Alternator troubleshooting & adding an ammeter

So I made a short 15 minute flight earlier this week repositioning following a transponder check. Upon landing, I noted that the voltage indicator was showing just at 12V (analog indication, so nothing too precise). I finished taxing and watched the Voltage indicator and noted some fluctuation between just under 12V to around 13V. Increasing the engine RPM resulted in the higher indications (which I am not sure I understand because it's an alternator, not a generator...)

I already had an ammeter that I was wanting to install so I took some time to wire it in today. I was planning to use the ammeter to help me troubleshoot the alternator and I temporarily placed the sensor prior to the main CB that then leads over to the main bus.

I started the engine noted the battery voltage was 12.6-12.7, turned on the alt switch, and watched the voltage "increase" to 12.9. I saw no reading on the ammeter...

I would have thought that I'd have seen at least something on the ammeter, I verified the resolution of 0.3A on a bench before putting it in the plane....

There were three large gauge wires coming through the firewall. One goes to the battery (largest), one goes to the "Main" CB (medium), one goes to the main bus (smallest of the three). I expect that the largest goes to the starter, the next is from the alternator, and the third is___ (I plan to trace this tomorrow)...

Questions:
I am wanting to mount the hall effect sensor aft of the firewall, shouldnt the line leading to the main CB have been the B line from the alternator?

Any suggestions on what to look for while double checking the alternator? (I am guessing this is a loose wire somewhere as the issue appeared intermittent.) The analog voltage sensor has displayed just over 13V when in use as long as I've owned the plane. I confirm the voltage jumps up when I turn on the ALT switch during preflight.

I'll stick with the alternator debug for the moment.

Voltage is the primary measurement to determine the output of the alternator. It should be measured where the B lead attaches to the battery, as that is what the battery sees and what the alternator is producing. Connectors notwithstanding.

If the voltage is low (less than 14.0-14.6) but rises with speed, then I would first look at the belt slip. You said the voltage was 12.7 static, indicating a fully charged battery. Higher amps mean higher torque on the alternator, and higher speed will allow lower torque to produce the same amps.

Since you have voltage for the alternator health, I would recommend the ammeter to be placed in the wire from the battery feeding your electrical system. This way if the voltage drops, then you can actively manage the amp draw by shutting down loads until you reach desired (and predetermined) minimum.

Thanks Bill.

I am going to be taking the bottom cowling off this afternoon to assess belt tension.

Problem found. There was an intermittent connection with the VR.

The belt is due for replacement, but tension was appropriate and I could not force a slip. Thankfully the prop is coming off next week anyways.

johnny11b said:

Problem found. There was an intermittent connection with the VR.

The belt is due for replacement, but tension was appropriate and I could not force a slip. Thankfully the prop is coming off next week anyways.

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Good find! Thanks for letting us know.

Yeah, changing an alternator belt us really hard. Stretching it over the second blade is a real bear. I especially hate three blade propellers.

BillL said:

Good find! Thanks for letting us know.

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You couldn't force a slip? That statement tells me something is wrong. Unless your "belt" is a chain, you better be able to "force" a slip on an alternator locked with a screwdriver or fully loaded or you're ruining bearings somewhere. In the GA world, the alternator belt should "slip" when you put a socket on the end of a torque wrench and hold onto the prop to keep anything from spinning. The belt should slip around 10 pounds for a used belt and ~12 for. a new belt. Be careful and don't ruin your alternator because you thought your airplane needed to withstand the g-forces of a re-entry from outer space.