PA38 low voltage light

[mad_jock]

The saga started 4 months ago with a new alternator.

Low voltage light began to flicker on and off. Ammeter was still reading normally

Regulator was replaced no change.

On the last annual low voltage light was replaced and new battery was installed (non gel)

Now the bloody thing is on all the time.

At all times the Ammeter shows current, jumps up etc if the nav lights are turned on. No dead bat episodes in the morning or radios dropping out etc.

Anyone else experenced this?

I haven't a clue how a low voltage system is set up.

Is it a wheatstone bridge and we need to tune it for zero difference?.

I might add its on a Public CoA and the engineers that look after it give sterling service. But they are flumouxed as well.

MJ

[rustle]

Check the link in the other "warning light" thread - it seems to go into a lot of detail about how the LV light should work and be connected etc...

[Vizsla]

I hope the "head scratching" engineers are not billing you.

[mad_jock]

H'mm bit of a touchy subject with the boss at the moment.
As you can imagine a new voltage regulator,battery and LV system ain't cheap

Had a read of the link but it doesn't go into enough detail.

Thanks to the ppruner that PM me who suggested that I take the bulb out. But it doesn't really fix the problem.

If the low voltage system detects current flowing out of the battery maybe its picking up a residual eddy. So a small resistor in series will sort that out.

To be honest after my fights with various electrical problems with cars I can understand the problems the Engineers are having.

I have tried swearing at it in Doric, English and German, unfortunately it didn't work.

MJ

[A and C]

The first thing I would look at is the alternator bearings , if the rotor shaft is moving about on worn bearings the brushes wont make a good contact hence the flickering lo volts light. cost to fix two hours labour plus £ 300 for the alternator

If the shaft is good then check the brushes are in good order ,if you are lucky it will just be a brush change ( one hour of labour plus about £ 10 for the brushes)

I have a charging system analyzer that picks up most problems with the alternator or regulator system first time and with about 10 min work.

[mad_jock]

Its a brand new alternator from factory

Shall i try to get the Engineers to swap it out?


MJ

[Monocock]

Sounds to me like a similar problem I had with a 172 I used to own.

One new alternator was installed followed by a new battery, followed by a new warning light and the problem was still there.

The engineer removed the cowling, scratched his head for a few minutes and checked the wires that go through the firewall to discover a chafed wire.

Problem solved.

Hope this helps.

[Circuit Basher]

Mad Jock - I'm not an aircraft wiring expert, but I am an electronics engineer and have done a lot of electrical work on cars / motor bikes. I understand an aircraft wiring system to be closely related to an automotive one... so I've written this as if it were an automotive system. I may be wooofing up the incorrect arbour. ;)

The Low Volts light will be on the regulator output to ensure that regulator is in 'charge' mode when the engine is running, as per the ignition light in a car. First test I would do to find out what's going on is to measure the battery voltage without the fan running and then with the fan running. I would expect 12V off line and 13.8V when charging. Turn all the electrics you can think of on - I would not expect the battery voltage to drop below around 13.3V with the engine running. If it does drop below that, then there is a fault in your charging circuit. I took your comment:
At all times the Ammeter shows current, jumps up etc if the nav lights are turned on to mean that the ammeter is showing a discharge current, which increases with electrical load.

The symptoms of this would typically suggest a duff alternator / regulator, which have already been tried. One option may be to take the aeroplane to your nearest Lucas dealer / auto electrician and get them to give it a once over.... ;) As you have a new one of each of these and ssuming that each is serviceable with good brushes, etc (is there a similar system that they can be put on to prove / deny servicability??), then in order of likelihood, I'd vote for the following:
Poor connection between aircraft wiring and alternator / regulator
As suggested by Monocock, chafed wiring in the loom
Alternator field connection intermittent - check crimps, etc (if fitted)
It's broke!
Hope this is of some help.

I would offer to fly up from Perth to offer moral support to your engineers, but our a/c is on yet another extended C of A / Annual and we aren't really sure when she'll be fit to fly again. Seriously, email / PM me if you want a contact number so that we can talk it through.

[mad_jock]

Thanks for that CB.

I think I will have a dabble with a volt meter next time its down for a service (under supervision of course).

And if it dosn't work I will definately PM you.

Failing that 2 of our planes will be down at the PFA flyin on the 29th in Perth. And if its still not working by then, bring your tool kit :D

MJ

[Keef]

If you have a reasonably accurate voltmeter, connect it to a convenient source (the cigar lighter socket would do if you have one), and observe the readings with engine off, engine running, various loads turned on (as CB suggests). 13.8 is a good number for "engine running, no load." If it's much lower than that, I'd suspect regulator and/or alternator.

If the LV warning device is based on a zener and a lamp, it's possible that the thing itself is faulty.

Certainly something to investigate.

[Circuit Basher]

Mad Jock - check your PMs for a response in a bit more detail. I was searching for a PA38 wiring diagram on the Web, but the bu$$ers want to sting me over $200 for the privilege, so I went looking and found a Cessna 140 diagram (well, it isn't going to be that much different!).

Cessna 140 Wiring Diagram - see Page 3 (http://www.cessna120-140.org/Library/337_Generator.pdf)

I would check that the alternator field winding is powered and that all the cables are OK. Other than that, I've PMed a Web link for a UK second hand instruments supplier who has a couple of Low Voltage Warning Lights for £60+VAT +P&P each.

Seriously, if you need help, drop me an email or PM - I'm afaraid I didn't understand the reference to PFA flyin on the 29th in Perth - which month?, as Jun is the closest where 29 is at a weekend and neither the PFA or Scottish Aero Club Web sites show a fly in at Perth that day.

I do come up to Arbroath (the gliding school) occasionally, so I could always give you a shout if I'm going that wey.

[mad_jock]

Sorry i got my dates wrong its the 24th next month. I think its a Scottish aero club event.

http://www.scottishaeroclub.org.uk/sac/flyin2003.html

I have had a good chat with the engineers who have been very helpful and are organising and spare everything for the next service.

The plane is not going to win this time!!!!

MJ

[mad_jock]

A wee update.

Plane went down for service, engineers let me loose with a voltmeter. Concluded that the bus was running at 13.7V so boosted it back up to over 14V. I didn't think this was going to solve anything because if a voltage regulator is altering its set voltage over night once it's going to do it again.

I was right it was on again the next morning. I might add it was fine on the positioning flight back that day.

Engineers were as pissed off as we were so 2 days later one of them turned up with so he thought everything.

Armed with a circuit diagram we started checking every wire for continuity and earthing. Wires OK.
Engineer looses plot and starts swapping out everything.

New alternator
New overvolt regulator
New Low Voltage light
New capacitor on the back of alternator.
New regulator.

On tracing through the circuits we found that a 50ohm resistor linking the alt warning light to earth was dead. The engineer hadn't brought one of them.

Thankyou to NATS tels engineers who did have one in there jar of resistors.

The end of the story is that I have done 6 hours in it today and the light hasn't come on once.

We still don't have a clue what actually solved the problem.

In the end we had

3* alternators
2* voltage regulators
3* low voltage lights
1* over voltage regulators
1* cap
1* 50 ohm resistor

I hate electrical problems

MJ

PS If anyone wants a fax with the wiring diagram of the PA38 send me a PM