Intermittent electrical problem

[andy]

I have an intermittent electric problem with my 1986 MX-7-180 that's difficult to troubleshoot. My JPI EDM800 engine analyzer and Apollo GX60 GPS/Comm randomly restart during flight (never on the ground) within about a 20 minute period. I hear a slight click in the headset and occasionally see a small indication on my VOR CDI. Once my GPSMAP 396 which was plugged into a 12V cigarette lighter receptacle detected a power interruption and started a shutdown sequence but resumed normal operation within 3 seconds. I did a major overhaul on the engine including a new alternator in 2004 but this problem arose about 3 months ago. The only maintenance activity prior to the occurence of the problem was to clean the contacts on the plug for the JPI fuel flow sensor. None of the other avionics in the airplane have a visible startup sequence, so they aren't affected.

Initially, I thought the problem was within the EDM800, which is wired to the GX60 through the serial port and could have caused both to restart. After alternating between shutting off each of them to see if the problem went away, I removed the EDM800, sent it to JPI for inspection and repair and pulled the circuit breaker that fed power to it. The GX60 still occasionally restarted but not as frequently as the EDM800. JPI sent me a loaner EDM800 which also restarts occasionally and they do not think the problem could be caused by any of the JPI sensors, including the fuel flow sensor. I think I've eliminated both the EDM800 and the GX60 as possible causes.

It's a nuisance to have the EDM800 restart, but it messes up the calculations on fuel used and fuel remaining during flight. The GX60 restarting is more of an issue since it's happened during an IFR approach. I don't want to fly in IMC until I get it fixed.

In an effort to troubleshoot and repair the problem:
- Alternately shut off the EDM800 and GX60 to see if the other one continues to reset - they do;
- Removed the EDM800 and flown without it, as well as installing a loaner from JPI;
- I've had inspections by 3 experienced A&Ps of all the wiring from the alternator back looking for loose or corroded connections;
- Inspected and tightened any loose switch and circuit breaker connections in the panel;
- Soldered in a new wire section to replace two Butt spliced connections in the alternator field wire;
- Replaced the alternator control unit.

I'm beginning to think I that the alternator is the problem. During the annual in June we added some washers to the alternator bracket to have the belt ride more evenly in the pulley and retensioned the belt but otherwise it's been working well for the last 5 years.

The only other possibilities that we can think of are the alternator switch and avionics master switch.

Has anyone seen a similar problem and how did you fix it?

Andy

[maules.com]

You could try running power to the JPI direct instead of through the radio master buss to eliminate that circuit.
Most likely a bad ground somewhere. Try cleaning the airframe to engine ground wire at the right rear of the crankcase. Often there is paint interference on the engine mount end of that wire.

[a64pilot]

Most likely a bad ground somewhere.

That is where my money is, start removing and cleaning them, they can be tight but due to corrosion, still make an open circuit ocassionally, the click you hear is a circuit opening, I believe

[aero101]

I would suggest the same- wire directly to main solenoid and a good ground preferably on the engine. I had several indicator problems with the panel in my MX7-180 and all turned out to be related to bad grounds of panel. If you wire direct as stated you will illiminate everything but battery / charging system and if that takes care of problem, further troubleshooting of buss and panel grounds would be in order? good luck!

[a64pilot]

I posted here many times that every time I chase intermittent electrical problems it's usually a loose or otherwise bad ground.
I use to chase a few electrical gremlins on Apache helicopters every now and again, so I've chased a few. There are a few wires in an Apache

[Kirk]

Grounds are a pretty common culprit, but it seems strange that 2 unrelated components fail ate the same time.

One common source they have is the avionics master switch. I'd wonder about the switch, contacts at the switch.

Kirk

[diebroke]

Grounds are a pretty common culprit, but it seems strange that 2 unrelated components fail ate the same time.

One common source they have is the avionics master switch. I'd wonder about the switch, contacts at the switch.

Kirk

I will second that. My JPI's in my C340A kept resetting in flight. Finally chased it down to a bad Avionics Master switch.

Good Luck

[akloon]

I've had both bad master switch and a bad avionics switch as culprits.

[andy]

Replaced the alternator control unit, avionics master switch, alternator switch and closely inspected all wiring. Looks like the problem is the alternator, which has about 550 hours on it. My A&P measured the resistance of the brushes and said it's too high, which means they are probably glazed.

Andy

[andy]

I replaced the alternator and that was the problem. What I learned from this experience is that you can troubleshoot the alternator by disconnecting the wires connected to the field and attaching an ohmmeter to the field posts on the alternator. The resistance should be less than 100 ohms and should not vary appreciably as the prop is rotated. My alternator had a little over 500 hours on it when I replaced it. My A&P said it wasn't unusual for one to last about that long. If I had known how to isolate the alternator as a potential source of the problem, I could have saved about a month and avoided the cost of replacing the avionics master switch and alternator switch.

Andy

[benflyn]

500 hours huuuuummmm?????

I have a delco 12SI, good old common chevy alt. on irrigation engine that has over 23,000 hours on the original brushes and bearings.

Why are the failure rate sooo high on aircraft?
Cessna and others used the ole ford and prestolite alternators.

[a64pilot]

Bet the irrigation alternator has very little load on it, that and temp / vibration would be my guess. Honestly who knows?