Generator Current Issues

[brandon7766]

I recently purchased a C140 and noticed that the generator is not outputting a constant current on the ammeter when loads are not changing. In flight (~2400 rpm), the ammeter is bouncing from 0 to +15 amps rapidly. Any thoughts as to what is wrong with it?
My initial thought was something with the voltage regulator. Would a faulty regulator make the current fluctuate from 0 to +15 amps?

[6643]

The ammeter should show current into or out of the battery. It is not a load meter.

Do you see this behavior all the time? How much load are you running? Have you tried switching everything off, or adding more load to see if the behavior changes? What is the rated current of your generator? Does the regulator match? How old and what condition is the battery? What voltage is the bus running at?

Generators are current limited by the regulator, If the total current (battery charging plus load) exceeds the rating, the regulator will turn the generator output down, which lowers the output, which causes the regulator to turn it back up, which exceeds the rating, which causes the regulator to turn it down; wash, rinse, repeat.

Fifteen amps into the battery is often a sign of a battery with a weak cell Add to that 10 amps for radios and strobes and you've overloaded a 20 amp generator.

[6597]

Sticking contactor points on an old vibrating regulator? Sort of same symptoms I had eons ago.

Faulty old rotating Grimes beacon motor contacts (again, actually happened to me)

[brandon7766]

The behavior was all the time no matter the load when the generator was working (above ~1200 rpm or so).
My A&P noticed that the gaps internal to the voltage regulator were not up to spec so we just replaced the voltage regulator with one he had laying around the shop (they have done a lot of alternator upgrades over the years). Things seem to be working much smoothly now. I noticed that my grimes beacon is rotating rather slowly as well. I will likely try and take a look at it next time we are doing maintenance because it draws a lot of power when on.
Thank you guys for your recommendations and help!

[6597]

Check out the winter 2020 newsletter for a ditty I wrote on the coffee grinder Grimes rotating beacon.

https://cessna120140.com ->Resources->Newsletters->2020 Winter Newsletter

I also put in LED's and it reduced the draw substantially.

[wingspinner]

The ammeter should show current into or out of the battery. It is not a load meter.

Hi John,
Supporting your comment and to explain why: Based on the schematics the generator is wired in parallel with the aircraft loads on the same side of the amp gauge as the loads making the gauge neither a true load meter or alternator gauge. Given the factory wiring, it only displays the the discharge or charging rate of the battery only.

For airplanes with low capacity generators like mine (12.5 amps) it’s particularly useful because it displays when the battery is discharging due to the aircraft electrical loads exceeding the capacity of the generator. This is a good indicator that the pilot should consider turning off non-essential loads or risk running down the battery prior to landing.


- Ron

[ghorn]

A “quick and dirty” method to troubleshoot a generator/regulator system…is to use a jumper and “ground” the Field terminal on the generator. (this will be the small terminal on the generator.)

If you get a steady good charge with the engine running near cruise power….the generator is good. If you do not get a charge the gen is likely failed….or you have a failure of the wiring system (gen/reg/master-sw wiring).

An easy place to ground the field terminal can also be to ground the regulator “F” terminal…which does the same thing…but also checks out the wiring thru the cockpit master switch. (The cockpit master sw connects the gen to the regulator “Field” terminal…..that’s how the regulator controls the generator output.)

If a generator HAS been working…and suddenly is not… (maybe you changed the battery…or the airplane has sat for awhile)…after confirming all wiring is intact…but before starting the engine… “Polarize” the system by:
1- Master Sw On (but engine NOT running)
2- MOMentarily connect the regulator ARMature terminal to the Regulator BATtery terminal. Only momentary …not continuous. (A visible “spark” is OK.)
3-Start the engine and perform a run-up to see if the system now charges.

[6930]

Brandon,

I had a similar issue. When all was working as it should I was seeing a relatively constant plus 5 or so amps with some jiggling as the Whelen flashers cycled (two installed, top and bottom). However, the system developed an intermittent problem where the ammeter would momentarily spike down to minus 10 or so amps. My transponder and GNC 250 XL would intermittently recycle too. Obviously, something was wrong, and it was really baffling.

I replaced the voltage regulator with a new Zeftronics unit which, as it turns out, I needed to do anyway. Unfortunately, the spiking ammeter issues were still there.

Turns out the problem was corrosion in my belly Whelen flasher. Changed the base and bulb and all is good.

Hope that helps.

[bill_e]

I am also having charging system issues. Low bus voltage while in flight that seems enough to run the lights and radio, but isn't enough to charge the battery. In starting to troubleshoot, I'm seeing some creative wiring behind the panel that really needs to be cleaned up. Something about 77 years of swapped radios, intercoms, and who knows what else. I'm currently referencing the Cessna 100 series maintenance manual schematic for the 150 electrical system since it appears roughly similar. Is that my best reference for how things should have been from the factory, or is there a better one out there? Obviously, I'll have differences between it and my specific airplane but am looking for the best starting point of what it should have been originally.

[6643]

It's not a proper schematic, but figure 19 of the Operation Manual is close.