We’ve been scratching our heads over this one. On startup, both volts and amps are normal. After some period of time (it varies), amps go to zero and volts drops to battery voltage. By the time we get the airplane in the hangar and uncowled, all works fine again! We’ve replaced the alternator, the alternator electrical plug and pins, replaced the alternator belt, replaced the switch. We have run it for what seems forever with VOM hooked to alternator to see if alternator is actually losing field voltage and we cannot get it to fail. It’s been suggested that we change the shunt before we have to start deep diving the wiring. I did clean both terminals on the shunt to D-180 wiring and the wiring appeared ok but each wire does have a long piece of insulation which I did not remove. Can a shunt actually cause this problem?
D-180 ammeter and voltage drop
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kurtfly
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It may help if you tell what brand of alternator (external regulator?) and how the shunt is connected. Is it configured as an alternator load meter or a battery charge/ discharge indicator. I assume it is in a Vans RV7 ?
But it sounds like something in the alternator circuit is intermittent. I have seen circuit breakers do this as they get a little warm in normal use. But yes, the shunt can cause this but a close visual inspection should confirm if you have a cracked /broken shunt. Check for bad / loose crimps. Regulator would be suspect also.
But it sounds like something in the alternator circuit is intermittent. I have seen circuit breakers do this as they get a little warm in normal use. But yes, the shunt can cause this but a close visual inspection should confirm if you have a cracked /broken shunt. Check for bad / loose crimps. Regulator would be suspect also.
Kurt, thanks for the reply. Yes, RV-7, Lycoming io-360, Plane Power internally regulated 60 amp alternator. The output line goes from the alternator to an ANL 60 then directly to the shunt. The D-180 is then taking its pickups from the shunt. I need to determine if the alternator output is dropping off or if this is an indication problem. Had it connected to a VOM and ran it for almost an hour but could not duplicate the failure. Looks like I’m going to decowl it and hook it back up. Once I get it to fail, I’ll know if the alternator is truly failing (in which case I’d suspect the input field voltage wire or circuit breaker) or an indication problem which could ge the shunt or the shunt wires/pins going tonD-180. Agree?
kurtfly
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Rob, Based on your description the problem has to be bad connection on the alternator side of the shunt, the ANL 60 fuse/ holder, alternator itself, or field wiring / field circuit breaker. Check all the connections between these points. Just because the alternator has been replaced does not completely exonerate it. You may want to also replace the 60 A fuse just in case it has a crack inside it. I doubt they are really designed for aviation environment.