Trouble shooting a low voltage

[Tommy]

Hi guys!

 

Today I had a a reasonably long flight so I got the chance to test my new EDM 700. All was well with engine side of things but the low voltage BAT was flashing 20 minutes into 2 hours of flight showing 11.8V for more than an hour and gradually down to 11.6V on the last 30 minutes of the flight. The announciator panel High / Lo Volt also flashes. Ampmetre is deflected slightly to -ve (-5 to -10 when previously at zero). The high/lo volt has been flashing lately but seems to go away by itself. The other thing I noticed was the cluster gauge seems to flicker in turbulence (none before).

 

I flew all the way to my destination and landed without any incidence. No CBs were popped. I didn't restart the engine and simply put it away in my friend's hangar who happens to be a A&P previously. My own A&P is on holiday and my home hangar is now 2 hours away.

 

I am just wondering

 

1) what would be the most likely cause of these symptoms?

2) how would I go about diagnosis it with my friend's help? (where is the field wire of the alternator and where is the VR? Pictures would be great!)

3) if I can't rectify the problem, is it safe to fly the aircraft for 2 hours back home?

 

I had researched - including watching Mike's webinar on the subject - so hopefully I am not going to be too lost on this...

 

Thanks guys!

 

ps. now I know why my A&P was smiling when he realised that I was going to put in an engine monitor...

[Tommy]

So far I have

 

1) loose alternator belt

2) frayed alternator field wire

3) bad master/alternator switch

4) dead alternator

5) dead battery

6) adjusting VR

 

Anything else?

[N201MKTurbo]

Bad engine ground strap.

 

Any whining in the audio?

 

Does the voltage change when you change the electrical load? Like turning on the landing light or pitot heat.

[Tommy]

No whine.

Where is the engine ground strap located on a 1981 J?

I dare not touching the landing light but will give it a try tomorrow. What does it mean if the voltage drops when it's turned on?

[tony]

Adding to your list:

 

over voltage relay

 

Remove from your list:

 

dead battery

[robert7467]

I would check the belt.

[larryb]

I would start with the field wire at the alternator. It's the smallest gauge wire connected to the alternator. The small size + vibration make it susceptible to failure. Take the cowling off and physically inspect all of the wires attached to the alternator. Tug at each one to make sure it's tight and secure.

 

With the engine off but master on you should find battery voltage at the field terminal.

 

Since you flew for a while, your battery is likely quite low. I would put it on a charger for a few hours before you try an engine start.

 

Larry

[RobertGary1]

First step is to check the quality of all connections. I'd start with the alternator. 

 

If you fire up the plane now do you still experience the issue or is it intermittent? Intermittent is the hardest to solve. I had a similar issue for about 6 months. Frustrating. Turned out the bearing in the back of the alternator had worn and would occasionally, slightly, ground out the armature. So I'd get low alternator output for a while and then the alternator would come back. Sometimes changing prop RPM could fix the problem.

 

-Robert

[robert7467]

Or you can just convert to Plane Power and be done with it.

[N201MKTurbo]

If your alternator is working it will regulate the buss voltage. When you turn on the landing light the alternator will automatically increase its output to compensate for the increased load and the buss voltage will remain the same. If the alternator is not working the increased load will cause the buss voltage to drop.

 

The ground strap goes from the engine to the firewall. When I bought my 201 it had none. I added a length of 4 GA wire from one of the prop governor mounting bolts to an unused nut plate on the firewall. Things behaved much better after that. The plane started better and the audio was quieter.

[BigTex]

Mine did that and it turned out to be the panel switch for the alternator.  In my case it mainly did it when the landing light was on and the engine was at low RPM.

[Guest]

Loose belt as first check, the fact the battery did not die completely suggest this as a possibility.

The alternator is an electro magnet, clip 2 paper clips together, turn on the master switch and alternator field switch, with the cowls off hold the paper clips near the drive pulley if the core is magnetized the paper clips will be drawn to the pulley.

If the core is not magnetized look for battery power at the alternator field terminal, if there is power at the terminal, check for good ground wire from the second field terminal to the alternaor case. If both wires are good, remove and check the brushes they may be worn. If the brushes and wires are good the problem may be within the alternator itself.

Clarence

[Tommy]

Ok I ran another test after examine the alyernator it seems like it's working.

So the voltage was ok initially at 14v then it dropped to 12.2v and the get was showing very slightly discharge then intermittently the voltage is all good again but the ametre showed 30amp charging rate.

Doesn't matter if I load the system it seems like it has a mind of its own.

Does anyone have any schematics for a presalite al-8420 so I can measure voltages and resistances correctly? Thanks

[jetdriven]

The field wire lots of times breaks right where it is crimped into the ring terminal that fastens on the alternator.  Thats happened 3 times in 500 hours.  Look at that.  Also, follow the POH guidance and reset the ALT circuit breaker when the voltage light comes on. If it restores electrical power you may have a regulator that is tripping off on high voltage.  Resetting the C/B resets the crowbar circuit.

[Tommy]

Another observation.

The voltage drops under load actually - I forgot that te landing light was disconnected 0.1 to 0.2 with strobe and nav light

[Will W]

Let me answer number 3. DO NOT FLY IT LIKE THIS!!!

You currently have an unairworthy airplane and flying it would be illegal. After flying it for so long you have clearly drained your battery and may need to trickle charge the battery. The alternator itself is clearly inoperative due to the reasons listed above. Do not fly until this gets fixed.

Ever see the speech that the Kings did for their crash? Sounds like you wanting to fly home was exactly what they tried to do... Except it didn't work so this is your black and white warning that most accidents don't get to have. You were lucky the battery had enough juice to get the gear down, you probably won't be able to power everything, takeoff, and get the gear up before you have a complete electrical failure due to the dead battery.

https://m.youtube.com/watch?v=RHKbeT-EZHM

Their story like yours starts at 19:50

the voltage drop is your lead acid battery slowly having a voltage drop as you are draining it. With your alternator working you will always see close to 14+/- .5 volts. Your battery is 12 volts...see where this is going?

[Will W]

Ok I ran another test after examine the alyernator it seems like it's working.

So the voltage was ok initially at 14v then it dropped to 12.2v and the get was showing very slightly discharge then intermittently the voltage is all good again but the ametre showed 30amp charging rate.

Doesn't matter if I load the system it seems like it has a mind of its own.

Does anyone have any schematics for a presalite al-8420 so I can measure voltages and resistances correctly? Thanks

This happened to me. The answer isn't a grounding wire since it does come on. You may have to pull off the alternator to see this but there are bolts that hold the alternator together and sometimes, the overhauled models are done by a cheap shop that over torques the bolts. Those bolts eventually crack and then the alternator literally splits in half. As it splits, you'll get intermittent charging as it's together then rattles apart again. The only solution in this case is a replacement alternator.

[Tommy]

Thanks for the advice!

I am having a local A&P to take a look now.

[Will W]

Tommy, I absolutely love your signature!

[Tommy]

Ok it looks like there is no voltage on the field wires. So it's most likely the voltage regulator!

Blut where exactly is the VR on 1981 J?

Under the glare shield on co-pilot side?

[Will W]

Ok it looks like there is no voltage on the field wires. So it's most likely the voltage regulator!

Blut where exactly is the VR on 1981 J?

Under the glare shield on co-pilot side?

A voltage regulator isn't going to go on an off like this, it won't regulate to 12 volts.  If it was your voltage regulator taking your alternator offline, it's because your alternator is spiking to a higher than ~14.8 volts.  Look at the alternator.

 

Maintenance Manual found Here

http://mooney.free.fr/Manuels%20M20J/M20J/Mooney%20M20J%20Illustrated%20Parts%20Catalog.pdf

 

Look on 39-40-01 number -13 for the Over Voltage Protection (If this is what you're thinking)

Look on 71-00-00 number -14 for the Voltage Regulator

[jetdriven]

We also recently changed the field wire on our plane. The fresh alternator OH still wouldnt charge. Investigation determined than there was no 12V on the field wire terminal. Squeezing different parts of the wire would intermittently show 12v on the terminal. Turns out, the wire is shielded, but the inner wire had so many cracks and a few bare spots, it would ground to the shield. The OECO voltage regulator on the early J's seems to tolerate this, but other V/Rs and alternators get damaged.

 

4 feet of 14 gauge shielded wire fixed it. The cannon plug can be disassembled at the firewall to re-solder the wire to the pins inside.

 

So, in short, new field wire, new PIDG crimp terminal with heat shrink extending over the wire and the terminal. I also added a new clamp at the alternator case (WDG2 size) to absorb the vibration in the field wire.

 

Pic shows the old crappy field wire, I replaced it with the newest stuff.

[Tommy]

My understanding is that if you have a regulator failure (assume it's open circuit) then no field current will be supplied to the alternator magnet and therefore no output will be generated. So the voltage bus will be the battery voltage.

 

The next step is for us to locate that VR on my J (I just don't want to oepn teh glareshield if it's not the right place to look) to see if it outputs anything. If it does then there is something wrong in the wiring, if it doesn't then it's likely failed.

 

Is my reasoning correct?

[tony]

Your reasoning is correct.  Put 12 volts on the field pin and the alternator outputs a voltage, take it away it stops outputting.  That's how it regulates. 

 

What Byron is trying to tell you is the voltage regulator might be fine and the wire going from the regulator to the field pin on the alternator might be broken; causing the intermittent operation you described.   He is suggesting you check that wire before you replace the VR.  That wire is small and is prone to breakage. 

[Tommy]

That's how I read it too. I reason if there is no output from VR at the VR's field terminal then the wires are potentially okay.

 

So I need to ge to that VR!  But where?