nightmoves

Instrument lighting issue - M20F

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Hello All.

I've had my plane for almost a year now and the instruments have never illuminated, even though I have the factory knob at the top/left that turns them on/dims. The plane has overhead LED lighting which works fine but I've had a few times flying longer stretches at night where I've thought having illuminated gauges would be easier than the broad lighting.

Anyway, are there any suggestions on where to start? I promise I did a lot of searching on the forum and couldn't find a specific answer. It appears there are some 5A fuses, somewhere, that could be the problem. Is that true, and if so where would one look for those? This is a 1969 F.

++ may or may not be related, but I had a Garmin 347 audio panel installed about 6 months ago and the backlighting does not illuminate. I am assuming the pins were properly done so that it would turn on lighting when the cabin lights were turned on. I'm hoping this is all related. The unit does briefly illuminate when it powers on during the self test ++

 

Thanks everybody.

Inst light.JPG

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1 hour ago, nightmoves said:

Hello All.

I've had my plane for almost a year now and the instruments have never illuminated, even though I have the factory knob at the top/left that turns them on/dims. The plane has overhead LED lighting which works fine but I've had a few times flying longer stretches at night where I've thought having illuminated gauges would be easier than the broad lighting.

Anyway, are there any suggestions on where to start? I promise I did a lot of searching on the forum and couldn't find a specific answer. It appears there are some 5A fuses, somewhere, that could be the problem. Is that true, and if so where would one look for those? This is a 1969 F.

++ may or may not be related, but I had a Garmin 347 audio panel installed about 6 months ago and the backlighting does not illuminate. I am assuming the pins were properly done so that it would turn on lighting when the cabin lights were turned on. I'm hoping this is all related. The unit does briefly illuminate when it powers on during the self test ++

 

Thanks everybody.

Inst light.JPG

I can't remember for sure, but I do recall some of those lights may have an inline fuse found under the panel. When I had my circuit breaker panel redone, we pull out a number of inline fuses and put them on breakers. Check under the panel for any of those barrel fuse holders.

Another thing to check is the connectors. On my F, there are black Amp (I think they are Amp versions) connectors that twist to open/close. Make sure there is no corrosion in them and they are completely latched.

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This is all from memory so be warned. There are two 5A miniature dipole fuses in the back of the light control box. They are underneath two individual screw cap protectors on the light control box (behind the dimmer/photo cell/press-to-test in your picture. Left hand side). Take the glare shield off. The screw cap tops will almost interfere with your (original) defrost strip. Very doubtful you can see the fuseable link when installed so pull them out to check through the "window" on top of the fuse. One protects the "dimmable" instrument light terminal strip. The other goes to cluster and overhead lights and others, IIRC. 

 From your description, It sounds as though one fuse is out. Buy yourself a handful of the industrial equivalent fuses for trouble shooting. There's a very good chance you have a short in the AC wiring (outside of the control box).If so, the fuse will blow just after the pot is cracked "on".  The AC approved fuses are expensive so troubleshoot with the cheap ones.  This will really test your patience. Don't ask me how I know.  Good Luck.

I can try and send more info if I get to the hangar tonight. Let me know if you need it.

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1 hour ago, Freemasm said:

This is all from memory so be warned. There are two 5A miniature dipole fuses in the back of that battery control box. They are underneath two individual screw cap protectors on the light control box (behind the dimmer/photo cell/press-to-test in your picture. Left hand side). Take the glare shield off. The screw cap tops will almost interfere with your (original) defrost strip. Very doubtful you can see the fuseable link when installed so pull them out to check through the "window" on top of the fuse. One protects the "dimmable" instrument light terminal strip. The other goes to cluster and overhead lights and others, IIRC. 

 From your description, It sounds as though one fuse is out. Buy yourself a handful of the industrial equivalent fuses for trouble shooting. There's a very good chance you have a short in the AC wiring (outside of the control box).If so, the fuse will blow just after the pot is cracked "on".  The AC approved fuses are expensive so troubleshoot with the cheap ones.  This will really test your patience. Don't ask me how I know.  Good Luck.

I can try and send more info if I get to the hangar tonight. Let me know if you need it.

Thanks for the replies thus far. My plan is to try and get to the hangar tomorrow afternoon to try and chase this down. Anything you have to add will be helpful. I do have 3 or 4 spare fuses that came with the plane when I purchased it. I was wondering what they were for, maybe this is it.

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In my 76F the instrument lighting rheostat failed.  It only lasted 41 years.  A&P installed a new one and all was good.

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Just now, kpaul said:

In my 76F the instrument lighting rheostat failed.  It only lasted 41 years.  A&P installed a new one and all was good.

At what point does urs start dimming. Mine is on 50%ish from the first click to the halfway point then begins getting brighter. 

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Just now, DualRatedFlyer said:

At what point does urs start dimming. Mine is on 50%ish from the first click to the halfway point then begins getting brighter. 

It dims throughout the entire range now.  Honestly I am not sure how the old rheostat behaved, I only had a couple of night flights before it gave up. 

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OK. I remembered it mostly correctly.  The safe bet is one of the fuses went bad. This is what you want. the light control units have two or three power transistors. If one goes bad (which would be rare as they're old solid state/rare earth type) it would get stupid expensive very quickly. Mine was so f'd up years back I spent a lot of time to learn and troubleshoot that box. Here's a couple of pix I hope will help. Followed by a link to discussing the fuse proper.

 

A side not to some other comments. They put a resistor in parallel with the Potentiometer/rheo and another circuit which controls current via the photocell input. These reasons and the component being older than dirt is probably why the adjustment is "non-linear."  Any EE's out there, please don't condemn my response if I termed anything wrong but I'm most positive on the functionality having lived with a lot of pain it induced. Best of luck.

 

Cheers Boys 

IMG_2691.thumb.jpg.ac2f026063e93b4b8cf6d56c94db70c4.jpg

IMG_2694.thumb.jpg.878644ad71d5a5172138230041f402f2.jpg

 

 

 

IMG_2691.jpg

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Please let us know what you find.  I have not been able to troubleshoot my lighting issue.  This thread has at least given me a place to start!

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@M20FTraveler,(or anybody else) if I can be of any help, just ask. I had to take mine out and test everything pin-to-pin a couple of times over the years to get it figured out and working.  Still have some notes/schematics as I didn't want to live that full pain again.   This assumes your’s is the same applicability or similar vintage to my 69C magical light control box. 

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