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Alternator Drops out at 900 rpm on Mooney Acclaim.

M20S Driver

By M20S Driver
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M20S Driver Experienced

M20S Driver

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My mechanic is working on a puzzling electrical problem on a Mooney Acclaim (2009?) that I find interesting enough to post and see if anyone has seen this.

 

This is what I know about this problem: (no it is not my M20S!-- fortunately)

 

1- the Alternator drops out when the rpm drops below 900.

2- re-setting the alt-field or the circuit breaker will bring the alternator back on-line at 1000 rpm

3-Rebuilt alternator did not solve the problem.

4-Bypassed the alt relay and did not solve the problem.

5-checked for loose wires and none found.

6-Voltage regulator on the list to be replaced but not done yet.

7-The line voltage is always greater than 28.6 volts.

8-Charging and alternator performance appears normal at 1000 rpm and above. 

 

 

Any ideas would be appreciated :)

 

M20S Driver

ArtVandelay Grand Master

ArtVandelay

Posted
Posted

I assume #4, you mean bypass the alt-field relay, if not, try that, or at least clean it with electronic cleaner.

2- re-setting the alt-field or the circuit breaker will bring the alternator back on-line at 1000 rpm

3-Rebuilt alternator did not solve the problem.

4-Bypassed the alt relay and did not solve the problem.

.

philiplane Mentor

philiplane

Posted
Posted

This is not puzzling, it's known to those mechanics with experience in late model airplanes (2001 and newer) with Continental engines. Or those who read the maintenance manuals... :)  It's normal for gear drive alternators on Continental engines to having trouble keeping up below 1000 rpms. It's just a function of the load on the system and the rpms. At 700 rpm, your 100 amp alternator can only put out about 10 amps. Any draw above that will drag the voltage down. You'll only get full output above 2200 rpms. Below that, output drops on a curve. Below 1300 rpm, the alternator can no longer maintain 28.5 volts, and it will decrease to 25.5 at 900, and drop offline around 650 rpm.

 

Cirrus uses the same base engine, and this condition is described in their flight manual.  Mooney should do the same.

M20S Driver Experienced

M20S Driver

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  • Author
Posted

This is not puzzling, it's known to those mechanics with experience in late model airplanes (2001 and newer) with Continental engines. Or those who read the maintenance manuals... It's normal for gear drive alternators on Continental engines to having trouble keeping up below 1000 rpms. It's just a function of the load on the system and the rpms. At 700 rpm, your 100 amp alternator can only put out about 10 amps. Any draw above that will drag the voltage down. You'll only get full output above 2200 rpms. Below that, output drops on a curve. Below 1300 rpm, the alternator can no longer maintain 28.5 volts, and it will decrease to 25.5 at 900, and drop offline around 650 rpm.

 

Cirrus uses the same base engine, and this condition is described in their flight manual.  Mooney should do the same.

Thanks for your post. In this case, the drop outs happen at 28.6 volts at 900 rpm which is 3 volts higher than 25.5 volts you mentioned. Does the higher voltage point to any component issues?

philiplane Mentor

philiplane

Posted
Posted

That's actually a little better than expected for that alternator. You should see 28.5 to 28.7 volts in normal operations with a fully charged battery, under full load, above 2300 rpm.

 

Acclaims came from the factory with a Kelly Aerospace (now Hartzell Engine Technologies, HET) ES series lightweight alternator. It is 8 pounds lighter than the old reliable TCM 100 amp unit. Like all electrical generating units, lightweight is never better than heavyweight. Kelly produced it to help with the forward CG issues Cirrus & Mooney had with the turbo IO-550 with a/c. They rarely make the 500 hour brush inspection before failing. It's best to check the brushes at each annual, or no more than 250 hours. When the brushes get worn or the braided contact wires shred, the alternator will work OK but it puts strain on the regulator. When the brushes start arcing, it usually kills the regulator first. These alternators are also known for weak rotors. There is a resistance check that is done as part of the 500 hour inspection. If the alternator has been replaced since mid-2011, it will be a little better. If it has been changed after that date with the HET alternator, it should make 500 hours with no problems. It is still recommended to replace the brushes in these lightweight alternators every 500 hours. They will not make 1000, and when the brushes go, the alternator and regulator need replacing to the tune of $2000 plus labor. 

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M20S Driver Experienced

M20S Driver

Posted
  • Author
Posted

That's actually a little better than expected for that alternator. You should see 28.5 to 28.7 volts in normal operations with a fully charged battery, under full load, above 2300 rpm.

 

Acclaims came from the factory with a Kelly Aerospace (now Hartzell Engine Technologies, HET) ES series lightweight alternator. It is 8 pounds lighter than the old reliable TCM 100 amp unit. Like all electrical generating units, lightweight is never better than heavyweight. Kelly produced it to help with the forward CG issues Cirrus & Mooney had with the turbo IO-550 with a/c. They rarely make the 500 hour brush inspection before failing. It's best to check the brushes at each annual, or no more than 250 hours. When the brushes get worn or the braided contact wires shred, the alternator will work OK but it puts strain on the regulator. When the brushes start arcing, it usually kills the regulator first. These alternators are also known for weak rotors. There is a resistance check that is done as part of the 500 hour inspection. If the alternator has been replaced since mid-2011, it will be a little better. If it has been changed after that date with the HET alternator, it should make 500 hours with no problems. It is still recommended to replace the brushes in these lightweight alternators every 500 hours. They will not make 1000, and when the brushes go, the alternator and regulator need replacing to the tune of $2000 plus labor. 

I think the regulator is next on the list since most every thing else is ruled out so far.  

philiplane Mentor

philiplane

Posted
Posted

If the regulator keeps steady output of 28.1 to 28.7 volts and sufficient current above 2000 rpm, there's nothing wrong with it. It can't force the alternator to make current at low rpm, that's a function of rotor speed, Less speed, fewer amps, and less voltage. Everything described sounds perfectly normal.

carusoam Grand Master

carusoam

Posted
Posted

When you get a chance... Do a search on regulators.

There was a post a while back regarding the dual alternators with a single regulator (twin?) device.

This is from an aging fuzzy memory, so it may not be fruitful...

Best regards,

-a-

M20S Driver Experienced

M20S Driver

Posted
  • Author
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From Astel regarding the Bravo's twin alternators...

Different, but possibly helpful...?

http://mooneyspace.com/topic/3843-alternator-balance/?hl=%2Bvoltage+%2Bregulator+%2Bcontroller#entry47878

Best regards,

-a-

Thanks carusoam.   I asked if it has dual alternators and the mechanic said that there are two alternators but the arrangement is different.  One is a back up that has to be turned on and it is not regulated as a dual alternator.  I did not know that this option existed!

 

M20S Driver

philiplane Mentor

philiplane

Posted
Posted

The Acclaim is not like any other dual alternator Mooney. The alternators are brand new designs for this model.

 

As for the "troublesome" drive couplings: they fail because mechanics do not have, or do not use the correct tools to install them. Improper installation causes partial disbonding of the elastomer to the housing, and it will fail under heat and full load some time later. It will work at low loads and fail under torque.  When these couplings are installed correctly they will outlast the alternator, and usually the engine as well.

  • Like 1
  • 3 weeks later...
Piloto Grand Master

Piloto

Posted
Posted

Check for drop out under no load. Pull as many circuit breakers except for the alternator field. Check the voltage with a portable DVM instead of the panel voltmeter. It is not unusual at low rpm for an alternator not too handle the load. I remember the case where a 252 owner used to keep the RPM high while taxing at night and burn up a tire inner tube due to heating of the brake disk.

 

José  

M20S Driver Experienced

M20S Driver

Posted
  • Author
Posted

Check for drop out under no load. Pull as many circuit breakers except for the alternator field. Check the voltage with a portable DVM instead of the panel voltmeter. It is not unusual at low rpm for an alternator not too handle the load. I remember the case where a 252 owner used to keep the RPM high while taxing at night and burn up a tire inner tube due to heating of the brake disk.

 

José  

Last informaion that I have is that the no load test and switching  the battery did not do much.  The owner is taking it to Lake Arrow for the repair or he may end up going to the factory in Texas.  This is pretty ugly and costly!

carusoam Grand Master

carusoam

Posted
Posted

One more time before he goes...?

1) An alternator does not keep up while taxiing slowly... The light comes on, on the instrument panel..to let you know. Dual batteries will last through a really long taxi...

2) Keeping the rpms up to douse that light results in fast taxi speeds...

3) dragging the brakes to maintain control of that speed leads to other issues, Wear and heat related.

I believe those are in the normal operations category.

These are observations from my single alternator O.

Best regards,

-a-

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