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Panel light dimming affects gauge indication


wpbarnar

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When I turn on the instrument panel lights,  several of the gauges in the cluster panel will decrease in indicated value.  There is a noticeable jump or needle flicker when the lights are first turned on.  Increasing the light brightness will  decrease the gauge indication a little more.  Total needle movement is probably less than 2 needle widths.   Although the movement is minor,  it definitely correlates with panel light brightness.   Increase the brightness and the needle moves down.  Dim or turn the lights off and the needles return to the earlier position. The fuel gauges have the most  noticeable moment.

The lighting loads are controlled by a set of transistors mounted in the back.   The rheostats in the panel control their output.  As the affected instruments basically just read resistance,  I am at a loss trying to understand how one impacts the other and where to look.

The rheostat for glare shield lights does not cause any change.

Bill

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Exactly the same thing happens with our '79 J. I emailed our avionics shop last week about it.

Easy way to excite everyone by dialing the fuel guages to zero.

I impacts our fuel gauges, fuel pressure and oil pressure.


Sent from my iPhone using Tapatalk

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Sounds a lot like a grounding problem... electrons not being drained fast enough...

There are many grounding straps and challenges that come and go over the decades...

See if your panel is still grounded... the bolts that support it are typically inside a rubber shock adsorbing mount... not very good at self grounding...

PP thoughts only, not a mechanic...

Best regards,

-a-

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Best guess is that the resistance to ground has increased somewhere. Panel lights inside the cluster and the gauges appear to use the same return to ground according to my schematic.  If there is resistance in the shared ground, turning up the lights will cause a voltage increase on the ground side of the gauges and decrease their indication.

The ground is a wire from the connector on the cluster to a ground plug. Problem could be there or inside the cluster. 

Edited by PT20J
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Look up cleaners for electrical components... a common challenge is small amounts of corrosion on connectors and connections...

If the challenge is affecting a few devices at the same time... focus on the grounds that are common to all of them...

PP thoughts only, not a mechanic..

Best regards,

-a-

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The problems described by @wpbarnar and @pwhicks are: (1) indicative of old wiring, which means high potential for poor contact due to oxidation, poor/loose connections, and may indicate wiring mistakes; (2) tricky to diagnose; and (3) a symptom that your aircraft's wiring  is not working correctly should be looked at before flight.  

When we re-did the panel in our '67C last year, we replaced all the breakers and re-did all the electrical connections.   That cleared up some strange behaviors.  We did install an engine monitor which allowed us to simplify the wiring some.

The anti-vibration lugs used to hold the pilot's side panel on have a tendency to sag over time, which may cause grounding straps to fail.

Lots can go wrong.  Best to get it looked at.

By analogy, your aircraft's nervous system is suffering from a serious malady, and nothing good can ever come from it.   It doesn't get better on its own- it can only get worse.   Get it looked at ASAP.

My PPSEL non A&P advice is worth the bits it is encoded in.  B)

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Problem resolved!

As most suspected it was a grounding problem.   Grounding  problems can be like looking for the needle in a haystack.  Thankfully @PT20J comments pointed me in the right direction.    

The SigmaTek cluster gauges  use the case for lighting ground.  The problem turned out to be a poor ground between the cluster gauge case and instrument panel.

@pwhicks you most likely  have the same problem.   The cluster gauge case has a hard paint coating and the electrical  contact area between it and panel is small.  Be meticulous and clean the back of the aluminum instrument panel  and the cluster gauge case face and you should be good to go.

Bill

 

 

 

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