The International Cessna 170 Association

I had the altimeter rebuilt and while I had the front panel out I decided to rewire the master light. I’m having trouble with the wire running to the oil pressure gauge. When I jumper an alligator clip ground the light works when I remove it it will not illuminate when the master is switched on. I’ve cleaned the surface of the post at the oil gauge and I know the new wire when grounded correctly makes the light illuminate. Can any tell me how that post connection on the oil gauge works. I don’t have me 170 electrical book nearby.

Happy new year and thanks!

It sounds (reads) as if you have a different gauge than original. Do you have an automotive gauge with internal lighting? If so, is your panel adequately grounded?>

Looks original but I’d have to go look for a P/N to be sure, problem is that it was working fine before I updated the wire, I just thought that since I had to pull the master light and disconnect the wires that it was an easy “preventative maintenance” action...didn’t intend to make more work for myself! I’ll have to dig around some more and check grounding paths...

I'm curious as to what exactly you are referring when you mention "master" light?

I talking about the big red diamond bulb at the 6 o'clock position on the instrument cover right below the DG. When the master switch is on it illuminates. I thought it went out when the volt meter went to charge, but i was a little taken back when i saw the one post of the light went to the oil pressure gauge. Now that i look at the IPC for the '50 A model, I can't find it anywhere.

I guess the 100 series manual refers to it as the Generator Warning Light after further review

That "Master Light" is not original, which is why George asked. I assumed you were referring to a low voltage light which would light when your master is on and no voltage was being made. One might think of it as a "Master light" but when it comes on and your flying you better realize your generator or alternator just quit. BTW a low voltage light is also not original.

Could this warning light been a option, I “had” exactly the same originally. Same position etc etc. I replaced all the wiring due to type and age and a EI volt/amp gauge was fitted . This warning light used to bug me as it came on at idle and got my attention on approach . Now my new alternator puts out a healthy charge regardless of engine speed.

The problem with warning lights is.... the lack of a light... doesn't mean A-OK. It only means the light isn't illuminated for some silly reason. (burned out lamp, broken wire, burned-up regulator, etc.) I once ferried a 175 from Ardmore to Dallas Love and lost radios when approaching Love because the generator/regulator burned up and the lamp didn't detect that sort of thing. The only light that was useful... was the green light from the control-tower cab.

Voltmeters and Ammeters are a lot more useful.

The low voltage light was never an option in the 170. However at some time I believe it became common in newer aircraft and it is easily retrofitted

Well, I guess I won't worry about it any more , other than to leave it there so there's not a big 1" hole in my panel. Looking through the 100 series manual, someone must have adapted the early 172 set up at some point in the life of the airplane...probably the same time that altimeter that had the Mooney label installed (and the reason i had it removed and overhauled). Thanks everyone!

My "master light" was always just to leave the anti-collision lights on. It's hard to walk away from an airplane with them running.

rupertjl wrote:Well, I guess I won't worry about it any more , other than to leave it there so there's not a big 1" hole in my panel. Looking through the 100 series manual, someone must have adapted the early 172 set up at some point in the life of the airplane...probably the same time that altimeter that had the Mooney label installed (and the reason i had it removed and overhauled). Thanks everyone!

A 1" vacuum gauge could be put in the hole.

Richard, if i do that, then I'll have a hole to replace my larger vacuum gauge, which i could probably use to put the remote head unit of the ADS-B system i bought, which will require the addition of the the GPS antenna and more wiring change. Which will ultimately lead to a full panel rework- and heck -while I"m at it might as well throw that 180 HP engine on the front that i have for it...

I think i'll just leave it alone for right now!

The generator system on the 172 trainer I bought has been converted to the Jasco alternator.... but the idiot light doesn't work any more. I've got to get under that panel and see why (but the center-mounted avionics are not friendly for that as I'll have to remove the box-frames todo that. grrr

If the lamp is installed.... IT MUST BE OPERATIONAL....according to FARs. The Jasco install instructs I've been able to find do not mention it's removal so I'm guessing it should work. Anyone with a Jasco in a later idiot-light-equipped airplane have any input on this?

rupertjl wrote:Richard,
I think i'll just leave it alone for right now!

I hear you, the age-old question, Where do you stop? I like flying better than working so I know what I would do. Good choice!