172M panel light question

Jdm

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Jdm
Does anyone know if the 172M engine and fuel gauges are internally illuminated? The POH says that it is supposed to be, and that it’s controlled by a rheostat.
The maintenance manual however does not address it. The wiring schematic doesn’t show any wiring that connects these instruments to a rheostat, or any other source for lighting.
Looking at the back of the gauges I see absolutely nothing that indicates a way to power internal lighting, or that any internal lighting even exists.
If anyone has this same cluster (see pic) I’d like to know if yours has an internal lighting or not.
Thanks.
 

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The wiring schematic doesn’t show any wiring that connects these instruments to a rheostat, or any other source for lighting.
FYI: most instrument lighting has its own schematic separate from the system indicator schematic. In most Cessnas I believe you'll see it grouped with the compass/radio lighting diagrams. And if I recall the lighting diagram only refers to the "instrument cluster" vs each individual indicator.
 
The 172M has two lighting circuits, with a tandem potentiometer and two knobs. One controls the internal radio lighting and the other runs the overhead panel floodlight and a tiny light inside the magnetic compass.
 
FYI: most instrument lighting has its own schematic separate from the system indicator schematic. In most Cessnas I believe you'll see it grouped with the compass/radio lighting diagrams. And if I recall the lighting diagram only refers to the "instrument cluster" vs each individual indicator.

Yep, you’re right, 2 of the 3 compass/instrument diagrams shows a cluster. One of the 3 diagrams doesn’t show a cluster. Pretty sure my s/n doesn’t have it.
With there being no place for a wiring connection behind the cluster I’m guessing this one doesn’t have internal lighting?
 
The 172M has two lighting circuits, with a tandem potentiometer and two knobs. One controls the internal radio lighting and the other runs the overhead panel floodlight and a tiny light inside the magnetic compass.

Yep, I’m familiar with the double knob circuit. Just not sure if it included the cluster originally. The original compass was swapped out with new internal lighting connected directly to the rheostat that your describing. The dome circuit seems untouched.
The panel lighting has been changed to ring lights with it’s own new rheostat.
 
If anyone happens to remember how or where the lighting circuit actually connects to the cluster that would be helpful. Just not seeing anything.

Thanks.
 
Thanks, that’s the manual I have actually.
 
From that manual:

upload_2021-8-31_10-5-43.png

That little Note 7 says that the cluster lights are applicable to the FR172 only. Those are the 172s made in France. Won't see many of those here.
 
Does anyone know if the 172M engine and fuel gauges are internally illuminated? The POH says that it is supposed to be

Are you sure you have the right POH for the correct model year?
 
Are you sure you have the right POH for the correct model year?
Good question. Some folks use more generic POHs from aviation publishers rather than the manufacturers.
 
That little Note 7 says that the cluster lights are applicable to the FR172 only. Those are the 172s made in France. Won't see many of those here.


Yep, I saw that one, and it makes sense, but it’s gotten a bit confusing in regards to applicability with there being 3 compass & instrument light diagrams.
 
Are you sure you have the right POH for the correct model year?

Yep, that is a great point. I’ll compare the current POH to the original. Thanks
 
Looking at several other diagrams it appears your S/N and even later models did not have internal cluster lighting. There was a post light option that would put an independent light on the cluster and was noted on some later model diagrams. And given there was no AFM requirement for your model its possible as mentioned earlier your POH may offer generic info for many different variants.
 
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FAA TCDS:
17263459 through 17265684 (1975 model)
17265685 through 17267584 (1976 model)


You have a 1975 model. The 1976 model is the one that has integral lighting on the fuel and engine gauges. You have the wrong POH.
 
Looking at several other diagrams it appears your S/N and even later models did not have internal cluster lighting.

Alright then, that adds up with my research and what I’m seeing behind the panel. Thanks for taking a look it. Greatly appreciated.
 
FAA TCDS:
17263459 through 17265684 (1975 model)
17265685 through 17267584 (1976 model)


You have a 1975 model. The 1976 model is the one that has integral lighting on the fuel and engine gauges. You have the wrong POH.

There’s absolutely no doubt about it being a 1975. All paperwork shows it as such. I’ll definitely take another look at the one in the airplane when I return. As far as I know it’s the only POH available for this model.
 
As far as I know it’s the only POH available for this model.

By pure coincidence I happen to have a 1975 POH and a 1976 POH on my bookshelf, they are completely different.
 
By pure coincidence I happen to have a 1975 POH and a 1976 POH on my bookshelf, they are completely different.

Great information! I’m looking forward to checking this when at the airplane next time. Thank you much.
 
By pure coincidence I happen to have a 1975 POH and a 1976 POH on my bookshelf, they are completely different.
Didn't the POH for '76 get real thick? To keep Cessna's butts covered. Got to warn about any and all possible risks.
 
Ok. The POH in the airplane is correct and is a 1975M version. The one with the incorrect lighting info is also for an M but is a copy that someone had.
Hate it doesn’t have the internal lighting, but thankful it all adds up with all the other information. I was already behind the panel for other reasons and really wanted to get to the bottom of this one while in the area.
Lessoned learned. Thanks for the great help!
 
Didn't the POH for '76 get real thick? To keep Cessna's butts covered. Got to warn about any and all possible risks.
It got thicker in 76 because the industry went to a GAMA format with a lot more data... a good thing!

Cessna originally used Leigh gages with internal lights that snapped in from the back. Leigh went bankrupt due to ELTs with lithium batteries that kept catching on fire. So Cessna switched to Stewart Warner mid model year... no lighting.

Paul
 
Thanks! The panel is back together and doing well now. Needed to clean up a few simple wiring/routing issues. It was a great time to get to the bottom of this lighting concern. Amazing how much a couple of days attention can help when straightening up behind an old panel. Also a great time to inspect during annual for any problems.
 
We'll, not during the production run for the subject 1976 aircraft, in any event. The Rochester retrofits came along around 1984 IIRC.
Yup. Cessna had several SBs on various gauge and sender replacements for the earlier models, as the old SW stuff failed. The systems were not compatible, and if an SW sender was replaced with Rochester, the gauge had to be replaced as well.
 
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