Generator Light Issue

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reecewallace
Posts: 165
Joined: Tue Jan 05, 2021 1:34 am

Generator Light Issue

Post by reecewallace »

Tonight in cruise flight, the generator indicator light on my panel turned red, indicating the generator was not working at cruise power which is usual.

However, my ammeter was still showing a positive deflection with most of my electrical system/lights on—this gave me conflicting messages.

I decided to land to be safe. As soon as I landed, the generator light turned off and all seemed to be OK.

I did a ground run and the generator light turned off around 1100 RPM as per usual.

What could be the culprit here?
- Faulty generator indicator light?
- Ground wire?
- Voltage regulator?
- Generator on the way out?

I know, I know, buy an alternator everyone will say...but I can't afford it right now.

Thoughts? I don't want to be flying around with a generator which isn't working properly and may discharge my entire system.
- Reece
1956 Cessna 170b
Nanaimo, BC Canada
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GAHorn
Posts: 20968
Joined: Fri Apr 12, 2002 8:45 pm

Re: Generator Light Issue

Post by GAHorn »

I am not a fan of alternator conversions…in most cases…since the advent of LED lighting and solid-state avionics. The generator system is more durable/tolerant of mistreatment, easier to fix at home if you know the system well, and a LOT less expensive.

From the Assoc’n Electrical Service Manual (written by an idiot, but the following paragraph might be helpful):

“ GENERATOR WARNING LIGHT. Cessna 170’s were not factory equipped with generator warning lights. However a few owners desired this feature and adapted it from other Cessna models. The following description of this system is provided for those aircraft so modified. The generator warning light is designed to indicate when the generator is not charging the battery. The light is electrically connected across the cutout relay contacts of the voltage regulator. Whenever the cutout contacts are open the light will measure the potential difference between the battery and the generator armature. When the battery voltage is in excess of the generator output (such as an idling condition) the light will come on. As the generator speed (output) is increased, the potential difference across the light will diminish and the light will grow dim. When the cutout relay connects the generator output to the battery the light circuit is bypassed through the voltage regulator and the light will go out.”
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons. ;)
voorheesh
Posts: 586
Joined: Tue Jan 04, 2005 5:22 am

Re: Generator Light Issue

Post by voorheesh »

“Tonight in cruise flight, the generator indicator light on my panel turned red, indicating the generator was not working at cruise power which is usual.

However, my ammeter was still showing a positive deflection with most of my electrical system/lights on—this gave me conflicting messages.

I decided to land to be safe. As soon as I landed, the generator light turned off and all seemed to be OK.

I did a ground run and the generator light turned off around 1100 RPM as per usual.

What could be the culprit here?
- Faulty generator indicator light?
- Ground wire?
- Voltage regulator?
- Generator on the way out?

I know, I know, buy an alternator everyone will say...but I can't afford it right now.

Thoughts? I don't want to be flying around with a generator which isn't working properly and may discharge my entire system.


You mention that the generator light is on in cruise “which is usual”. Does that mean the light is usually on during cruise in this airplane, or are you saying that the light being on usually indicates the generator is not working? If the light is usually on, but the battery is charging, then the problem is most likely in the voltage regulator (circuit that turns light on).

Based on George’s post, the light being on indicates generator voltage (output) is less than battery voltage by some set amount. So it seems possible that voltage difference might turn the light on even if the generator is still providing enough charge to show movement of the ammeter. Once on the ground, I would check battery voltage. If it’s dropped, I would suspect a problem on the generator side. Maybe start with condition of generator (brushes, etc). Otherwise, I would try and find some fault that is keeping generator voltage lower than it should be. Maybe a bad ground somewhere. Stuff like this can be a real pain. I recently was involved with a similar but intermittent problem(Cherokee 180). After much checking, the problem turned out to be an old, worn out master switch. Who would have figured? The master seemed to work just fine.
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n2582d
Posts: 2808
Joined: Mon Apr 29, 2002 4:58 am

Re: Generator Light Issue

Post by n2582d »

You'll find some useful troubleshooting information from Zeftronics. I'd check resistances to what is shown to be typical values on page 3.
Gary
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