Intermittent Alternator

[NealRomeoGolf]

Our alternator has become intermittent. A few weeks ago, all was fine on take off and during some local flight and then the alternator light came on. Cycling the switch didn't help. Went back and landed and at low power the alternator came back. Replaced the voltage regulator. Took plane up today. Again, fine on the ground. But in the pattern at full power, the alternator light came on. Cycled switch. Nothing changed. Landed and alternator was back on.

Any experience with an intermittent alternator that works at low power but kicks off at high power?

 

[Domenick]

What sort of troubleshooting was done to determine the issue was the VR?

Check the alternator field connection. The field wire/connector at the alternator experiences much vibration. The internal conductors at the connector can work harden and break inside the insulation with no external indication. Simply replace the connector. If this is the problem, it's an almost free fix.

 

[NealRomeoGolf]

What sort of troubleshooting was done to determine the issue was the VR?

Check the alternator field connection. The field wire/connector at the alternator experiences much vibration. The internal conductors at the connector can work harden and break inside the insulation with no external indication. Simply replace the connector. If this is the problem, it's a almost free fix.

I wish I knew what was done. My mechanic isn't usually big on details. I had checked the field wire connection with a voltmeter and it tested ok.

He just told me to get a new alternator and I feel like we're just throwing money at problems.

 

[GeorgeC]

Put the parts shotgun down... I had an intermittent alternator issue that turned out to be a loose field wire. Could be different harmonics at different RPMs rather than high power vs low power.

 

[Ed Haywood]

Check connections and replace the brushes.

 

[asicer]

Put the parts shotgun down... I had an intermittent alternator issue that turned out to be a loose field wire. Could be different harmonics at different RPMs rather than high power vs low power.

I was once a passenger on board a Bonanza where someone overtorqued the nut holding the field wire to the alternator, causing the stud to spin. It too, was intermittent and we had to make a decision whether we wanted to risk going home NORDO with the wheels down all the way.

BTW, the pilot was the owner of the plane and the FBO that maintained it.

 

[RyanB]

He just told me to get a new alternator and I feel like we're just throwing money at problems.

Classic. I agree, I wouldn’t jump to replacement. How much time is on it? It might be a loose connector at the voltage regulator, or the alternator power cable has an issue where it’s connected at the terminal. Is the ammeter showing a discharge when this happens?

 

[NealRomeoGolf]

For those that like data, here is the original flight where it went out. RPM doesn't appear to matter in this case. I need to go pull the data from today.

Magenta line is RPM. Yellow line is voltage. White line is manifold pressure.

 

[NealRomeoGolf]

Classic. I agree, I wouldn’t jump to replacement. How much time is on it? It might be a loose connector at the voltage regulator, or the alternator power cable has an issue where it’s connected at the terminal. Is the ammeter showing a discharge when this happens?

Just have an amp indicator. It goes to zero when it happens. Alternator has 300 hours.

 

[Ed Haywood]

... someone overtorqued the nut holding the field wire to the alternator, causing the stud to spin.

Super easy to do. The stud is anchored in a flimsy plastic housing. Basically any tightening without holding the jam nut is going to spin it. If you don't have a set of jam nut wrenches handy (thin box wrenches made from sheet metal), it is tempting to try to work around the problem rather than doing it right. Bad idea. Ask me how I know.

Unfortunately, pilots are not generally well trained on spotting intermittent alternator issues before they become a mission issue. Knowing how to interpret your ammeter and voltmeter together, and keeping an eye on them, will usually give early warning so you don't get stuck AOG away from home. Again, ask me how I know.

 

[NealRomeoGolf]

Here is today's flight. All good until right when I add full power to take off and then comes back when I take power back out to land. I feel like I should have left the pattern and gone and played around with RPM and MAPs but the volt thing flashing at me just made me want to get back on the ground.

 

[Dan Thomas]

He just told me to get a new alternator and I feel like we're just throwing money at problems.

That's already started. It has a long way to go, if you keep using the same mechanic.

Check connections and replace the brushes.

The brushes should be fine at 300 hours. But I had numerous problems with rebuilt alternators. The overhaulers have a real bad habit of putting way too much grease in the rear bearing, which then gets extruded out when the rotor shaft goes in during reassembly, and the grease then gets flung around by the rotating rotor and it gets on the slip rings and brushes and creates a resistive sludge that reduces the alternator's max output by limiting the field current. Easily diagnosed by measuring the field resistance, with the field wire disconnected. Should be low, a few ohms, and it should remain steady as the propeller is turned. Disconnect those sparkplugs, please, before doing this.

Super easy to do. The stud is anchored in a flimsy plastic housing. Basically any tightening without holding the jam nut is going to spin it. If you don't have a set of jam nut wrenches handy (thin box wrenches made from sheet metal), it is tempting to try to work around the problem. Bad idea. Ask me how I know.

I never had any problems with that, and I've had MANY alternators off for brush inspection and repair. That nut only needs tightening with a 3/8" nut driver, not a ratchet and socket. Just a few inch-pounds. Before putting the terminal on the stud, see that the stud retaining nut isn't loose.


There should be a set in every toolbox. They don't need to be expensive; they're not dealing with lots of torque. I used them on the 7/16" hex nut on the output stud, too.

That voltage falling off after some time is indicative of a heat-induced problem, like a failing diode in the alternator. That can be hard to isolate.

 

[NealRomeoGolf]

That's already started. It has a long way to go, if you keep using the same mechanic.

He is essentially my only option. I've started taking my plane to a different one for annuals. But unplanned maintenance leaves me with just him.

 

[Chip Sylverne]

I've found the best way to troubleshoot the alternator diode bridge is with an oscilloscope. Check the trace for stable voltage without significant ripple and symmetric wave form.

 

[Magman]

I’ve found somewhat similar problems on Cherokees were

caused by high resistance in the Field Circuit Breaker.

 

[Bell206]

Any experience with an intermittent alternator that works at low power but kicks off at high power?

The fact both graphs show the alternator output returns to "normal" once back on the ground is interesting.

Is this a front or rear mounted alternator?

And the only thing you changed is the regulator?

What year aircraft/engine?

What's the condition of your alternator belt/mounts and does it have the proper tension?

 

[NealRomeoGolf]

The fact both graphs show the alternator output returns to "normal" once back on the ground is interesting.

Is this a front or rear mounted alternator?

And the only thing you changed is the regulator?

What year aircraft/engine?

What's the condition of your alternator belt/mounts and does it have the proper tension?

Front mounted alternator.

Only thing changed between those graphs is the voltage regulator.

1977 Piper Lance. Lycoming IO-540-K1G5D

Not sure I'm qualified to talk about belts and mounts, but I tug on the belt during every preflight. Seems nice and tight as always.

Here's the back of the alternator. I checked all the wires at the time and they seemed secure and normal. But I'm no mechanic.

 

[Chip Sylverne]

Gotta tellya, I'm not liking the corroded look of the 90 bend in that field wire. I would ring that with a VOM to the regulator and wiggle the heck out of it to see if it loses continuity, then replace it anyway on principle.

 

[Bell206]

I checked all the wires at the time and they seemed secure and normal.

Have you tried running the aircraft on the ground to full static power to see if the alternator drops out?

I only have my phone at the moment, but does the alt wire with the red boot have a sharp bend in it? It seems to be a bit odd from what I can tell.

Also pull back the red boot and look at the terminal.

One difference in the graphs that I can tell is the original failure with the original regulator was well after you were at the higher RPM. There also seemed to be spikes of voltage output as well. Then the voltage returned once you landed.

Where with the new regulator the failure was immediate with the RPM increase and there was no apparent volt output spike except one.

Without chasing some voltages, etc. tough to make a call but I'd take a good look at that field wire from the alternator to the regulator.

Perhaps call the guy who does your annuals for some input? He knows the aircraft and maybe you can get him to look at it?

 

[NealRomeoGolf]

I would ring that with a VOM to the regulator

I don't know what this means.

 

[NealRomeoGolf]

Have you tried running the aircraft on the ground to full static power to see if the alternator drops out?

I only have my phone at the moment, but does the alt wire with the red boot have a sharp bend in it? It seems to be a bit odd from what I can tell.

Also pull back the red boot and look at the terminal.

One difference in the graphs that I can tell is the original failure with the original regulator was well after you were at the higher RPM. There also seemed to be spikes of voltage output as well. Then the voltage returned once you landed.

Where with the new regulator the failure was immediate with the RPM increase and there was no apparent volt output spike except one.

Without chasing some voltages, etc. tough to make a call but I'd take a good look at that field wire from the alternator to the regulator.

Perhaps call the guy who does your annuals for some input? He knows the aircraft and maybe you can get him to look at it?

I have not gone past runup power on the ground. The runup I did last weekend (without flying because it was LIFR) went to 2000 rpm, I loaded it up with pitot, all the lights, all the GPS and radios, everything I could think of, and it held steady. Same thing today (regular runup - didn't load the electrical) and it held.

Yes the field wire has a bend in it. Any coloration on it is probably grime/oil. Before we replaced the regulator, I pulled that boot back and checked it with a voltmeter and got 11.6 volts with the battery on.

I can call my annual guy. He gets the plane in 2 months so he can get a head start on it.

The position of live-in mechanic is still open for you.

The pain with alternators on Lances is you have to pull the bottom cowl every time you want to get at it. That's a good 45 minutes to an hour with two people. Yay airplanes.

 

[Bell206]

Same thing today (regular runup - didn't load the electrical) and it held.

Well that doesn't follow the drop off from your 2nd graph....next.

I pulled that boot back and checked it with a voltmeter and got 11.6 volts with the battery on.

If you're handy with a VOM... with battery on, alternator switch on, check voltage at battery, battery voltage IN at regulator, voltage OUT at regulator (field), and voltage again at alternator field terminal post on alternator (not ring terminal). Post those values.

The position of live-in mechanic is still open for you.

Ha. No hablo ingles. Hace demasiado frío allí.
Besides think of all the money I'm saving you by simply posting free help here!

 

[GeorgeC]

I never had any problems with that, and I've had MANY alternators off for brush inspection and repair. That nut only needs tightening with a 3/8" nut driver, not a ratchet and socket. Just a few inch-pounds.

Yep. Rookie mistake: try to remove corroded nut with nut driver, no joy. Break out the ratchet and watch the stud start thinking about spinning. Change undergarments, then do it more carefully...

 

[Dan Thomas]

I've found the best way to troubleshoot the alternator diode bridge is with an oscilloscope. Check the trace for stable voltage without significant ripple and symmetric wave form.

That's the way, but if it's intermittent and only fails when hot, it gets a lot harder to isolate it.

 

[Jim K]

Can you make a graph with just rpm, amps, & volts?

Weird that it looks like it just cuts out, but isn't popping the breaker. I believe there's an overvoltage relay... Could that be tripping?

 

[Chip Sylverne]

That's the way, but if it's intermittent and only fails when hot, it gets a lot harder to isolate it.

The new battery powered digital tablet handheld models are under $100 now. Don't see why you couldn't hook one up to the main bus and monitor it it flight.

 

[Chip Sylverne]

I don't know what this means.

It means disconnecting both ends of a wire and using a jumper connecting each end to the other through a VOM, essentially forming a ring to see if there are any breaks or high resistance in the wire.

 

[NealRomeoGolf]

Besides think of all the money I'm saving you by simply posting free help here!

Yeah but I'll be waiting two weeks for the next step to get done by the A&P, whereas you and I could knock it out Monday morning. Just think of all the things you could do with your savings when you have free room and board.

 

[3393RP]

No need for all the complexity. That kink in the field wire has compromised the insulation, you can see the conductor strands. It's almost certain some of those wire strands have parted, causing intermittent field voltage and thus alternator output.

Repair it and the problem goes away.

Your mechanic is lazy.

 

[NealRomeoGolf]

No need for all the complexity. That kink in the field wire has compromised the insulation, you can see the conductor strands. It's almost certain some of those wire strands have parted, causing intermittent field voltage and thus alternator output.

Repair it and the problem goes away.

Your mechanic is lazy.

I think the field wire just looks bad at that angle. Here's a different angle, same day.

 

[cowman]

I've been down this rabbit hole twice with two different Pipers, it could be a dozen different things but based on my own experiences it's probably going to be an alternator. An intermittent problem is hard to track down, it might pass every test on the ground and not fail until put under the heat/vibrations/g-forces experienced in flight.

On my Archer we tested several things and ended up replacing the alternator. Tested good on the ground, I took the plane out and it was good... flew a couple laps in the pattern and I left the airport to do some maneuvers. On the first steep turn three things happened at once- ALT light came on, the field breaker popped, and I smelled something hot and electrical. After as quick of a return to the airport as I could the plane was down for what felt like forever trying to test anything we could. After what felt like forever and following multiple "could be this/that" rabbit holes that got us nowhere my A&P pulled the alternator and took it apart- one of the needles in the needle bearing was laying flat in the case instead of oriented correctly on the shaft. This had been allowing the shaft and armature to wobble a bit but it didn't become obvious until I put some g-forces on it in the steep turn and shorted it out. The second reman alternator we received solved the problem for good.

Then a couple years ago on my Lance I had a more straightforward failure, again testing indicated it needed an alternator. We got another reman unit and wouldn't you know it this one was also defective! Second replacement cured the problem.

Just based on my own experiences with these Chrysler 60-amp alternators that were on both my planes and yours I'd just upgrade to a plane power unit.... and make steep turns part of your post-repair test flight. It might have been a fluke that I got two defective ones but I am not rolling the dice on a three-peat.

 

[Bell206]

whereas you and I could knock it out Monday morning.

Monday? You may want to stick to your current plan as I would be lounging in your/my basement penthouse sipping hot toddies until the OAT hit at least 60F. So that would be when, first Monday in April?

 

[Jim K]

Monday? You may want to stick to your current plan as I would be lounging in your/my basement penthouse sipping hot toddies until the OAT hit at least 60F. So that would be when, first Monday in April?

58 on Friday. I'm sure Neal could follow you around with a propane heater.

 

[NealRomeoGolf]

Monday? You may want to stick to your current plan as I would be lounging in your/my basement penthouse sipping hot toddies until the OAT hit at least 60F. So that would be when, first Monday in April?

Well then, Thursday. After the insane January weather it decided to go straight to spring around here.

 

[Domenick]

I wish I knew what was done. My mechanic isn't usually big on details. I had checked the field wire connection with a voltmeter and it tested ok.

Put the parts shotgun down... I had an intermittent alternator issue that turned out to be a loose field wire. Could be different harmonics at different RPMs rather than high power vs low power.

Yes, what George said. A ground test does not replicate the brutal conditions in the engine compartment during flight. At the least, test the connection while pulling, wiggling, and generally abusing the wire. Remove the VR end of the wire before testing continuity.

What model plane and year? Some had overvoltage protection as mentioned by JimK.

Maybe it's not something simple, but at least rule out the simple, inexpensive stuff first.

 

[Ed Haywood]

This is what the backside of the field stud looks like. If the jam nut is not tight, the only thing holding it is a square hole in 1/8" plastic sheet. Note the crack on the edge.

If the stud spins, the copper wire can break. It will still make intermittent contact. Maybe not the problem but easy to check and costs nothing.

 

[SoCal 182 Driver]

Yes the field wire has a bend in it. Any coloration on it is probably grime/oil. Before we replaced the regulator, I pulled that boot back and checked it with a voltmeter and got 11.6 volts with the battery on.

Is 11.6 volts normal?

 

[Bell206]

I loaded it up with pitot, all the lights, all the GPS and radios, everything I could think of, and it held steady.

Since this ground attempt with a full electric load didn't get the same result as your 2nd graph above, maybe try it again with your normal elec load for flying along with a similar throttle movement as though you're taking off? Be interesting to see if the alternator drops off like graph B with throttle increase and recovers when throttle pulled back. It should the way I see it.

I'm sure Neal could follow you around with a propane heater.

Ha. No. Might resurrect old nightmares of being tethered to a Herman heater with 2 inch scat ducting....

Well then, Thursday. After the insane January weather it decided to go straight to spring around here.

Still didn't see anything starting with a "6" on that chart. When I 1st moved south I would be in shorts and T-shirts when it was 40s. 40+ years later I got a jacket on when it gets below 70 and looking to put the flannel sheets on my water bed when it gets below 60.... just saying.

 

[keen9]

I think the field wire just looks bad at that angle. Here's a different angle, same day.

View attachment 125051

Have you pulled back that boot the check the wires? I had my alternator cable work loose from its connector. Its a big fat wire hung out where there is a lot of vibration.

 

[NealRomeoGolf]

Have you pulled back that boot the check the wires? I had my alternator cable work loose from its connector. Its a big fat wire hung out where there is a lot of vibration.

Yes. I moved the boot and tugged on it a lot. Didn't move.