M20K 14V Dual Alternator trouble

[juergenklicker]

My M20K-231 has undergone a "Trophy" conversion including an engine upgrade...

...and a second alternator. Both alternators are feeding just one 14V battery.

Has anyone an idea how these two alternators are supposed to share the load during regular operation and how the other one is supposed to keep the show going in case one of them does south?

If I switch the original alternator off, all is fine, the second alternator keeps charging as expected. However. if I switch off the additional alternator, the system keeps working but voltage goes down from 14.4 to 12.7V at regular cruse rpm and charge/discharge amps start dancing around somewhere between plus and minus 40amps. I have no schematics for this STC, the STC provider (Mooney Mart) is not really responsive, and I'd like to find out if I have a problem or no.

Can anyone help me out here?

Juergen

 

 

 

[Bennett]

As I recall this conversion used two separate voltage regulators. Over the years I replaced them more than once. They should be "balanced".

[juergenklicker]

Yes, there are two separate regulators, the original one has a separate OV protector and is now regulating to 12.7V with dancing amps). The second one (Zeftronics R15300) includes OV protection and is working just fine at 14.4V).

That being said, this means right now the second alternator takes all the load until it can't keep up. Only then the voltage will start dropping and at 12.7 the original alternator will start contributing. That doesn't sound right.
I guess this means tossing the original regulator and OV protection will have get replaced both with just a second R15300.

[ArtVandelay]

14.4v is a little high, I believe 13.8-14.2 is the preferred range...there is an internal pot you can adjust the threshold voltage. If it were me I would get a 2nd 15300, open then both up, adjust each one for 14v unless you want set of them to be considered secondary, then I would adjust it as 13.9.

[Bennett]

The regulators I remember were stamped Ford, and then a Mooney part number. I replaced them in pairs to maintain the same output from each alternator. I also seem to recall I replaced the second alternator with a larger capacity unit than the original conversion. Even with both alternators on line, I would sometimes get a low voltage warning when I added the landing lights to all the avionics in the panel. I sure like the new LEDs, and low draw modem avionics in my current Mooney.

[ArtVandelay]

Even with LEDs all around and modern avionics but no glass, I added up my max/peak current requirements and came out to 46.5A. Realistically I won't be transmitting on both radios, I've calculated my minimum draw in case of power loss to be 13A, not counting flaps, gear motor.

[juergenklicker]

I guess I am going to end replacing the Mooney original alternator controller with a second Zeftronics 15300 and yank out the separate OV in the process.

That'll do it.

Thanks much everyone

Juergen

[Guest]

Why not switch the field wires between the 2 alternators to determine if it the alternator or regulator.  Have you checked that the drive coupling on the primary alternator is not slipping?

Clarence

[juergenklicker]

Up to now I was just trying to find out if what I observe might be normal for that configuration. Now that I know it's not, let the troubleshooting begin...

[juergenklicker]

Replacing the original alternator with a second Zeftronics 15300 didn't work. The Output Voltage on the15300 can't be adjusted, so now both alternators ar3e trying to adjust to 13.8-14.2 and are fighting for the load. 

Now I am guessing the idea behind this mod was to have the second 80Amps Alternator take the entire load as long as it is alive and kicking. That would make sense because the second alternator can chip in 10 more amps (80) than the original and can start chipping in at lower engine rpms because it's belt driven.

Once it dies, the voltage will drop to whatever the voltage regulator of the original alternator has been adjusted to. That works now and we got rid of the dancing amps by adjusting this one to 13.1 Volts, but I am still not sure if this is how this solution is supposed to operate.

If anyone has schematics / wiring diagrams / anything describing this modification, that would be greatly appreciated.

Juergen

[ArtVandelay]

Replacing the original alternator with a second Zeftronics 15300 didn't work. The Output Voltage on the15300 can't be adjusted, so now both alternators ar3e trying to adjust to 13.8-14.2 and are fighting for the load. 

The Zeftronics 15300 is the voltage regulator, not an alternator.

And it can be adjusted, remove the 2 screws, take off the cover, you will see a pot with bit of caulking on it, remove the caulking, use a screw driver, with the other field CB pulled to disable the primary alternator, adjust as desired, replace cover.

However, I would leave them alone, one or the other will be slightly higher voltage and will take over the charging. If you want to force one or the other to be not charging , just pull the Field CB.

Having "fight" each other is not going to harm anything, although it could create radio static. Personally I'd just pull the CB to disable one or the other, be sure to alternate (no pun intended) which is active, so you know you have a reliable secondary.

[juergenklicker]

Voltage regulator is what I meant, of course. Sorry for that.

Right now I have the primary regulator at 13.1-13.2 and the secondary is as it came at 13.6-13.7. That appears to be working.

If the primary regulator is adjusted to any point close to where the second one is, the green light starts flashing, back to 13.2 or so, and the green light goes back to steady.

So, even though it's working now, I am still curious how this was meant to be...

Juergen

[ArtVandelay]

Have you done a survey of your load requirements? Count up the Amp draw of your avionics and lights. You may not ever have enough of a load to need the second alternator.

[juergenklicker]

With Avionics, Pitot Heat, TKS, landing lights, nav lights, strobe, cigar lighter charging ipad and stratus, it gets close. But that's not the main reason for the second ALT, of course.

The reason is I am a scardy cat. Two alternators, two vacuum pumps, two ILS receivers, ...you get the picture. 

Juergen

[kortopates]

Both Voltage regulators are intended to be set to the same bus voltage. On  a 14V system they should be set in the range of 13.8 to 14.1 V for battery longevity. That said, Alternator #2 will carry most of the load because it is belt driven and running at a higher RPM than the engine driven #1. But during your run up, part of your checklist is to fail #2 (by turning off the field rocker switch for #2) and you should see #1 pick up the load. Bus voltage should remain in the proper range above on either alternator alone.