Alternator slow to come online

BLUF: Automotive alternator was slow to come online at idle RPMs, but rock solid once it was up. I later discovered it may be directly tied to a minimum crank RPM. My previous experience with this setup did not have a minimum RPM for positive charge; this behavior is new and I’m not sure why.

I had an issue I hadn’t encountered before on my flight today. I started the engine as normal with the alternator switch off. Once started, I turned the alternator switch on and nothing happened. The voltage did not change from what was supplied to the bus with just the battery (approximately 12.5v). After cycling the field breaker and the switch a few times each, eventually the voltage came up to 14.9 and then slowly went down as the battery charged from the start. Although I gave it some time between each action, I could not positively identify if either action caused it to successfully come up. Engine RPM was just above 1100 this whole time. I thought perhaps the alternator or voltage regulator had just been cold soaked and I taxi’d to the fuel pump after letting the engine begin coming up to temp.

After I filled up and started again, same thing. Just as I was turning to taxi back to my hangar the voltage came up appeared steady. I opted to take the plane for a lap in the pattern to see what it did. I ended up flying for almost 2 hours and the voltage was steady the entire time. After I landed, the alternator appeared to drop off again as I taxi’d in at about 950 RPM. In front of the hangar I advanced the throttle to achieve 1200 RPM and the voltage came up again.

In my past experience with the plane the alternator has not been noticeably sensitive to RPM once it was turned on. Ambient temperature at the time was about 50*F on the ground, but it’s been in the 20s at night for the past several days. The alternator is an older automotive brand (Mitsubishi I think, I’ll have to double check) in a belt driven configuration off the flywheel.

I can gather more information as needed on the external regulator and the alternator if needed, but since the operating concepts should be the same for any alternator I’m wondering if anyone has some insight into what’s going on here. Is it talking to me?

Just me

Since it was fine at lower rpms, and now it is not, I would think a belt of pulley slipping could cause this.

Or the voltage reg is failing.

Or a diode set broke, so there is not enough umph at low rpms to charge the battery anymore.

I decowled today and checked all the connections. The belt and wires were visually intact. The alternator is a Motorola 8AL2095K, that goes to some sort of unknown box that is just labeled “Short Circuit Protected.” I believe this box is the overvoltage regulator given it has the typical four wires. There is also a Prestolite X16799 overvoltage box in the circuit. I’ve linked the engine monitor data from the last flight below, to show how the voltage appears to come in and out.

https://apps.savvyaviation.com/my-flights/32653/adbe3e3d-409b-4766-b2cf-7f63dd02413b

I’m not familiar with how to troubleshoot each item in this system. How would I check each item beyond randomly replacing things?

One possibility is that a brush hung up and lost contact with the slip ring. Vibration freed up the brush and the voltage came back.

00Dan said:

I decowled today and checked all the connections. The belt and wires were visually intact. The alternator is a Motorola 8AL2095K, that goes to some sort of unknown box that is just labeled “Short Circuit Protected.” I believe this box is the overvoltage regulator given it has the typical four wires. There is also a Prestolite X16799 overvoltage box in the circuit. I’ve linked the engine monitor data from the last flight below, to show how the voltage appears to come in and out.

https://apps.savvyaviation.com/my-flights/32653/adbe3e3d-409b-4766-b2cf-7f63dd02413b

I’m not familiar with how to troubleshoot each item in this system. How would I check each item beyond randomly replacing things?

Click to expand...

Visually intact does mean that it won't slip. Place a socket on the pully nut and rotate CCW with a torque wrench. The belt should not slip under 9ft. lbs.

Alternator

It really sounds like you blew a diode in the alternator. If it was me, I would pull the alternator and take it to an Autozone or similar auto parts store. They should be able to test for free.