ALTERNATOR TROUBLESHOOTING ?

[DCarlton]

My M20F alternator warning light came on during my last flight.  I have a Plane Power STC Kit (SAL12-70) that was installed around 2013 (ALT, Brackets, Voltage Regulator).  Since the shop was busy, I went ahead and replaced my 7 year old battery this weekend.  Here's what I'm seeing with a fresh battery:  

- the battery tested 12.9 V and 100% charged on the bench

- after battery installation with the engine off, I was seeing 12.7 V on my EDM 730 (avionics fan was running along with the EDM)

- after engine start, with the ALT field switch off, with the warning light illuminated, I was seeing 12.4 V  

- with the ALT field switch on, the warning light was still illuminated; I was initially seeing 14.2 V with the engine idling around 1000 rpm

- I flipped the ALT field switch on and off a few times; the voltage dropped to 14.0 with the field switch on, engine was idling around 1000 rpm

So... I'm wondering if my alternator is just getting tired, or if the voltage regulator is crapping out (perhaps the warning circuit is just bad).  I may try to do some more troubleshooting tomorrow (before the game) using the Plane Power guide.   Appreciate any tips.  Also wondering if folks have success just replacing brushes or is that not worth the effort.  New alternator is over a grand.  Thanks! 

[Fly Boomer]

12 minutes ago, DCarlton said:

My M20F alternator warning light came on during my last flight.  I have a Plane Power STC Kit that was installed around 2013 (ALT, Brackets, Voltage Regulator).  Since the shop was busy, I went ahead and replaced my 7 year old battery this weekend.  Here's what I'm seeing with a fresh battery:  

- the battery tested 12.9 V and 100% charged on the bench

- after battery installation with the engine off, I was seeing 12.7 V on my EDM 730 (avionics fan was running along with the EDM)

- after engine start, with the ALT field switch off, with the warning light illuminated, I was seeing 12.4 V  

- with the ALT field switch on, the warning light was still illuminated; I was initially seeing 14.2 V with the engine idling around 1000 rpm

- I flipped the ALT field switch on and off a few times; the voltage dropped to 14.0 with the field switch on, engine was idling around 1000 rpm

So... I'm wondering if my alternator is just getting tired, or if the voltage regulator is crapping out (perhaps the warning circuit is just bad).  I may try to do some more troubleshooting tomorrow (before the game) using the Plane Power guide.   Appreciate any tips.  Also wondering if folks have success just replacing brushes or is that not worth the effort.  New alternator is over a grand.  Thanks! 

Seems like 12.9 is excellent, and 14.0 or 14.2 are also good.  Maybe the warning light circuit?

[AIREMATT]

If you run the electrical system enough to run the battery down to 12v and then run the engine and alternator showing 14.2v for a while is your battery actually charging back up? Is it possible the alternator output isn’t getting to the battery and that’s why the warning light is on?

 

Do you have the manual for the Plane Power alternator? It might help checking the warning lamp circuit. I have it if you need it.

 

Edited February 11, 2024 by AIREMATT

[philiplane]

if you have 1000 or more hours on the alternator, you should replace the brushes.

[DCarlton]

57 minutes ago, AIREMATT said:

If you run the electrical system enough to run the battery down to 12v and then run the engine and alternator showing 14.2v for a while is your battery actually charging back up? Is it possible the alternator output isn’t getting to the battery and that’s why the warning light is on?

 

Do you have the manual for the Plane Power alternator? It might help checking the warning lamp circuit. I have it if you need it.

 

I have the Flight Manual Supplement, A Brush Assembly Inspection / Replacement guide, and a one page Troubleshooting Guide for externally regulated alternators and the part numbers for the replacement parts.  Not sure about a manual beyond the schematic in the Troubleshooting Guide.  Plan to check the warning circuit next.  Thanks. 

[DCarlton]

12 minutes ago, philiplane said:

if you have 1000 or more hours on the alternator, you should replace the brushes.

Less than 500 hours.   I wouldn't think they should need replacing.  

[Fly Boomer]

4 hours ago, DCarlton said:

So... I'm wondering if my alternator is just getting tired, or if the voltage regulator is crapping out (perhaps the warning circuit is just bad).

As always, please let us know what it turns out to be, and what the cure was.

[AIREMATT]

50 minutes ago, DCarlton said:

I have the Flight Manual Supplement, A Brush Assembly Inspection / Replacement guide, and a one page Troubleshooting Guide for externally regulated alternators and the part numbers for the replacement parts.  Not sure about a manual beyond the schematic in the Troubleshooting Guide.  Plan to check the warning circuit next.  Thanks. 

Here’s what I have if it helps.

SAL12-70 Alternat Install and ICA Instructions.pdf

[Ragsf15e]

6 hours ago, DCarlton said:

My M20F alternator warning light came on during my last flight.  I have a Plane Power STC Kit (SAL12-70) that was installed around 2013 (ALT, Brackets, Voltage Regulator).  Since the shop was busy, I went ahead and replaced my 7 year old battery this weekend.  Here's what I'm seeing with a fresh battery:  

- the battery tested 12.9 V and 100% charged on the bench

- after battery installation with the engine off, I was seeing 12.7 V on my EDM 730 (avionics fan was running along with the EDM)

- after engine start, with the ALT field switch off, with the warning light illuminated, I was seeing 12.4 V  

- with the ALT field switch on, the warning light was still illuminated; I was initially seeing 14.2 V with the engine idling around 1000 rpm

- I flipped the ALT field switch on and off a few times; the voltage dropped to 14.0 with the field switch on, engine was idling around 1000 rpm

So... I'm wondering if my alternator is just getting tired, or if the voltage regulator is crapping out (perhaps the warning circuit is just bad).  I may try to do some more troubleshooting tomorrow (before the game) using the Plane Power guide.   Appreciate any tips.  Also wondering if folks have success just replacing brushes or is that not worth the effort.  New alternator is over a grand.  Thanks! 

If you’re seeing 14v at the bus, the alternator is working.  Which implies the vr is also working.  You need to look at the wiring for your alternator light and see what exactly it indicates and why. If you’re concerned about the battery not getting a proper charge, (safely) have a friend hold the brakes and start the engine.  Use a digital voltmeter to measure voltage back at the battery. It should be ~14v just like your bus is indicating.

[DCarlton]

17 hours ago, Fly Boomer said:

As always, please let us know what it turns out to be, and what the cure was.

As folks expected, turned out to be a broken AUX wire at the alternator.  Inexpensive fix.   My favorite.  Appreciate the help! 

[BRBENNETT]

I had this problem recently and solve it by going from prop to tail checking all of the alternator related connections, ie voltage regulator, battery cables, related grounds, etc.  Found some that were questionable.  After torquing everything down....problem solved itself.  Note, this was AFTER I spent $1300 on a new alternator.  The same model you have!  Hope this helps!

[Jim Peace]

On 2/10/2024 at 7:50 PM, philiplane said:

if you have 1000 or more hours on the alternator, you should replace the brushes.

I have the plane power for about 10 years now...close to 1000 hours.  I want to be proactive but they say no mx required until it fails.  I am not to knowledgeable about my alternator.  May be a stupid question....do I have brushes?  is there really nothing I can do until it fails short of just buying a new one?

[PT20J]

All alternators have brushes. I have never replaced brushes in any of my automobile alternators. My airport car is a 2004 Volvo XC90 with 120K miles. That’s probably about 4000 hrs and 20 years and it has never failed.

[MikeOH]

4 hours ago, PT20J said:

All alternators have brushes. I have never replaced brushes in any of my automobile alternators. My airport car is a 2004 Volvo XC90 with 120K miles. That’s probably about 4000 hrs and 20 years and it has never failed.

My understanding is that alternators have slip rings vs. brushes.  Further, I believe while slip rings do wear, it is much less than brushes. A slip ring is typically a spring loaded contact that rides against as smooth metal ring.  A brush is pushed against a serrated commutator that is not smooth, especially as it wears.

[PT20J]

It’s terminology. A slip ring is a continuous rotating metal contact. A commutator is a segmented rotating metal contact. The stationary conductor that makes contact with either a slip ring or a commutator is a brush. Commutators are filled with an insulating material between metallic segments, so they are pretty smooth. But, because of the commutating function they arc more than slip rings and I believe this is why generator brushes wear faster than alternator brushes.

[MikeOH]

Sorry, I can't agree that two completely different design approaches are merely "terminology." Clearly we agree that the brush technology wears more.

[N201MKTurbo]

26 minutes ago, MikeOH said:

Sorry, I can't agree that two completely different design approaches are merely "terminology." Clearly we agree that the brush technology wears more.

They are both called brushes. They both are constructed very similarly. The brushes in a generator are usually a lot beefier than the brushes in an alternator. In an alternator only field current goes through the brushes. In a generator the entire output current goes through the brushes. That is why they are bigger and wear faster. 

[MikeOH]

17 minutes ago, N201MKTurbo said:

They are both called brushes. They both are constructed very similarly. The brushes in a generator are usually a lot beefier than the brushes in an alternator. In an alternator only field current goes through the brushes. In a generator the entire output current goes through the brushes. That is why they are bigger and wear faster. 

The higher current makes sense for the more rapid wear; thanks for that explanation.

I do maintain that that slip ring construction is quite different from commutator construction, however.  Same purpose: conduct current across a rotating boundary, but very different design.

[jamesm]

I believe this statement to be correct... The ol Delco Remy 50 amp  generator is pushing 10 times plus the amount of current through it's spinning brushes and  commutator circuitry making it heaver but produces cleaner power (without less external filtering circuitry less whining) but  also needs to be  spinning faster (coming in speed) for it to charge the battery , as oppose to an Alternator which spinning the field circuit  much lighter in weight  and pushes something way less than 5 amps of current for the field circuitry.

James '67C

[EricJ]

The other difference is that the contact on a slip ring is continuous and flat, while the contact on a commutator is segmented.   I've always thought the smooth surface is less prone to wearing the brush (and it's a "brush" on either), which is partly why they don't wear as quickly on an alternator.

On most alternators the brushes are easily accessible and servicable since they're essentially the main wear component in the unit, along with the diodes or regulator or whatever else may be integrated into the unit.   Usually they're easy to pop off the back of the alternator and swap out.   Unless a bearing fails, this makes repairing the alternator in place (without removal) often possible.

 

[philiplane]

an alternator brush is only about an eighth of the size of a 50 am generator brush.

a typical 70 amp alternator brush will wear to half length in 800-1000 hours. 

Once it wears below half length, it begins to bounce at higher RPM. That bouncing disrupts the rectifier circuit and soon fries the rectifier.

Brushes are less than $60 in most cases.

You choice. $60 brushes at 1000 hours, or a $1200 alternator soon after that point.

[cliffy]

I have machined several alternator slip rings when found very worn but the wear may have come from (as noted above) 

wear caused by worn brushes and the concomitant weaker spring tension pushing the brushes onto the slip rings

thereby causing wear from more arcing.  

Nothing is ever mentioned about replacing brushes at an hour mark but maybe, just maybe, it should be considered 

a preventative maintenance item in reality.

No reason why an alternator shouldn't make TBO with proper maintenance.

[N201MKTurbo]

8 minutes ago, cliffy said:

I have machined several alternator slip rings when found very worn but the wear may have come from (as noted above) 

wear caused by worn brushes and the concomitant weaker spring tension pushing the brushes onto the slip rings

thereby causing wear from more arcing.  

Nothing is ever mentioned about replacing brushes at an hour mark but maybe, just maybe, it should be considered 

a preventative maintenance item in reality.

No reason why an alternator shouldn't make TBO with proper maintenance.

Instead of replacing them by time, why not pop them out and measure them and when they are worn beyond the wear limit replace them? And, what is the ware limit? I don't remember seeing it anywhere. It only takes about 10 minutes to R&R the brush holder. Once the cowl is off anyway.

[DCarlton]

3 hours ago, N201MKTurbo said:

Instead of replacing them by time, why not pop them out and measure them and when they are worn beyond the wear limit replace them? And, what is the ware limit? I don't remember seeing it anywhere. It only takes about 10 minutes to R&R the brush holder. Once the cowl is off anyway.

See attached.  "11. Replace brushes if they extend less than .250” from the edge of the case."

AL12-24-Brush-Block-Replace.pdf