Betty_the_Mooney

I tried something new - I gave all of my symptoms to ChatGPT, and this is what it said. What does Mooneyspace think about the diagnosis and suggested remedies? Reported Symptoms Faulty grounding previously discovered and treated - last few weeks (headset noise). Annunciator light alternates between solid (OVER voltage) and flashing (LOW voltage). Flashing annunciator (voltage light) is erratic (sometimes steady, sometimes fast). Annunciator bulb runs hot. Transmitting on COM1 (Garmin 430W) causes EGT rise on engine monitor (returns to normal after transmit). Radios flickered off and back on briefly a few weeks ago. Annunciator push-to-test inconsistent - 5+ years of ownership. Ammeter usually ~0, slight flicker ±, steady most of the time. Voltmeter (cigarette-lighter plug) shows 13.2–13.7V in cruise (normal after 5+ years), except when multiple high-drain USB devices (~4–4.5 A combined) are plugged in at the cigarette plug — then the measured voltage drops to ~12.7–13.0V. Diagnostic Interpretation (updated) Alternator/regulator likely healthy: Voltmeter steady at 13.2–13.7V in cruise indicates normal alternator output. The temporary drop to ~12.7–13.0V under substantial accessory load is expected to some degree, but the magnitude and where it occurs (cigarette outlet) provide useful clues (see below). Ammeter behavior: “0 with slight flicker” indicates alternator is carrying the load; not a gross failure. False warnings: Annunciator showing OV/UV while voltmeter steady = suspect bad grounds, faulty annunciator sense/wiring, or intermittent wiring. Radio transmit → EGT rise: Grounding/shielding problem. RF energy or voltage drop in a ground loop causing interference with engine monitor. Radios flicker / annunciator test inconsistent: Intermittent power bus or weak ground connection. Voltage sag under USB load (12.7–13.0V at cigar outlet): Could be normal small drop across wiring/connectors for a 4+ A draw, but if the drop is larger at the outlet than measured at the bus (or if it causes annunciator behavior), it suggests high resistance in the accessory circuit or a weak ground/positive connection at or near the cigarette outlet or its feed. Also worth checking whether the voltmeter in the cigarette socket is measuring at the socket (subject to local wiring voltage drop) rather than at the main bus — if so, the bus may be fine while the outlet wiring/connector is marginal. Common Grounding & Connection “Hot Spots” (M20F) Airframe & Engine Grounds Engine-to-firewall ground strap (braided, often left side) — check for corrosion, fray, looseness. Battery-to-airframe ground — check both battery box and airframe lug, clean to bare metal. Avionics ground bus bonding — ensure solid tie to airframe ground. Alternator & Regulator Alternator ground lug (often overlooked). Voltage regulator ground (older external types very sensitive). Alternator B-lead & field connections (tight, corrosion-free). Annunciator & Panel Wiring Annunciator connectors: look for heat damage, loose/corroded pins. Annunciator ground integrity (many were daisy-chained). Avionics Wiring Garmin 430W: confirm shield grounds for COM coax & audio wiring are terminated properly. Engine monitor: ensure probe/return grounds not tied into RF grounds (to avoid false EGT rise). Accessory / Cigarette Outlet / USB Load Inspect the accessory feed wiring, connector, and ground at the cigarette outlet — high resistance here will cause localized voltage sag under 4+ A loads. Confirm whether the cigarette outlet’s wiring runs through any intermediate switch, breaker, or connector that could add resistance. Check for a poor ground return at the outlet (loose ring-terminal, corroded lug, paint under terminal, etc.). Miscellaneous Avionics master switch: inspect for arcing or looseness. Circuit breaker crimps (especially avionics & alternator field). Summary for A&P (updated) Alternator & regulator appear to be functioning normally in cruise. Annunciator panel may be partially failed, but the root problem still strongly suggests ground integrity and/or annunciator wiring/sense circuit faults. The observed voltage sag at the cigarette outlet under heavy USB load suggests local wiring resistance or poor connection at the outlet or its feed — check outlet wiring, connector, and ground return in addition to the main grounds.

I heard the entire situation while flying from KSAC to southern California. I heard 85U depart Los Banos, check in with NorCal, and then for over an hour try to coach the crippled Cessna to a better resolution. The pilot(s - names broadcast on ATC but omitted here) in 85U were incredibly calm, thoughtful, and appropriate in their coaching, guidance, and recommendations. The pilot of 03H (name broadcast on ATC but omitted here) was also calm, focused, and determined. After a series of electrical system (breakers; autopilot; master switch) tests were performed, all pilots involved became confident this was a mechanical problem. Listening to this was one of the most helpless feelings I can describe; knowing that another aircraft launched in the hopes of helping the situation was heartwarming and instilled a great sense of faith in the commitment of our aviation community. The end to this story was tragic. After more than 25 years of flying, this is one of the most difficult days I can recall. Blue skies and tailwinds to the aviator of 03H and to the kind souls of 85U.

A big issue these days is that the older generation are semi-retiring and leaving it to the younger generation who may or may not be as involved. I didn’t see Don much, until the plane he sold me had a $20k in mistakes. Before then, it was all his unsupervised apprentices. After that, he slowly showed up.

Field voltage is 0 My A&P thinks the VR is the problem, that its enough putting out enough to 'excite' the alternator (Pardon my lack of electrical knowledge - definitely not my area of expertise!)

Alternator Yes on the breakers

So the saga took an interesting twist Battery was charged for five days on a Chargeminder, and then capacity tested. Feedback from two different A&Ps/two different shops was: Battery is in great health. Good! Battery went back into plane. Now, the alternator isn't charging the battery. Start-up is fine, but the volt meter (cigarette lighter) never gets above 12.5V. If I put any load on the plane (avionics master ON), the amp meter shows a discharge. A&P tested the current at the alternator and it showed zero. What's weird is that NOTHING was wrong with the charging system (VR, alternator, etc.) before the battery discharge incident that triggered all of this. Local shop thinks its a bad voltage regulator - we think its the 1976 original. I've asked them to check some simpler things too, such as the wiring at the Master switch (maybe old wires final bit the dust after me cycling the Master switch a few times? Fluke chance?) Overall: I'm perplexed how an issue of a drained battery that was re-charged and re-installed, could affect the charging system forward of the firewall? Opposite ends of the plane

It was the perfect storm of events: the week before, the bulb in my rotating beacon burnt out. On or off, I wouldn’t have told me anything. Trust me, it doubles my annoyance at myself….

M20F with a Concorde RG35AXC. 2.5-3 year old battery. After 24 years of flying, I finally did the “leave the master on” goof-up. Drained the battery to dead (whatever the lowest symbol on a Chargeminder is - the very unhappy territory). Local shop put it on a Chargeminder for a few days, and it showed good health. First attempt at flying it: after start, annunciator flashed Voltage and voltmeter in plane showed 12.3-12.5. From other threads, I suspect that’s the end of my battery. I could spend hours and $$$ chasing it - or do I just bite the bullet, accept the price of my stupidity, and buy a new battery? (Side note: I don’t profess to be handy at electrical things, so returning the battery to the plane was my first time doing that… but I’m pretty sure that’s hard to screw up as long as you put it back how you found it? )

I’m based at SBP, have an F model, and would be happy to chat Mooney. PM me and we can arrange something.

Same problem here in the Central Coast. Overcast 018 and freezing level at 030 with rain forecast. Sorry to miss this - but let’s keep the repetition going with another one in April or sooner!

Anyone around SBP/PRB/SMX interested in "Mooney pooling" up there? Happy to pick others up in the area.

26th is great. I'm hoping my Mooney will be back in the air by then. But if not, if anyone wants to stop in San Luis Obispo (SBP) and pick me up, I'll gladly share the fuel bill with you!

Thanks - do you remember what your symptoms were going into the process?

Update on this - I did a few small checks based on my very novice (but learning ) electrical skills. Checked the voltage on the field wire with battery/alt switch on. Was showing about 11V. Checked the resistance on the field wire (removed one of two wires; batt/alt off; resistance checked at terminals). Showed between 0-1 ohms. (Mostly 0 but occasionally bounced up a bit toward 1 ohm, then steady at zero) Its been suggested that this indicates issues with the brushes on the alternator that need to be replaced. What do others think? I have photos of the alternator area if that matters.

Wow… I don’t want to get too excited at the coincidences here, but… here goes: I’ve only had 4 electrical ‘moments’ with this plane: 1. ALT FIELD CB popped on a long flight from TX to DC. Reset. No issue for years after. 2. Ever since buying plane, the volts were low: 12.8-13.2. Finally at annual had the VR adjusted up. Produced 14-14.4 thereafter. 3. VOLTAGE light after lower landing gear on approach to KHHR. Local A&Ps couldn’t find any issue. Home mechanics said it was the field wire faulty. (40 hours ago) 4. The present issue in thread above. Interesting that on my first flight, the plane was damp after two weeks of heavy rain on the ramp (albeit covered but still…). On the second flight, the issue was gone: until I lowered landing gear on a bumpy approach back home at KSBP. the parallels are striking: wet/moisture and/or landing gear lowering, seems to be happening around the same time as this issue appears. I’d be curious to check out the diode resolution. does anyone have a photo of their wiring with a diode installed?