SR22 2002 Alt 2 light intermittent

Just got plane up after annual. About 10 minutes of flying the alt2 light came on during cruise. It went off when I went from 20 inches to 23 inches then it came back on. I turned the alt2 switch off and back on and it was ok for a while. The ammeter does not show discharge when the light comes on, but ammeter does go down when I turn off the alt2 switch. On the ground after the flight light comes on under 1700 rpm and it turns off over 1700. Worked normally on the ground.


Give me a call at 703-946-9376 and I will walk you through my Alt 2 saga. Several of us have experienced this and there are several old posts. Could be one of many parts…troubleshooting is required.

If you have the original MCU, that has been an issue.


Paging Craig Albright…

I do have the updated ECU.

When i first got my Cirrus i had the same issue. An incompetent shop looked at it and said they had to replace the alternator. When i got it back, SAME ISSUE was there… THEN, I met Alexander Wolf. He cleaned the connectors and everything has worked normally now for almost 2 years. Not saying thats your problem, but corroded connectors in need of cleaning cost me ALOT of money.

On a side note, always use a service center, even if your local shop swears up and down they know cirrus’s…

I would qualify that statement. There are a ton of great A&P’s out there I would trust my plane to that don’t work at a SC. And some SC’s I have seen some real shoddy work. It depends what your dealing with. With a cylinder issue you need Continental not Cirrus knowledge. With some specific airframe issues Cirrus knowledge is critical. MCU’s are one of those unique items, but many SC’s only want to replace it not dig around inside it. Not all SC’s have the same skills. Just say’n.

What you want is a knowledgable mechanic, and don’t take their word for their competence because like pilots we are all above average in our own mind. Have to wonder where the self confessed below average pilot or mechanic is, right?

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I agree Roger. Thanks to the wealth of knowledge and experience of COPAns such as yourself, along with Cirrus Tech Pubs and Tech Support, I troubleshot and repaired my MCU120 that was having the same issue under the supervision and signoff of my A&P/IA.
My experience with my MCU issue tells me that even though the troubleshooting guide is good, the culprit (if not the alternator itself or the field control module power connect to the alternator) is difficult to isolate. There is little to no way to determine if the culprit is the Field Control Module, the Logic Unit, or the current sensor. One suggestion is to swap FCMs and swap current sensors to troubleshoot, then order the appropriate failed part.
If the J111 power pin connector to the PCB is fried, obviously that is a clue to high current draw in the field current circuit due to something (corrosion, bad MCU component, or ALT2 switch on at low RPM on the ground), but even after changing the J111 harness and cleaning up the pin on the PCB, replacing the current sensor and logic unit as a matched set (since both parts have rolled their dash numbers a few times in recent years telling me their design has been problematic) is what finally solved the problem. I had replaced the Field Control Module first to no avail.
I know various members have different levels of interest or free time to undergo such a tedious trial and error process because it requires potentially a few iterations of disassembly, component replacement, reassembly, and functional checkout, but MCU repair is very doable by the average mechanically inclined owner/pilot. I do it because it keeps ownership costs lower, and I enjoy it, but the downside is the airplane was down for almost two months while I troubleshot, waited for parts, did the repairs, and tested the fixes.


Good for you. Taking this on is not easy, but as you proved it can be done. It is just machinery. If a person designed and built it, it will fail and it can be fixed.

I went thru the CSIP training and there was no mention of leaving Alt 2 off while on the ground. Is that proper procedure and if not followed will that damage anything?

If you spent a lot of time at low RPM, like taxiing or holding for takeoff, keeping the Alt2 switch on apparently generates heat that can damage the pins or connector J111 in your MCU. The result of that failure is the loss of Alt2, and in many cases a lot of futzing around with your SC to find and fix the problem; my SC at the time had me replace both the Alt2 and FCU2, both of which were fine, before they would agree to open up the MCU and replace the connector, which was fried.

Leaving the Alt2 switch off during low RPM operations doesn’t effect the electrical generation as displayed on the MFD; it should be on and checked during your run up, and of course switched on once you are ready for takeoff.

I make a habit of turning on both boost pump and Alt2 prior to crossing the runway threshold (before takeoff checklist), and turning off Alt2 after clearing the runway (after landing checklist).

Not in the POH nor will it ever be. It is a trick long term owners have discovered that reduces MCU problems.


It was an item I learned the hard way after finding my J111 connector fried. The windings in the rotor are energized by “field current” to create the magnetic field that then creates current flow as the rotor passes each stator. The field current is regulated by the field control module. That current passes through the J111 connector inside the MCU and on to the wire with a connector on the back of your alternator. The way folks like Jim, Alex, and Roger explained it to me, the type of “molex” (a brand name) connector inside the MCU, and IDT connector, is prone to failure because the contact surface is so small. The wires are jammed into “blades” in the connector which pierce the insulation and create the metal/metal contact. (These connectors are now obsolete because their is so little contact area, but somehow, the brand new replacement J111 harness came with the exact same connector…Cirrus must be sourcing them from overseas). At low RPM the alternator is not turning fast enough to create the required voltage, so the FCM attempts to send more current to the rotor field to boost the voltage, which sets up the J111 connector for this failure mode, especially if there is some corrosion present. So, the habit pattern of leaving ALT2 off on the ground has been passed down through COPA from those (including me now) who have experienced this failure mode and have had to do expensive repairs to their MCU!


Hello just saw this post. So can you give me a bit more info on keeping alt 2 of when on ground? Should it be turned of right after start up? If not gonna be taxing or on ground long is it necessary to turn off? Does it effect any electronics when turn back on prior to take off, i.e. Pfd, mfd ahars? Is there a rule of thumb on when you should do this or do you do it all the time? Many thanks on future help.

Note: we are talking about Avidyne planes here; Perspective Cirrus have a different Alt1 and Alt2.

There is no point to turning Alt2 on just after startup, since the engine is not developing enough RPM to generate power. You first turn Alt2 on during run up, to check that it is powering the EBus AND that you have higher voltage on the EBus than you do on the MBus, which tells you the isolation diode is working. You can then turn Alt2 off until just before adding takeoff power, when once again it will be generating electrical power. Watch the voltage on your EBus and MBus, and you’ll see no difference with Alt2 on or off during ground operations, therefore impact on your electronics is nil.


Perspective planes still have a MCU. And all the associated FCM’s, solenoid’s and other stuff.

But the alternators and are different on perspective, so I assume the advice doesn’t apply to them, right?

That is correct. This specific issue does not affect Perspective aircraft. Just making sure it didn’t start a rumor [;)]

Ok now what. The J111 connector is not burnt and I still have an intermittent alt 2 light. All voltages are normal. What else should we check next?

Current sensor swap with ~1 alternator… 2nd step is the logic module