Alternator INOP annuciator issue

[ronnie27478]

Hi,

I could not find a section specifically for electrical system questions, so I'm putting this in the general airframe section. Please move it if appropriate.

My 1978 Navajo Chieftain has an intermittent right alternator INOP annunciation. Some times the alternator INOP annunciator does not go out when the right engine is started, while other times it does. Also, in cases when is stays on after starting, it always goes out when the engine is brought up to flight level RPMs and remains off for the duration of a flight.  When power is reduced, it may stay off or come back on. During flight, the stock ammeter shows little indication on either battery, left or right alternator, so I really can't tell from that if the alternator load is balanced or not.

When I first got the airplane, it had a poor field connection on the right alternator, but that has been addressed.  Also, the belt tension looks good.

At first, I suspected a bad alternator INOP switch, but after reading the maintenance manual I am now wondering if the voltage regulator paralleling system is causing the issue, i.e., the right voltage regulator is set low as compared to the left one.  The system bus voltage is correct.

I plan to put a good millivolt DMM in place of the ammeter and operate each engine individually and together to see what the actual load current on each alternator is to make sure I don't have an issue with the right alternator.  Beyond that, I can run through the service manual procedure for adjusting the voltage regulators so they operate properly individually and in parallel.

I would appreciate comments from those who have experienced a similar issue and what you found as root cause.

Thanks,

Ronnie

[roger]

If you run only the right alternator (left one shut off or engine not running), does the inop light behave the same, or does it go out and stay out? If the inop light goes out and stays when the left alternator is out of the equation then you just need to do a balance. To do a balance, you'll need an analog multimeter in addition to you digital one. Digital to set the right alternator, and analog to balance the left one. I'm sure there's a bazillion different techniques out there but I've had great luck following the MM procedure to the letter.

Also, don't put too much stock in how that belt tension "looks". I've found that it can look and feel fine, but then you do the torque slip check from the MM and find that it's too loose and will slip, especially in rain. You need an indicating torque wrench and I think a 1" socket. Also when you do it, make sure that you rotate the alternator opposite the direction of rotation (to put the slip load in the direction that it is with the belt driving it). Weird as it sounds you'll often get different readings in the two directions.

Don't rule out the possibility of more bad wires in the engine bay either, it's a terrible environment for wiring with vibration, heat, and oil exposure. For the love of God if you do get any wires repaired have it done by somebody who knows how to use a crimper properly.

So yeah, I would exhaust the cheap, easy stuff first (belt tension per MM, load balance, and wiring) before you go spending a bunch of dough on parts.

[ronnie27478]

I have not had a chance to pull the airplane out of the hangar and run the right engine to check it, but I did have a chance to check the belt tension via the alternator pulley torque method as per the MM. I measured the following: (all my CW and CCW are referenced from a point of view of looking into the nacelle open from the front at the alternator pulley; CW in this reference is the direction the right engine turns the alternator).

CW   direction: 107 in-lbs. (8.95 ft-lbs)
CCW direction:   75 in-lbs. (6.25 ft-lbs)

The MM calls for turning the alternator pulley in the CW direction and these torques:

  3/8" wide belt:  new 11 to 13 ft-lbs;  used 7 to 9 ft-lbs

  1/2" wide belt:  new 13 to 15 ft-lbs;  used 9 to 11 ft-lbs

The belt installed is a Lycoming LW-18129, which the parts catalog says is an 11 mm wide belt. This is about halfway between the 3/8" and 1/2" belt widths listed in the MM and the belt is definitely used.  It has at least 300 hours on it. 

So based on the your comments above, the CCW direction is the way the alternator would slip when being driven.  In that case, it appears to be below MM specs for torque, but within specs when turned in the CW direction.

[roger]

Oct 10, 2017 16:41:37 GMT -9 ronnie27478 said:

I have not had a chance to pull the airplane out of the hangar and run the right engine to check it, but I did have a chance to check the belt tension via the alternator pulley torque method as per the MM. I measured the following: (all my CW and CCW are referenced from a point of view of looking into the nacelle open from the front at the alternator pulley; CW in this reference is the direction the right engine turns the alternator).

CW   direction: 107 in-lbs. (8.95 ft-lbs)
CCW direction:   75 in-lbs. (6.25 ft-lbs)

The MM calls for turning the alternator pulley in the CW direction and these torques:

  3/8" wide belt:  new 11 to 13 ft-lbs;  used 7 to 9 ft-lbs

  1/2" wide belt:  new 13 to 15 ft-lbs;  used 9 to 11 ft-lbs

The belt installed is a Lycoming LW-18129, which the parts catalog says is an 11 mm wide belt. This is about halfway between the 3/8" and 1/2" belt widths listed in the MM and the belt is definitely used.  It has at least 300 hours on it. 

So based on the your comments above, the CCW direction is the way the alternator would slip when being driven.  In that case, it appears to be below MM specs for torque, but within specs when turned in the CW direction.

Personally, I would crank it up to 108 in-lbs CCW.    

[ronnie27478]

Just a quick follow-up. Finally solved the intermittent issue with the right alternator circuit. Turned out to be a non-soldered, press fit terminal on the printed circuit board in the voltage regulator. Soldered it to the PCB and now works great. Did a full adjustment an balance as per the MM and have not had any issue since.

[klausnw]

Thanks for the update. Troubleshooting common problems is bad enough, having an uncommon problem... Good find.