Overhead lights problem

[Brett Kallish]

Hi folks,

I have a 67 m20C and I can turn the white overhead light on and off with no problem but when I turn the overhead knob all the way, for the red lights, the circuit breaker pops. I looked for frayed wires but still cannot find anything out of the ordinary. I also searched through this website but could not find anybody with a similar problem. Any ideas would be greatly appreciated.

Brett

[Andy95W]

Originally, that rheostat only drove 4 lights- 2 overhead red spotlights, the compass light, and the light over the pilot's left knee that shines on the fuel selector.

Over the years, as avionics shops and mechanics want to add more and more instrument back lighting, they keep splicing more and more lights onto that circuit, usually from a location near that light over the pilot's knee.

This ends up being a lot of current flowing through that rheostat which leads to its failure, and more and more places where a chafed wire could short to ground and pop the circuit breaker.

Finding the offending circuit means tracking down every single light, isolating it from the system, and see if that fixes the problem.  Good luck, you'll need it.

This is how I finally got mine working right:

 

[carusoam]

If the total wattage for all the lights being on doesn't exceed the the CB rating.... it is possible that a new CB of the same rating can be swapped in.

The landing light CB/switch is known to wear out and trip early.

If you desire to lighten the load, LED lights have a magical way of producing good light at lower wattage.

Best regards,

-a-

[Hector]

I converted the overhead torpedo lights to LED's. the knob used to get so hot you could hardly touch it.

[DXB]

4 hours ago, N1395W said:

Originally, that rheostat only drove 4 lights- 2 overhead red spotlights, the compass light, and the light over the pilot's left knee that shines on the fuel selector.

Over the years, as avionics shops and mechanics want to add more and more instrument back lighting, they keep splicing more and more lights onto that circuit, usually from a location near that light over the pilot's knee.

This ends up being a lot of current flowing through that rheostat which leads to its failure, and more and more places where a chafed wire could short to ground and pop the circuit breaker.

Finding the offending circuit means tracking down every single light, isolating it from the system, and see if that fixes the problem.  Good luck, you'll need it.

This is how I finally got mine working right:

 

Seems a little extreme to fix a dimmer 

[Brett Kallish]

Thanks for the suggestions, I am working through them but no luck so far but the journey continues. I did notice that when I turn up the rheostat to its highest level about 3 seconds later the wet compass starts to swing then the circuit pops and the compass returns to where it was.

Brett

[Andy95W]

Definitely sounds like a short to me.  Somewhere, most likely, a wire is chafed and touching a piece of metal.

[Yetti]

Could be overcharging the Flux capacitor. They will do that to the compass ya know

[Jerry 5TJ]

10 minutes ago, Yetti said:

Could be overcharging the Flux capacitor. They will do that to the compass ya know

Time to open the Flux Gate and let some out. 

[Lance Keve]

On 9/29/2016 at 11:34 PM, Hector said:

I converted the overhead torpedo lights to LED's. the knob used to get so hot you could hardly touch it.

Hi Hector - My overhead torpedo lights are also incredibly hot to the touch.  Curious how you switched to LED's.  Are they direct inserts that fit in the topedos?  Where did you source them from, etc.?

Thanks,

Lance

[cctsurf]

Search Function is marvelous...

Here's the link to a former post on the matter:  http://mooneyspace.com/topic/1396-led-overhead-instrument-lights

It's a long read through, but good information.

 

[Lance Keve]

4 minutes ago, cctsurf said:

Search Function is marvelous...

Here's the link to a former post on the matter:  http://mooneyspace.com/topic/1396-led-overhead-instrument-lights

It's a long read through, but good information.

 

Thanks.  Usually my first method and I swear I've read 90% of all historic posts by now.

[Hector]

Sent from my iPhone using Tapatalk

[Brett Kallish]

Well I finally pulled the overhead down and started to figure out which wires of the 6 up there was the problem. I ruled out 3 of them and the 3 left went along the roof up to the front. I started to disconnect them one by one and then see if the breaker pops. The first wire disconnected was the culprit. So now I have to figure where this wire leads to. The light at the pilots knee seems to work fine, there are still no lights for the instruments or the compass. The light on the compass is in the inside of it, my question is are the instrument lights on the inside of them and which ones are usually lit. Again thanks for everybodies help as you gave me some good ideas on how to search for this problem.

 

Brett

[Andy95W]

Okay, so I did a search for your N# and it looks like you own a 1967 model (SN 670012).  I've attached a photo that shows the overhead wiring from the aircraft electrical schematics.

From the diagram, three wires come off the output side of the overhead rheostat: the fuel selector light (which you say works) and the two overhead "torpedo" lights.  This is actually good news because those are relatively easy to get to and troubleshoot.  Trace the wires for whichever of those lights doesn't work, looking or a chafed wire that is touching metal.  That should be the culprit.

[Brett Kallish]

Andy,

Thanks for the schematic but I don't think I explained enough. The 2 torpedo lights work and the fuel selector light works. The wire that I disconnected that stopped the breaker from popping, was one of three from the rheostat, that went along the roof of the cabin forward to the front, somewhere. What I am trying to figure out what it connects to. The compass light does not seem to work but from the schematic it seems to be connected before the rheostat. The instrument lights do not seem to work so if it is that, are there single lights in each instrument , if so then I guess I would have to check the wiring under the panel that would power each instrument.

Brett

[Andy95W]

In that case I'm going to speculate that your 1967, since it was one of the first from that year, still used the previous setup.  I have seen this before with older Mooneys: the manuals say one thing but the actual parts installed could differ.

I think it is probably your compass light.  The wires that run to the compass are very small gauge wires and are easy to kink or chafe.  I'm going to guess that you do not have a 201-style windshield, so one of your first places to look will be the aluminum u-channel that runs down the center of your windshield.

[Brett Kallish]

Yea, I kind of figured that I was going to have to remove the u-channel. How careful do I have to be because it seems to be part of the windshield structure. Is the u-channel a part of the sandwiching of the windshield and is there some kind of sealant under the u-channel?

Brett

[Andy95W]

Sorry, Brett, I haven't had a u-channel since about 1995 when I installed a 201 windshield on my first M20C.  My current C already had the 201 mod when I got it.

I don't remember any sealant on the inside, only the outside of the airplane.

Can anyone out there help with this?