Generator Voltage Fluctuations

[Bryce]

Following further problems with our generator the system voltages, as measured on a Pilot III GPS in flight, have become erratic.
Previously the voltages observed have steadily risen during each flight indicating a max of 15.2v after approx 1 hour airbourne.
Now after the generator has had new brushes fitted the generator light stays on until 5 mins airbourne and the voltages slowly increase but fluctuate from 13.1v to 15.1v in 3 to 5 second intervals. The battery was charged 4 hours flight time previous 5 days previous.

This overhauled generator was fitted approx 100 hours ago together with a new voltage regulator following previous system problems when the generator lamp would not extinguish until after 5/10 mins airbourne and the battery was not being charged.
The new brushes were fitted when the generator light would not extinguish and our engineer on inspecting the generator detected burning /electrical splashes at the brushes.
Has anyone any ideas about the brushes failing on a newly overhauled generator and could this have caused other damage to make the voltages fluctuate.
Thanks
Bryce

[Bruce Fenstermacher]

Something is not right. Your generator light should go off as soon as the generator has enough rpm to develop enough voltage to pull in the relay in the regulator.

These systems are very simple yet difficult to remember how they work to mechanics who don't see them every day.

The TIC170A has a REALLY GREAT book published (it's a chapter in a bigger project actually) on the 170 electrical system. How it works and how to fix it. All the information found in it can be found in various other sources but this one pulls it all together in one place.

You need to get a source of info and do some trouble shooting. I'd start by flashing the generator. It's a simple procedure which has been described here before. If the generator is found to be working then the issue is in the regulator. While these regulators are simple devices sometimes simple devices are the hardest to fix. This is the case most likely if you narrow it down the the regulator. Again replacement regulators have been discussed here many times.

Do a search. You will probably find info on regulators in the same thread as the procedure to flash the generator. And when you do find the thread you may note auto regulators are discussed as being cheap "diagnostic tools". They aren't so cheap anymore. A friend just bought one at NAPA. His discounted price was over $50. He said the price to a customer off the street was nearly $100.

[cessna170bdriver]

Bryce,

Check your wiring. It sounds like your regulator is not getting good feedback on battery voltage. It thinks battery voltage is low so it dumps more current to the field to get the voltage up. An intermittent connection at the battery terminal of the regulator will cause system voltage fluctuation. A poor ground between the regulator and airframe will cause similar symptoms. If you can, measure the voltage at the battery terminal on the regulator when you're getting system voltage fluctuations.

A friend here recently replaced his entire charging system one component at a time while troubleshooting, only to find a wire with intermittent continuity.

If you really want to do it right, join the Association and buy the Electrical System book.

Miles

[Bruce Fenstermacher]

Yes Miles is right. Look for the simple stuff first.

After installing a rebuilt engine after taking the time to clean and paint the firewall for which I removed the regulator, my charging system would not work. Local experts and I spent hours going round and round trying to figure out what was wrong till someone said "did you ground the regulator."

Sure enough that's all it took.

[jrenwick]

Zeftronics makes solid-state regulators. Their site, http://www.zeftronics.com/, has a "Troubleshooting" link with information not just about their products, but covering generator and alternator systems controlled by generic regulators. I've learned to look there first when I've got a charging problem, because the step-by-step instructions have served me well a couple of times now. Check it out; I hope it's useful.

Best Regards,

John

[blueldr]

Also, be sure that your generator and regulator have matched outputs.
There have been occasions where regulators have been replaced without proper consideration for matching the outputs.
Actually, I believe there are only two output sizes used, 25 and 35 amps,
but they require different regulators.

[GAHorn]

Note: This post was edited on 1/13/21 deleting the 20A regulator sold by Autozone due to changed specs.

Vibrating-point regulators are temperature sensitive. If your regulator is increasing voltage as the engine compartment temps warm it up......
After inspecting the entire system for integrity, I'd try another regulator. The AutoZone VR605 25A unit is $19, and the 35A VR699 is still $39. (These are only suggested for trouble-shooting purposes, as they are automotive units that perform exactly as the aircaft units. Be careful not to accidentally switch the cover plates, or the original Delco Remy cover might get onto the auto part and you'd then be confused as to which one is which.)
Don't forget to polarize it after installation.


1959 CHEVROLET TRUCK 3200 1/2 TON P/U 2WD
Normal Stock
Special Order



Brand/Amps/PN/Warranty/Price

WELLS
25 AMP GENERATOR VR605 3 MO $19.99

WELLS
35 AMP GENERATOR VR699 3 MO $38.99

[N1277D]

I've noticed that on long flights in the 170 like 6 plus hrs, that the charging rate tends to increase (long flight back from Alaska staying in front of a weather change). I've attributed this charging pattern to temperature effects. Finding the cause of electrical problems takes time. Some are very tough to identify.

We had an intermittant charging problem in the Stinson, same generator/regulator combination as the 170. We checked everthing, turned out to be a bad condensor - a noise filter someone had installed. Then the problem returned, then complete charging system failure. All the electrical tests checked ok. The only thing it could be was the generator shaft not turning, and sure enough that was it. Major panic set in, was there some problems in the accessory case, rotate the prop generator armature did not move. Yikes thinking major $$$$ to fix, but turns out it was the rubber coupling bushing between the generator and accessory drive gear that had failed. (Note - The coupling in the Franklin engine is different than the one in the C145) The coupling failure probably caused by the generator working properly again after the orginal fix.

In this case we brought one item back to specs and it stressed something else and caused a second failure of a marginal component. After these experiences we tend to inspect and look for what will also fail after correcting a marginal component.

[Green Bean]

Gahorn- Your not advising to install a non-approved part on an aircraft are you?

[GAHorn]

Of course not! That's why I warned against getting the virtually identical regulators mixed up with each other.

[Bryce]

Thanks
Bruce,Miles, John,BL and George.
Our engineer followed the advice and determined the year old voltage regulator was at fault.
We still cannot understand why the generator brushes required changing after 100 hrs flying on a completely overhauled unit but with new brushes and the new genuine Cessna voltage regulator all is well.
The gen light goes out at 1300 rpm and on a flight of 3 hours the GPS III Pilot reading remained at 14.3 to 14.6 volts.
The previous 15.6 volts readings did not occurr and this was something I was always concerned about.
Thanks again.
Bryce

[GAHorn]

Bryce, it's likely that your previous (faulty) regulator overworked your generator (resulting in the high voltages) and burned up your gen's brushes. That would explain their short life-span. (Although, it's also possible that a rough commutator and/either/or incorrect/too-strong/too weak brush springs would also cause excessive wear of the brushes and would also manifest itself in "jumping" bruses causing lots of sparking and flashing. Brushes are also sensitive to composition.... too soft brushes will wear rapidly and too hard brushes will cause excessive wear of the commutator. It's important to use genuine mfr. replacement parts for such items...not something the local garage might realize.)