Electrical Gremlin...

ARFlyer

ARFlyer

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So I’ve been trying to track down a electrical gremlin with no luck. The ammeter has been windshield wiping for awhile. The alternator failed a bench test and was replaced with a overhauled unit along with a new voltage regulator that had been fried by the alternator. However that didn’t fix the ammeter issue.

Engine off the ammeter shows proper discharge with everything turned on in the cockpit. However once you start the engine the ammeter starts bouncing around and won’t stop unless you turn the avionics bus off. I haven’t been able to isolate it to any one circuit on the bus. It seems to be something along the bus bar circuit.

Also the Aspen and 650 will show a GPS failure and a GDL 88 failure unless you cycle the power several times. But that might be a red herring.

Anyone have any ideas or a someway to better isolate the issue?
 
What happens if you pull all the breakers on the avionics bus before you switch on the avionics master? if you push each breaker in individually, does that show anything? Is the avionics bus powered up by a relay circuit?
 
Yep, if the breakers are not pullable, try turning off as much as possible to divide and conquer.

Is there a backup to the avionics contactor? (My plane has a secondary "always pulled" 25A breaker in parallel to it).
 
Chip Sylverne said:
What happens if you pull all the breakers on the avionics bus before you switch on the avionics master? if you push each breaker in individually, does that show anything? Is the avionics bus powered up by a relay circuit?
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flyingron said:
Yep, if the breakers are not pullable, try turning off as much as possible to divide and conquer.

Is there a backup to the avionics contactor? (My plane has a secondary "always pulled" 25A breaker in parallel to it).
Click to expand...

Tried that technique last night and I couldn’t narrow it down. Half the avionics are modern breakers and half are those horrible flush types.

JAWS said:
http://ronkilber.tripod.com/alternat/alternat.htm

Edit: The link above assumes a typical older Cessna single. The OP did not state what aircraft.
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It’s a 78 N model.
 
Bell206 said:
Does the system voltage fluctuate along with the ammeter indication?
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Nope it’s rock solid around 27/28 volts right where it’s suppose to be. The avionics guy thinks it might be a loose ground somewhere but said it will be like finding a needle in a hay stack.
 
ARFlyer said:
Nope it’s rock solid around 27/28 volts
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If it's still hunting between 27 and 28 in rhythm with the ammeter that can possibly point in one direction. Some older regulator systems share the same wire for both field voltage and voltage sensing. So if there is a build up of resistance (due to various reasons) in that circuit it can cause the charging system to "chase its own tail." But if the voltage is actually rock solid then the loose wire scenario makes more sense.
ARFlyer said:
Also the Aspen and 650 will show a GPS failure and a GDL 88 failure unless you cycle the power several times.
Click to expand...
ARFlyer said:
The avionics guy thinks it might be a loose ground somewhere
Click to expand...
To keep it simple, relatively speaking, start at the ammeter and work toward the buss. However, since you mention having to cycle power for your GPS perhaps look at the buss first. Don't know if your system has a remote shunt for the ammeter but in addition to connection security make sure the connector crimps are good and the wire braids are not starting to break. Unfortunately a conductor single strand can carry full voltage but not full current. And don't forget corroded connections, chafing, etc., you know the fun stuff. Get a copy of a wiring schematic for you S/N aircraft and chase each separate system connections using that as a guide and check off each on the diagram as you go. It may seem like a haystack but most haystacks don't come with a road map.;)
 
Google "Zeftronics+Cessna+troubleshooting". One of the first items should be a pdf, detailing how the system works and a troubleshooting guide.

A very good document to help find needles in haystacks.

I'd start at the connections: regulator plug, alternator, ground strap, etc. All these aircraft are getting older and older and all the pins and sockets are getting, well, older and older too. I know my pins and sockets are not what they used to be! :)

Ps Forgot to include the often overlooked master switch.
 
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Probably a dumb thought, but it doesn't pulse in rhythm with the tail beacon does it?
 
Update: With all the avionics off and breakers pulled it still windshield wipes. Now it just does it in a very fast motion within about a needles width back and forth. So I’m wondering if it’s really on the avionics bus or if the issue is the primary bus.
 
By all breakers pulled, do you include the primary bus breaker that feeds the avionics bus? What circuit might possibly include an auto-resetting breaker? When it wipes, what is the max amp draw before it resets?
 
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