mounted a short ground cable across the seam and got rid of some of the noise. The rest of it was coming through the portable intercom, which someone had helpfully mounted by running screws through its metal enclosure and into the pedestal structure. Ground current was getting into the system via the now-grounded headset jacks. I removed the screws and mounted the thing with Velcro and stopped the whine.

I have a small oscilloscope I occasionally used to see the alternator's waveform. A defective diode would be instantly visible. Never found one.

Awesome advice! I appreciate the reply!! I may poke around some more with the grounds to see what I can find. I would also assume if a bad diode was the cause wouldn’t I see some drop in voltage?

I see 13.8v at 2500 RPM and 13.2v at 1000 RPM.


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[flyingbrit]

I would also assume if a bad diode was the cause wouldn’t I see some drop in voltage?

I see 13.8v at 2500 RPM and 13.2v at 1000 RPM.


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No, under most conditions the voltage regulator would compensate for the bad diode and maintain the average/DC voltage at a normal level.

 

[Indiana_Pilot]

No, under most conditions the voltage regulator would compensate for the bad diode and maintain the average/DC voltage at a normal level.

I just ordered a small scope going to check it this weekend.. thanks!


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[3393RP]

I just ordered a small scope going to check it this weekend.. thanks!


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The cost of those handheld scopes allows one to place a really good diagnostic tool in play. When I purchased my four channel Tektronix 150 Mhz scope back in the previous century, I paid almost $4,000 for it.

The same trend has occurred in true RMS digital multimeters. My old Fluke 8060A cost just under $500 in 1986. That's almost $1,200 in 2021 dollars! Now one can purchase a full function Fluke DMM for around $100, and they have the ability to test capacitors too.

Back in the day, a single function capacitance tester was required for the job, and they were expensive. Now one can purchase a Fluke DT6013 capacitance tester is $19.99 on Amazon.

 

[mondtster]

I would also assume if a bad diode was the cause wouldn’t I see some drop in voltage?

I see 13.8v at 2500 RPM and 13.2v at 1000 RPM.

In all the years I’ve worked on cars, motorcycles, and airplanes I’ve only run across two alternators that truly had a bad diode (they were on cars). The voltage would continue to be regulated fine but there was a significant drop in current output/capacity. The diodes failed in a way that left them open, not shorted.

There may still be problems with the charging system that are giving you trouble but that is not the place I would start. Dan gave you some ideas to look at that I would investigate before I would dismantle a functional charging system.