" VOLTAGE " annunciator light flashing

[MooneyBob]

I noticed today on the approach my VOLTAGE light was flashing. The voltmeter read 13.1 Volts. It usually reads 13.7V. I shot off all lights, transponder and radio and it stopped flashing and voltmeter showed normal voltage. My plane is 1977 M20J.

Once on the ground I turned on all lights, radio, pitot heat and fuel pump. The voltage dropped again around or below 13volts and red light was flashing again. 

After shut off the battery voltage was 12.6 Volts.

I will search the forum for low voltage topics but if can pint me to the right direction I would appreciate it.

I can think about two possible issues :

1. Alternator belt is slipping under higher current draw conditions

2. Something is going on with the voltage regulator ( or whatever it is) 

Thanks

[carusoam]

Add to your list...

3. Strength/age of the battery.  Old Gil or Concorde?

4. Belt tightness. There is proper procedure to check for this.

 

what RPM do you usually use when on approach?

Does raising the rpm slightly make the light go out?

Which voltage regulator do you have?  (Zeftronics replaced my C's sticks, stones and springs model)

Best regards,

-a-

[MooneyBob]

29 minutes ago, carusoam said:

Add to your list...

3. Strength/age of the battery.  Old Gil or Concorde?

4. Belt tightness. There is proper procedure to check for this.

 

what RPM do you usually use when on approach?

Does raising the rpm slightly make the light go out?

Which voltage regulator do you have?  (Zeftronics replaced my C's sticks, stones and springs model)

Best regards,

-a-

Based what I found on the forum I will check recommended items tomorrow. Alternator, regulator, lose wiring, belt, battery, etc.

My approach RPMs on short final are around or less then 2200. Don't know if higher RPM shut off the warning.

Concord battery that looked and charged good so far. Don't know the age of the battery. Have to read the logs.

 Unknown voltage regulator. I will look for it tomorrow. 

Thanks Anthony. 

[jetdriven]

Could be a bad diode in the alternator. Then it will only put out about half its rated amps or less. Usually accompanied by a whine inn headset 

[MooneyBob]

Just now, jetdriven said:

Could be a bad diode in the alternator. Then it will only put out about half its rated amps or less. Usually accompanied by a whine inn headset 

That's what I have noticed. Unusually whine sound in my headset. Thanks. 

[jetdriven]

So, next step hook up a digital volt ohm meter to the alternator output post. Use test leads with alligator clips. Ground the other lead. Set the VOM to AC volts. Start it up and run at 1400 rpm.  If it shows more than .7 VAC suspect a bad diode. The Plane Power rectifier is listed now, it's 25$.  The Kelly Ford Prestolite  one will be similar. 

Edited July 16, 2016 by jetdriven

[MooneyBob]

17 minutes ago, jetdriven said:

So, next step hook up a digital volt ohm meter to the alternator output post. Use test leads with alligator clips. Ground the other lead. Set the VOM to AC volts. Start it up and run at 1400 rpm.  If it shows more than .7 VAC suspect a bad diode. The Plane Power rectifier is listed now, it's 25$.  The Kelly Ford Prestolite  one will be similar. 

Thank you very much. 

[N9201A]

I chased this for a while on my J, light first was intermittent then became solid. After ruling out moving parts and confirming voltage with voltmeter, followed procedure in service manual to adjust when light comes on accessible in back of annunciator panel.

Sent from my iPad using Tapatalk

[MooneyBob]

1 hour ago, N9201A said:

I chased this for a while on my J, light first was intermittent then became solid. After ruling out moving parts and confirming voltage with voltmeter, followed procedure in service manual to adjust when light comes on accessible in back of annunciator panel.

Sent from my iPad using Tapatalk

My low voltage is confirmed by the engine monitor that shows voltage drop. So it is probably real not just a false reading. 

[Yetti]

Bad diode in Alternator like Jetdriven said

[MooneyBob]

1 hour ago, Yetti said:

Bad diode in Alternator like Jetdriven said

So what needs to be done to fix it ?

Thanks 

[carusoam]

Someone around here posted the wiring diagram of something similar to a 3 phase system using diodes to turn it all into a single phase DC system.   Losing a leg of the system, because a diode goes dead, the design allows the system to continue working, just not very well....

The solution was to find where the diodes are mounted, identify the dead one and replace it.  Diodes are known to croak in the rugged environment they are in.  Designers tend to make them easy to swap out.

If I can find that post, I'll put a link here...

Try this link regarding noisy alternators...

For additional info search on the word diode.  There are a few threads...

Experience: Using knowledge found here, I have succesfully replaced a diode on my 12V lawn tractor/Tug, and I'm only a PP...

Best regards,

-a-

[jetdriven]

47 minutes ago, MooneyBob said:

So what needs to be done to fix it ?

Thanks 

Replace the rectifier.  The Plane Power one is The inr-720p.  From here http://www.aspwholesale.com/rectifier-nd-p1359.html

 

if you have the prestolite it's going to be a rectifier from like a 1972 ford f-100.  

Edited July 17, 2016 by jetdriven

[Yetti]

 

[MooneyBob]

Thank you all. I will work on it today. 

[MooneyBob]

On 7/16/2016 at 11:02 PM, jetdriven said:

Replace the rectifier.  The Plane Power one is The inr-720p.  From here http://www.aspwholesale.com/rectifier-nd-p1359.html

 

if you have the prestolite it's going to be a rectifier from like a 1972 ford f-100.  

So my alternator is Kelly Aerospace ALX 8521R. Which rectifier I should get? 

Thanks

[jetdriven]

I would take it to autzone and try to find one that's visually similar.  Start with the alternator for a 1972 ford.

anothet thing, open it up and try to find a number off the rectifier. Some may also have press fit diodes too, you can rob the ford alternator for the diodes, or the whole rectifier. 

Edited July 18, 2016 by jetdriven