Alternator problem and solution

I am writing this post in hopes that one day someone troubleshooting an alternator issue will find it and it will save them time and keep them from pulling their hair out. I used VAF archives and was able to find a solution to my problem.

Background:
RV-6A with Lyc O-360-A1A, 909.3TTSN, dual P-Mags, Hartzell CS prop, VM1000 engine monitor, seperate master and field switches, EXP2 Bus Module.

Problem:
Alternator has worked solid for the 3.5 years I have owned the plane. All records indicate it is the original alternator Mitsubishi A1T40791 50amp. Voltage has read between 14.1-14.4 over the years. Gradually creeping higher. (Quick talk with my A&P after solution found and I have learned this is high! oops)
Last Thursday coming home from a breakfast run, the alternator voltage began to fluctuate between battery power and 13.8. I began to troubleshoot.
On subsequent check flights the alternator would work great through ground checks, but as soon as takeoff power was applied it would die and I would be on battery power alone. (No fluctuations)

Troubleshooting:
I downloaded the plane power alternator troubleshooting guide as well as an electrical system troubleshooting guide (I did not build my plane). At first all signs pointed to my alternator (ie. 12V on power wire and 12v on field wire when activated via field switch). So, off comes the alternator and a quick stop at PepBoys for a bench test. Alternator tested GOOD! Now what?
I re-installed, shot some more wires and all appeared good. So I replaced all the electrical butt connectors on my system thinking that maybe the vibration of takeoff power was causing a short. This did not fix my problem. This goes on and on for 2 more days until I replace my field wiring completely. Still did not fix my problem. All signs still pointed to the alternator, so off it comes again and away to O'Reilly's autoparts I go. Bench tested GOOD, AGAIN!

Solution:
After some additional research on VAF, I came across and old post here describing an issue with the EXP2 and an over-voltage protection circuit. I had not thought of that. So I began to learn more about my EXP2 system and sure enough, a high voltage will cause the system to shut off the alternator input and requires a reset via the master switch to take it out of over voltage protection. (This is starting to sound familiar, alternator putting out 14.4v till full power is applied, then shuts off.)

With very little hair left and my guess that the alternator is original (1999) and has served the plane for over 900+ hours it is time for a new one. If it didn't fix the problem, it wouldn't hurt it. I was a little sketchy putting down $88.77 for a new alternator (Ultima 14694) when the core I was trading in had passed two bench tests. However, I installed the new alternator and she is humming right along at 13.6-13.8v. (More normal readings I have come to learn).

OBTW:
-Neither store could tell me at what RPM they ran the bench test.
-I now have a lifetime warranty on my '1985 Mitsubishi Mirage' alternator
-Car alternators have less strict tolerances for voltage output. (This may be also linked to internal regulators)

Dan

Dan, I would not say that 13.8 is the norm. Certainly if you have a PC680 that is low. They require a minimum of 14.1 and prefer 14.6 to get a full charge. There was just a thread about this a few weeks ago. Keep in mind that many EFIS based voltmeters read half a volt low for reasons someone else smarter will have to explain.
George

my new 55A ND alternator is set at 14.2 according to the test strip I got with it from NAPA. It was $20 more at NAPA but came with a test strip and the guy at NAPA was able to answer questions about it.

What happened to the day when you used to walk into an auto parts store with a part in your hand and before you could say a thing the guy behind the counter would say "Upper radiator hose for Jaguar XJ6 1974-1982. I'll see if her have one"?

When dealing with alt issues I found it helpful to get readings at the alt bat , and meter. There can variations between allocations.

I have noticed also that the EFIS reads voltage about a half volt low from what you can read at the battery. I also don't know why unless the shunt that is installed to read current flow is causing the voltage drop. After all it is a resister? Would be interesting on others comment / observations. I have a D180 and "1978 Suzuki" Advanced Auto 35amp alternator with a PC 680

DennisRhodes said:

I have noticed also that the EFIS reads voltage about a half volt low from what you can read at the battery. I also don't know why unless the shunt that is installed to read current flow is causing the voltage drop. After all it is a resister? Would be interesting on others comment / observations. I have a D180 and "1978 Suzuki" Advanced Auto 35amp alternator with a PC 680

Click to expand...

My understanding is that the low voltage reading at the EFIS is normal and due to the use of a diode in the EFIS. The diode is necessary to prevent a backward electrical flow when multiple EFIS power inlets are in use.

erich

lower voltage displayed by EFIS

I'm delighted this came up. I noticed it right away after installing my Dynon D120, and I asked Dynon about it.
I thought I was seeing poor battery charge retention because it habitually shows 12.3 when I turn the master on before start-up. But a volt meter on the battery shows 12.8.

Thanks!

If I ventured a guess as to what may have been wrong with the original alternator, a set of brushes would seem the likely culprit to me. At probably less than $5, it's worth trying before you plunk down more $ for a new alternator.

When brushes get worn, and they do so a bit quicker in our faster-turning installations than in cars, they can make intermittent contact with the rotor and create problems similar to what you described.

However, less than $90 for 900+ hours worth of juice isn't too shabby!

scsmith said:

I'm delighted this came up. I noticed it right away after installing my Dynon D120, and I asked Dynon about it.
I thought I was seeing poor battery charge retention because it habitually shows 12.3 when I turn the master on before start-up. But a volt meter on the battery shows 12.8.

Thanks!

Click to expand...

The GRT does the same on its power pins. A WS reads a little over .4v less than what is actually hitting the pin. An HX isn't quite as bad.