C150 discharging

Chilito

Chilito

Pre-Flight
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Aug 8, 2019
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MN
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Chilito
Shortly after takeoff today, I noticed a small discharge on the ammeter. Turned off what electrical I could and it continued. As I headed back to the airport, it swung back to charging for a few seconds, then returned to discharging. When I landed and tested the battery, I was sitting at 12.5v (at startup I had 13v). So I put on a charger for a bit.

Came back later and put a multimeter on the alternator. Battery terminal on alt showed 12.8v. Fld terminal showed 11.8v. Haven't tested the voltage regulator yet, but it was installed less than three years ago. Alternator had brush work done three years ago as well, maybe 200 hours, don't know if it was a reputable shop as it was the previous owner. Wondering where to go from here, besides my A&P. Thanks.
 
Doesn’t sound alarming yet based on the information you gave.

You need to approach with great care, but you need to get a voltage reading across the battery with the engine running. No big deal with a car, but think through carefully doing this test on an airplane so you don’t get a bad headache from the prop. If the alternator is charging, You will have that 12.8 VDC with engine off and if the alternator is charging, you will see something like 13. 5 to 14.5 with the engine running.
 
MBDiagMan said:
get a voltage reading
Click to expand...
from the cigarette lighter socket. It's easier.

Note: If you have an issue with the battery cables or connection at the battery, it won't show up at the lighter. But low alternator output will.
 
I'm guessing the OP was measuring with the engine running since there was 11.8V on the field terminal. This means the regulator was "full fielding" the alternator, i.e. trying to turn it on full blast. Since the battery voltage was only 12.8 it suggests the alternator is not functioning. Next step would be to disconnect the field wire at the alternator and measure the field terminal resistance to ground with an ohmmeter, engine not running. It should be around 6 ohms and not vary significantly when the alternator is rotated. An open or very high resistance indicates a problem in the field circuit, i.e. the brushes/slip rings/field winding.
 
flyingbrit said:
I'm guessing the OP was measuring with the engine running since there was 11.8V on the field terminal. This means the regulator was "full fielding" the alternator, i.e. trying to turn it on full blast. Since the battery voltage was only 12.8 it suggests the alternator is not functioning. Next step would be to disconnect the field wire at the alternator and measure the field terminal resistance to ground with an ohmmeter, engine not running. It should be around 6 ohms and not vary significantly when the alternator is rotated. An open or very high resistance indicates a problem in the field circuit, i.e. the brushes/slip rings/field winding.
Click to expand...
I usually found 3 to 5 ohms on a 12V alternator field. Anyway, you're right; if that field is getting 11.8 volts and the alternator isn't producing, it's a field circuit problem or the alternator isn't even turning. That alternator, in that airplane, is a gear-driven affair. The prop should be turned (backward! Or disconnect all the sparkplugs. Much safer.) and the alternator watched to see if the cooling fan just inside its front end is turning. The gear drive has rubber dampers in a thin metal cup that could conceivably fail and let the gear spin freely on the alternator shaft. Not likely, though.

If the field resistance is high or open, either a brush has failed or, more likely, too much grease was put into the rear bearing when the thing was apart for the brush change, and the excess grease squeezes out and sooner or later ends up on the slip rings. It contaminates the brushes and forms a resistive sludge on the slip rings that cuts the field current to almost nothing. That rear bearing needs very little grease.

The field wire needs to be disconnected to get a reliable resistance reading. Easiest is to pull the connector out of the regulator.
 
Update: Thanks for all of the responses. I test ran the engine again today, and everything was normal. Charging at 13.8v on the ground, so everything was doing its job. The short blip of charging during the previous flight in addition to a normal run up hints at a wiring issue, which, is itself 53 years old. Still going in to mx.
 
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