Intermittent charging failure M20J

[wishboneash]

Hi all

Yesterday on a flight from Avalon to Livermore I had a low voltage annunication towards the end of the flight. Never seen this happen before The ammeter was discharging at ~20A. The bus voltage was around 12.0V. I turned off a few loads but couldn't do too much to reduce the loads. The voltage held steady around 11.8V to 12V (was about 20 min from LVK) as I monitored the situation. It was VFR and plenty of places to land so no need to land immediately. A couple of weeks ago, I had to tighten the alternator belt (Kelly alternator, I am waiting for it to die, hopefully in benign conditions like this one).

 

Here's the strange thing. About 4 miles from LVK, I put the gear down. The ammeter needle spikes down as expected, but gear comes down without issues and immediately thereafter the ammeter spikes positive like a bat out of hell. The low-voltage light goes out and the bus voltage jumps to 13.9V. The rest of the flight which lasted a few minutes, the charging system is functioning normally putting out about 15A or so. On the taxi, I am getting 10A charging. So, I believe the alternator is OK. Something intermittent perhaps in the charging system? Anyone else seen this behaviour? Thanks.

[BigTex]

I had similar strangeness happen where my low-voltage lite flickered from time to time.  The issue turned out to be the alternator switch... go-figure.

[Smiles201]

See previous threads on charging system gremlins, especially wiring issues on master switch. There were a couple of threads earlier this year on this and other forums. I had similar problem turned out to be bad wiring.

[OR75]

Hi all

Yesterday on a flight from Avalon to Livermore I had a low voltage annunication towards the end of the flight. Never seen this happen before The ammeter was discharging at ~20A. The bus voltage was around 12.0V. I turned off a few loads but couldn't do too much to reduce the loads. The voltage held steady around 11.8V to 12V (was about 20 min from LVK) as I monitored the situation. It was VFR and plenty of places to land so no need to land immediately. A couple of weeks ago, I had to tighten the alternator belt (Kelly alternator, I am waiting for it to die, hopefully in benign conditions like this one).

 

Here's the strange thing. About 4 miles from LVK, I put the gear down. The ammeter needle spikes down as expected, but gear comes down without issues and immediately thereafter the ammeter spikes positive like a bat out of hell. The low-voltage light goes out and the bus voltage jumps to 13.9V. The rest of the flight which lasted a few minutes, the charging system is functioning normally putting out about 15A or so. On the taxi, I am getting 10A charging. So, I believe the alternator is OK. Something intermittent perhaps in the charging system? Anyone else seen this behaviour? Thanks.

just wondering why you had to tighten your alternator belt and how tight did you get it.

[wishboneash]

just wondering why you had to tighten your alternator belt and how tight did you get it.

 

The belt was quite loose before and a couple of the nuts securing the alternator could be spun by hand. The safety wire was still attached to one of the bolts. Since the voltage held at 12V after this happened for almost 20 minutes, I now feel that the alternator was putting out some charge to maintain 12V. When the gear went down, something changed. On second thoughts, I still suspect the alternator and its assembly. Should double check wiring, connections and switches. Thanks for everyone's feedback.

[OR75]

electricals systems are a pain to troubleshoot and mechanics tend to want to change (functionning) parts until the problem is gone (a few $100 each time).

 

1) Alternator: I believe your  alternator is working correctly. In general, they either work or are completely kaput. Since yours is producing electricity  (intermitently), I am thinking it is still 100% ok.

 

2) voltage regualtor / switch.   Read a great post in Don Maxwell website about the issue that some old Mooney switches can have because corrosion. 

 

 

3)  I am betting you have tighetened the belt too much.   A loose belt is not good. Being able to spin nuts by hand is worse. You can loosen it a tad and see what it does.

[larryb]

To adjust the belt:

ref Lycoming s. i. 1129A

Lycoming SI 1129A Accessory Drive Belt Tension

1. SLIP TORQUE METHOD: This method consists of installing a

torque wrench on the pulley retaining nut and measuring the amount of

torque required to make the pulley slip. Turn the torque wrench in a

clockwise direction, as viewed from the pulley end, and adjust belt

tension accordingly.

SLIP TORQUE SLIP TORQUE

BELT WIDTH.......NEW BELT........USED BELT

.....3/8 Inch .....11 to 13 Ft.....Lbs. 7 to 9 Ft. Lbs.

.....1/2 Inch .....13 to 15 Ft.....Lbs. 9 to 11 Ft. Lbs.

If a new belt is being installed the slip torque should be checked to the

used belt specification after 1 hour operation, at 25 hours, and each

100 hours thereafter. See Aircraft Manual or call Aircraft

Manufacturer.

NOTE: The higher torque value for the new belts is to compensate for

the initial stretch of the belt that occurs as soon as it is operated. Do

not use the higher torque value for a belt that has been previously used.

2. BELT TENSION METHOD: This method consists of installing a

belt tension meter on the fan belt at mid point of the longest

unsupported section of the belt and adjusting the alternator to obtain

the specifications listed below.

The following specifications are for a 3 to 1 pulley ratio with a belt

wrap of 140° on the alternator pulley.

TYPE................OUTPUT..BELT LOAD

ALE,.................40 Amp....50 Lbs.

ALH, ALT, ALZ....50 Amp....75 Lbs.

ALY, ANG...........60 Amp....75 Lbs.

ALU, ALX...........70 Amp....75 Lbs.

The meter used for these specifications was a Borroughs Belt Tension

Meter Model #BT-33-73F.

[laytonl]

Sounds like a high resistance connection in the field circuit. It's time to drag out the multimeter and check voltage across all the connections in the field circuit. Good luck, Lee

[John Sommerfeld]

One more thing to look for:

 

On my 1977 Model J, I continuously received a low voltage light at low engine rpm. When the rpm was increased above 1000 rpm the warning light went out. This was consistent from the time I purchased the airplane and I thought it was normal. Shortly thereafter I experienced an in-flight voltage failure and had to abort the flight.

 

The problem was in the main alternator cable. On the 1977 J Model the cable is routed in a manner that causes it to make a very tight 90 degree bend at the point where it attaches to the alternator. This resulted in a continuous strain being applied to the copper lug on the end of the cable, causing the lug to crack and eventually fail.  Inspection of the lug's fracture surface showed only a small area of fresh copper. The remaining area was blackened from continuous arcing along the crack which limited current flow from the alternator. The crack was not detected during routine inspection as the lug is covered with a rubber boot. The fix was to replace the broken lug.

 

To prevent reocurrance, I fabricated a strain relief from a piece of aluminum fuel line tubing. After forming a ninety degree bend in the tubing, I sawed the formed section in half along its lineal axis. Using the inner half of the formed section, I deburred all sharp edges, laid the cable along the I.D. surface, and secured it with tie wraps. The strain relief is positioned near the rubber boot allowing sufficient space to slide the boot back to fully clear the lug. This effectively transferred the cable strain from the lug to the aluminum tube section, and there has been no further problem.

 

John

[bumper]

Did you inspect the fuel pump diaphram for leaks? Check pressure? I'm not familiar with Continentals either, but it maybe that flow would appear okay on the bench with no head yet not do so well on the plane if the pump has internal leak that only shows up when pressure required?

bumper

[wishboneash]

Sounds like a high resistance connection in the field circuit. It's time to drag out the multimeter and check voltage across all the connections in the field circuit. Good luck, Lee

 

Thank-you Lee, you hit the nail on the head. I finally got the time to look at the alternator last weekend. While inspecting the connections to the alternator, the field wire was frayed to the point it was probably being held up with a single strand. The rubber cover prevented me from noticing this earlier. When I pulled on this cover, the thing finally broke off completely. I reconnected it and the tested and the charging returned to normal. I had planned on replacing the alternator anyway, so I had that done. I now had a Plane Power AL12-P70 which looks to have better cooling, and is a bit lighter than the Kelly ALX-8521R. If anyone is interested in buying this please send me a PM with an offer price. An overhauled one goes for around $400 and a new about $500. From what I can tell, it is working fine. 

[bumper]

I now had a Plane Power AL12-P70 which looks to have better cooling, and is a bit lighter than the Kelly ALX-8521R. If anyone is interested in buying this please send me a PM with an offer price. An overhauled one goes for around $400 and a new about $500. From what I can tell, it is working fine.

 

Did you remove the plenum on the rear of the alternator?

 

When I had my 87 201 re-engined, the shop screwed up and didn't remove the plenum. Took a few hundred hours, but the restricted cooling finally destroyed the alternator.

 

bumper

[wishboneash]

I am not exactly sure what you mean by a plenum. There were no cooling issues with my old alternator. It was just a badly frayed field wire. 

[bumper]

I am not exactly sure what you mean by a plenum. There were no cooling issues with my old alternator. It was just a badly frayed field wire. 

 

Poor choice of words on my part. Should have said rear cover with blast tube . . .

 

 Looking at the parts diagram on the Plane Power AL12-P70, they show a blast tube with bracket rather than a "cover", the bracket having stand offs so it shouldn't restrict cooling if the blast tube is unused  (as is the case on my 201 installation). This is unlike the alternator installed on my ship (don't know the mfg) which had a tightly fitting rear cover with blast tube that was intended for an installation with cooling air forced forward through the alternator with a centrifugal flow fan mounted on the front behind the pulley. On my Mooney, this more tightly fitting rear cover was left in place and this restricted the front to rear air flow that then caused the alt. to fail prematurely.

[wishboneash]

Poor choice of words on my part. Should have said rear cover with blast tube . . .

 

 Looking at the parts diagram on the Plane Power AL12-P70, they show a blast tube with bracket rather than a "cover", the bracket having stand offs so it shouldn't restrict cooling if the blast tube is unused  (as is the case on my 201 installation). This is unlike the alternator installed on my ship (don't know the mfg) which had a tightly fitting rear cover with blast tube that was intended for an installation with cooling air forced forward through the alternator with a centrifugal flow fan mounted on the front behind the pulley. On my Mooney, this more tightly fitting rear cover was left in place and this restricted the front to rear air flow that then caused the alt. to fail prematurely.

 

Yes, my Kelly alternator was the same way. It had the cover in place. I didn't however see any issues with it while it was working. The PP has a blast tube that directs the airflow perpendicular to the flow of air.