Inter Av Voltage Regulator Issues/Replacement

[bencpeters]

Hi all, I have a M20C with an alternator conversion. I recently overhauled the engine and installed a JPI 900 with better voltage monitoring. Since then, I've noticed that I don't get charging from the alternator after starting the engine until I run up the engine past a certain point, usually somewhere in the 2000+ RPM range. Before this, I'm seeing battery voltages below 12V with a 6-7 A draw, but after I run up the engine and hit the threshold, I immediately get ~14V, with an initial +20A current that eventually settles (presumably as the battery recharges). Voltages remain constant regardless of load or engine speed for the rest of flight.

Talking with my mechanic, it sounds like it's probably a case of a bad voltage regular. I have an Inter Av unit that I believe was installed with the original generator to alternator conversion (prior to my ownership). It's the same unit as before the overhaul, and I used to see power coming from the alternator on the amp meter immediately after engine start, but the JPI is installed in the "Ammeter Configuration" which measures current at a different place than the old gauge, so I can't say for sure that this issue wasn't happening before. Either way, it seems like the voltage regulator is probably ripe for replacement.

I was looking at options, and it seems like the consensus choices are Power Plane or Zeftronics. I found this thread comparing options:

I'm a bit confused on the Zeftronic part numbers though - looking at models on Aircraft Spruce (https://www.aircraftspruce.com/catalog/eppages/zeftronicsreg.php?clickkey=35105), it looks like the Mooney listed models have some kind of plastic barrel connectors which I'm guessing are the original wiring harness from the OEM part, which was replaced on my plane when the Inter Av was installed and the alternator conversion happened. My Inter Av just has 4 exposed wiring terminals (similar to the Zeftronics R15V0N). Which Zeftronic P/N would be appropriate to install? Do I need to get one of the Mooney ones (R15300 or R1530B), and then find/fabricate the plastic wiring harness? Or is there some other way to install it that I'm missing? It seems like others have installed these successfully on vintage Mooneys, does everyone else just still have the original harness?

Thanks!

[Andy95W]

Starting around 1970, Mooney started putting alternators on at the factory.  The Zeftronics are for those airplanes.

The only approved replacement for the Interav regulators is the Plane Power.

https://www.aircraftspruce.com/pages/el/voltagecontrols_regulator/planePowerSAL2.php

It’s an excellent unit.  I replaced my Interav regulator 11 years ago and my Plane Power voltage regulator is still rock solid, putting out 14.1 volts.

[David Lloyd]

I had a conversation with the former owner of Interav before he sold the company. He said the 75 ohm resistor on the back of the alternator provided initial voltage to the field winding at startup to get the electrons flowing. If the resistor was failed, there usually enough residual magnetism in the stator to begin producing voltage at a higher rpm than idle. Once producing power, it would all work like it should. Sounded somewhat logical to me as one of my resistor leads was broken and it would come on line about 1600 rpm and work fine until the next start.

One of you electrical guys can probably clean up what I wrote.

[Andy95W]

I think David’s explanation, above, is the most likely culprit for the charging issue.  It sounds like that resistor got damaged during removal/installation during the engine overhaul process.

I’m glad David posted this.

[bencpeters]

Interesting re: the resistor! Do you know what it looks like? These are some photos of the alternator, but I don't see anything that obviously looks like a resistor to me... unless it's under the shrink wrap on the connection between the two terminals near the red line here:

 

Or is the resistor over on the other side by the voltage regulator? I'm not totally sure where I should be looking, but none of the wires look obviously problematic here...

Thanks!

Other pics:

 

[David Lloyd]

The 75 ohm resistor is the short shrink-wrapped part between the two terminals.  When I cut the shrink wrap off mine, one of the leads had pulled out of the resistor.  The shrink wrap is there mainly to add mechanical strength to the flimsy little resistor.

The orange arrow is pointing to a capacitor.

While you have things apart, remove the brush assembly behind the fiber board rectangle.  Any auto alternator shop will have those brushes. May as well replace them and go another 500 hours.

In the download section there is an Interav trouble shooting and repair guide.  Parts are still available but can't remember the name of the company, it should be easy to find with an internet search.

[Andy95W]

I’m not 100% sure where the resistor is, but my guess is from the photo (like David said):

(I attached the Interav manual as well)

interav.pdf

Edited March 29, 2022 by Andy95W

[65MooneyPilot]

I have had an interav alternator for many years. It has always worked but has never come on after starting the engine. I always have to after starting the engine reset it buy turning the master off then back on and it comes right on and starts producing 14 volts. When I called years ago to get tech support the guy I talked to unfortunately had no answers. Instead of running the rpm up try the master off/on to see if it comes on line. 

[bencpeters]

Thanks for all the thoughts! I went flying again yesterday evening and took a multimeter with me to do a little debugging.

 

- @65MooneyPilot's suggestion to cycle the master, I tried that, but no luck unfortunately.

- I was looking at the troubleshooting guide in http://www.beechcraft.org/vtail/interav.pdf - it suggests measuring voltage to ground on the terminals with master on, engine off. I did that on the voltage regulator only (since it's pretty easy to get leads on through the oil inspection port), and found ~100mv on both (not the same value, but close) terminals - so close to ground. According to that sheet, that means an "open Reg or bad connection" (but not a bad resistor - according to the troubleshooting sheet, a bad resistor connection would lead to values of ~0.4 V on both terminals).

Kinda leaning toward replacing the voltage regulator with the plane power one regardless - I trust solid state electronics a lot more and it's not very expensive. But it does make me wonder whether there's bad wiring to the alternator or perhaps a bad connection on it?