Electrical problem in M20J

[Ario]

Gentlemen,

 

We have had the HighLowVolt light on the annunciator panel flashing transiently in flight in the past few days. This is the sequence of events:

 

1. The ammeter showed negative (discharging) on the ground even at 2000rpm. The output of the Volt.Reg. was very low. A new Volt.Reg. didn't solve the problem.

 

2. An (assumed) wrong grounding wire was removed under the dash and the voltage to the alternator became 9v and the ammeter started showing about +35. We thought the problem was fixed.

 

3. During the first couple of hours of flying, the ammeter slowly reduced to (and stayed at) about +15, and the HighLowVolt light transiently flashed during flight. We also noticed the lights on the autopilot (the HDG) was very dim during the flight. But it was back to normal when close to the ground for landing. It is normal on the ground.

 

4. We shutdown the engine on the ground and started again and ran the engine at different configurations, on the ground, including cruise config. No HighLowVolt flashing or dimming was observed.

 

5. We did another flight. On the ground everything was ok. At around 700 feet AGL, the HighLowVolt started showing transient flashing again. The flashing happens for about a couple of seconds, then it's off, and after about 10 seconds, it shows again. There is no exact timing of the flashing though. But it happens at altitude only.

 

The only difference to me seems the air pressure at altitude that is different from the ground operation.

Here is the list of what I think may be wrong. 

 

1. The battery is broken. Does the air pressure affect the battery?

 

2. The alternator is broken? Can it show the problem in low pressure air only?

 

3. The connections of the battery need cleaning/replacement? Why is it only affected at altitude?

 

Any other thoughts?

 

p.s. I'm going to connect a multimeter to the cigarette lighter in the next flight tomorrow and observe the voltage during the whole operation.

 

[Tommy]

It sounds like your have got an intermittent charging issue. When you are in the air, there will be more vibrations that exacerbate any loose / broken wire /brushings etc etc You need to trouble shoot this in a methodical way, starting from the alternator in the front and work your way back. A good guide will be Mike Busch's excellent EAA Webinar on troubleshooting electrical problem. Follow his instruction and you should be able to get to the problem eventually!

[Andy95W]

My suggestions: start easy and cheap first.

1.) clean all contacts, beginning with the battery

2.) borrow somebody else's battery and see if that fixes the problem

3.) run a new field wire from voltage regulator to the alternator (this is my guess as culprit)

4.) new master relay

5.) overhaul alternator

Strangest charging problems I ever saw were caused by a bad master relay. The relay made contact, but the contractor was so old and dirty it intermittently prevented full current flowing through the system.

Good luck.

[N201MKTurbo]

You never mentioned what the system voltage is. Do you have anything that displays the voltage? If not you need to put a volt meter on it.

It sounds like you may have a bad alternator and the wire change you made bypassed the regulator and put full field current to the alternator.

Hard to tell for sure without more troubleshooting.

[Marauder]

It really sounds like an alternator problem. When my plane is running at idle speeds (1000 RPMs) with the avionics running, I am not seeing a negative ammeter indication.

Sent from my iPad using Tapatalk

[OR75]

From all I have seen (not saying it is always the case), alternators are either dead or working normally. They rarely work intermittently .

I would check the alternator belt tension( you don't want it too tight or too loose)

[Andy95W]

From all I have seen (not saying it is always the case), alternators are either dead or working normally. They rarely work intermittently .

And if the field wire is making intermittent contact, then the alternator is turning off and on, but working correctly.

Field wires live a hard life with heat and vibration and are overlooked until they begin to break down.

30 minutes of labor and $2.00 in parts.

[Ario]

Problem solved.

 

The battery was faulty. We replaced it with a new one and everything became normal. No HighLowVolt flashing anymore, the bus voltage stayed at 13.95v, ammeter returned to near zero on positive side, and no low fuel lights came on at turns anymore.

 

Thanks to Clarence for his help.

 

Also thanks for the thoughts here.

[1964-M20E]

I had a similar problem on 5 mile final Sunday low bus voltage and ammeter showing discharge have a mx looking at it once on taxi to the ramp voltage returned to normal.

 

Gremlins???

[ArtVandelay]

I had a similar problem on 5 mile final Sunday low bus voltage and ammeter showing discharge have a mx looking at it once on taxi to the ramp voltage returned to normal.

Gremlins???

Reset the Alt field CB, if it fixes the problem you may have a bad circuit breaker.

[jetdriven]

Or a bad voltage regulator. Resisting the field resets the regulator, such as when a transient over voltage trips the crowbar circuit

[Andy95W]

I had a similar problem on 5 mile final Sunday low bus voltage and ammeter showing discharge have a mx looking at it once on taxi to the ramp voltage returned to normal.

 

Gremlins???

I had exactly the same thing 20 years ago on my first M20C. Bill Wheat at the factory helped me fix it.

Any chance you have electric gear and you also have an alternator conversion?

[Rik]

Afraid to know, but what's the cost for a battery in a Mooney?

[1964-M20E]

It turned out to be a loose field wire on the alternator.  The local MX at KEVV fixed it.  I'm not sure the cost yet have not been back to airport.

 

Oh yes I have manual gear so my gear extension was normal no problems with low bus voltage.

 

If I had more time I would have looked at it myself.

[Guest]

Afraid to know, but what's the cost for a battery in a Mooney?

Almost priceless if you're stranded.

Clarence

[yvesg]

Did anyone ever seen someone successfully swing the prop on a Mooney because the battery was dead?

Yves

[N201MKTurbo]

I hand propped my old F many times. I have tried with my J and have not been successful.

[DrBill]

Afraid to know, but what's the cost for a battery in a Mooney?

http://www.aircraftspruce.com/catalog/elpages/rg_25.php

$209

[N9201A]

Hand propping is DEFINITELY not recommended, and all of the usual caveats apply, but I MAY know someone who knows someone who MAY have seen someone hand prop their J with a dead battery.  This only occurred -- if indeed it DID occur -- with a qualified pilot in the left seat holding brakes with chocks in place, and an experienced, careful and agile pilot doing the hand-propping...so I heard, anyway.

[N9201A]

BTW Yves, the guy who told me the story is in the Caravan...ask me about it apres une biere ou deux.

[1964-M20E]

No need to hand prop you need to do is push start it.

[PTK]

That's why most runways are built with a slight downhill slope!

[MB65E]

I hate electrical systems!!! All the failure points of the system produce the same indications!

Lol.

-Matt