Electrical System

How to keep the Cessna 170 flying and airworthy.

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Electrical System

Postby N73087 » Thu Apr 22, 2004 10:50 am

I just re-read FAR 91-205, and I'm asking someone with wider knowledge to confirm what I think.
For day/night VFR, with a generator, there is no requirement for an ammeter.
I'm considering switching to a voltmeter.
Thanks.
Dave
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Postby gahorn » Thu Apr 22, 2004 2:47 pm

Keep in mind that all the electrical current generated and/or supplied by the battery for the main electrical buss passes through the ammeter via a shunt. Most voltmeters do not serve as a pass-through buss, but instead merely sample voltage as if an auxilliary circuit. So simply installing the voltmeter in the place of the ammeter will not work.
The simplest solution is to add a voltmeter such as one of the inexpensive models available from Spruce, to one of the small 2-1/4" locations on the panel.
If panel space is a problem, an elegant solution is to install a combination amp/volt gauge.
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Volt/Amp Meters

Postby N1277D » Thu Apr 22, 2004 3:58 pm

It is also my understanding that an amp meter is not necessary, but it is wise to have one. The 170 is configured so that the amp meter measures the charging rate of the battery and not the output of the generator. Failures in either the generator or regulator can result in an overcharged battery that a volt meter would be slow to detect, but would be easly identified by an amp meter. I would recommend that you keep the amp meter and add a volt meter if space allows. Some but not all amp meters use a shunt, I believe the orginal 170 amp meter does not use a shunt. (This was the case in my 170A).

If you have a high electrical load, dual amp meters would be useful too, one to measure the charging rate of the battery, the other measures the generator/alternator output current.

We also have a stinson 108 (1946) in which the amp meter only measures the output from the generator; with all the electrical stuff turned on (landing and nav lights, nav/com, transponder, strobes) the battery will dischage even though the amp meter is showing a positive output.
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Postby zero.one.victor » Thu Apr 22, 2004 4:05 pm

I have a Garmin GPS-3 hardwired into my 170 (via a half-amp CB). I set up one of the pages to have a "voltage" reading. If I'm in doubt of my power supply,I can punch up that page & check it out.

Eric
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Postby zero.one.victor » Thu Apr 22, 2004 5:42 pm

I just read 91.205, here's what I got re: required instrumentation:
For day VFR--flight instruments: ASI,altimeter,compass
engine instruments: tach,oil pressure/temp gages,fuel gage(s),
for night VFR: also position & anticollision lights (landing light if for hire),
"adequate source of electrical energy",spare fuses,
More stuff listed as required for IFR but no mention of ampmeter or voltmeter.

Eric
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Re: Volt/Amp Meters

Postby gahorn » Thu Apr 22, 2004 9:23 pm

N1277D wrote:It is also my understanding that an amp meter is not necessary, but it is wise to have one. The 170 is configured so that the amp meter measures the charging rate of the battery and not the output of the generator. Failures in either the generator or regulator can result in an overcharged battery that a volt meter would be slow to detect, but would be easly identified by an amp meter. I would recommend that you keep the amp meter and add a volt meter if space allows. Some but not all amp meters use a shunt, I believe the orginal 170 amp meter does not use a shunt. (This was the case in my 170A).

If you have a high electrical load, dual amp meters would be useful too, one to measure the charging rate of the battery, the other measures the generator/alternator output current.

We also have a stinson 108 (1946) in which the amp meter only measures the output from the generator; with all the electrical stuff turned on (landing and nav lights, nav/com, transponder, strobes) the battery will dischage even though the amp meter is showing a positive output.


The orginal 170 ammeter does indeed have an internal shunt. All ammeters are designed for either an internal, or external shunts. An ammeter without a shunt, or with an incorrectly/inadequately sized shunt will display the undesireable results such as a too-sensitive or too insensitive needle. One incorrectly installed may display like the mentioned Stinson, but more likely that battery has an internal short due to old age or rough handling, or it's regulator is maladjusted.
An ammeter will also show "no charge" if it's insensitive, or oversized for the circuit, such as a 60 amp meter in a 20 amp generator circuit. Your ammeter should be matched as nearly as possible to your maximum potential electrical load rather than your gen/alt capacity.
A voltmeter will show, say 16 volts, when your battery is boiling over from excess current due to a faulty regulator, while an ammeter may indicate only half-a-needle's width on the plus side. Not much warning from an ammeter. The most likely sympotom will be a "sulphur" smell like rotten eggs, consitently low battery water, or dripping acid from the battery overflow vent. (You DO check that yours is clear and free of insect nests dont' you? Pour some water into the battery box to check it. Better yet, pour some water into which a teaspoon of baking soda has been dissolved.)
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Postby zero.one.victor » Fri Apr 23, 2004 5:55 am

Quite often when flying with a certain friend (?) of mine, I notice a smell like rotten eggs. But I don't think it has anything to do with the battery.......
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Postby Harold Holiman » Fri Apr 23, 2004 6:15 am

I have an automotive type voltmeter (the type that plugs in the lighter plug). I usually keep it plugged in just to monitor the voltage. It is not mounted and can be removed at any time and indicates just the same as a mounted unit would.

Harold
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Postby N1478D » Wed Apr 28, 2004 4:33 pm

I was not able to find a replacement for the original 12 amp regulator. So, bought a 20 amp regulator and am in the process of trying to adjust it down to 12 amps so the generator does not overcharge the battery.

My question is this: In the mean time till the proper adjustment is reached, if during flight, more and more electrical load is added such as rotating beacon, nav lights, landing light, etc. till the volt meter reading lowers to about 14 - 15, instead of 15 -16, is this doing the job temporarily till the regulator is properly adjusted? Or, am I just fooling myself and the battery is still receiving too much juice?
Joe
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Postby gahorn » Wed Apr 28, 2004 10:56 pm

Joe, if you have a 12 Amp gen, and you install a 20A regulator, you stand the chance of burning up your generator because the regulator will keep demanding more and more output (especially if you keep turning on more and more appliances.) You are setting yourself up for an expensive repair.
You can safely install a smaller regulator on a higher rated generator, but not the reverse as you have done. (The lower rated reg will only request the most it's rated for, and the higher rated gen it's attached to will be loafing.)
Are you certain you have a 12 A gen? Those are getting more and more rare to acually see these days as most folks have upgraded to one of higher amps over the years.
I'm off on a trip today, but if you talk to AutoZone, (as opposed to their online store) I think they may have a 12-15 amp regulator they can order for you cheaply that is an exact replacement (except that it's only legal for the ground-testing that I know you'll restrict yourself to with it.)
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Postby zero.one.victor » Thu Apr 29, 2004 5:38 am

Joe,if you do indeed have a 12-amp generator,maybe you should consider upgrading to a bigger one,like maybe the 35 amp. I would think that one of those (and a regulator to go with it) would be easy to find,from someone who went the alternator route. I seem to recall someone on the board having one he wanted to get rid of,not too long ago.
From what I remember,though,you have to have a dampened (counter-balanced) engine -- one with a "D" in the serial number -- to run the 35 amp generator.
Then again,I imagine the 12 amp generator is smaller & lighter,so as long as it meets your electrical needs, maybe it is the best choice after all!

Eric
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Postby N1478D » Thu Apr 29, 2004 1:50 pm

Eric,

Yes, I have the 1101876 generator which I believe is the 12 amp version. Randall was explaining to me that both the 12 and 20 amp models are shorter than the higher amp models, which probably does mean they are lighter. I have been very happy with the performance, back when I wasn't having any problems anyway:lol: , so would prefer to solve the regulator problem and keep the system the way it is to save weight and have more room in the engine compartment.
Joe
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Postby zero.one.victor » Thu Apr 29, 2004 3:49 pm

Sound thinking,Joe.
I looked in my TCM parts book,the only generator listed is the 20 amp,p/n 534111 (comlete gen assembly is p/n 534208). 20 amp regulator is shown as p/n 534112.
Maybe someone on the board has upgraded,and has some 12 amp stuff in the spare parts bin?

Eric
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Postby N1478D » Fri Apr 30, 2004 3:52 pm

Since Delco-Remy no longer produces the mechanical regulators, what is the best way to get ahold of their service bulletins? If someone has the Delco-Remy Service Bulletins 1R-186, 1R-117, & 1R-188A, could they be made accessible thru our web site? These bulletins have the procedures and specs for air gaps, point gaps, etc.
Joe
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Postby gahorn » Sat May 01, 2004 3:25 am

Joe, the Delco-Remy 1101876 is indeed the 12 Amp generator. You should obtain a Delco-Remy 1118383 regulator which is the 12 Amp model, should you wish to stay with it. (Or the ElectroDelta VR300-14-12 is the current replacement. Try Aviall if you can't get it elsewhere.)
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