Generator problem

Delco 35A generator on a C172 with a TCM O-300D. On start up and take off, system voltage is at 14V. As flight continues, voltage slowly drops. After 15 min. it's 13.1V. After 30 min. it's 12.6V. After 45 min. it's 12.1V. After 60 min. it's 11.9V.
Ground adjustment after flying (still warm) requires 2 turns of regulator voltage screw to bring back to 14V. The next flight a week later, the voltage is now at 17V. Adjusting the screw again back to 14V brings back the original scenario. Voltage measured with external voltmeter to bus bar.
Have not trouble shot other than checking that the wiring is tight. Cleaned a few terminals. Grounding is good. Generator commutator and brushes look good. Battery is 4 yrs old, but looses charge after 6 weeks of sitting.

Whadda ya think ???

I'm guessing battery or regulator, as the generator will get to operating temperature a lot faster and thus the problem should exhibit earlier. Either might be warming up and changing its characteristics. I think it's less likely to be the regulator. The fact that the battery exhibits signs of ageing (losing charge when sitting) is probably another clue that it's the battery.

If the battery box is not under the cowling, of course, it's less likely to be affected by heat and thus less likely to be the culprit.

New battery or a new regulator are probably about the same price, but the battery change definitely falls under owner maintenance. I'd do the battery and see if it fixes it. If not, put the original battery back in and stick the new one in a cool place for use when the original battery DOES need replacement.

I've done a bunch of tron-chasing in my Fly Baby (similar Delco system, only lower current) and have put some information up on the Fly Baby web page:

http://www.bowersflybaby.com/tech/spark2.html

Ron Wanttaja

I had a C-125 with the same system. My advice is toss the regulator. The symptoms say the battery is not being charged. Likely the Delco-Remy regulator's internal springs are old and the contacts worn. A solid state Zeftronics regulator is an approved replacement and takes 10 minutes (maybe 20). Two bolts and 3 wires. Your friendly local mechanic should be happy to make the logbook entry.

That said, the battery could benefit from being connected to a battery tender/maintainer if you are going to let it sit for weeks at a time. It is getting old...

Best of luck,

Wes

Quote:

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Originally Posted by WLIU

I had a C-125 with the same system. My advice is toss the regulator. The symptoms say the battery is not being charged. Likely the Delco-Remy regulator's internal springs are old and the contacts worn. A solid state Zeftronics regulator is an approved replacement and takes 10 minutes (maybe 20). Two bolts and 3 wires. Your friendly local mechanic should be happy to make the logbook entry.

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I replaced my old mechanical regulator with a Zeftronics earlier this year, and it was as easy as Wes says. About $200 from Aircraft Spruce.

Zeftronics' installation instructions include some measurements you can take to verify your generator is OK:

http://www.aircraftspruce.com/catalo...icsinstall.pdf

Ron Wanttaja

+3 for get a new SS regulator.

I have a similar C - 172. After troubleshooting the voltage regulator and generator, I would look at a Cessna wiring diagram.

The factory routed the wire from the Field terminal of the voltage regulator through a normally open contact on the master switch and from there to the Field terminal of the generator. A bad switch contact could produce your symptoms.

Also, a bad bearing at the commentator end of the generator can cause these symptoms.

Don't ask me how I know this.

Interesting thing. Did a little more trouble shooting, today. Did a ground run and voltage measured 14.2V. As the run continued, voltage dropped to 13.8V after 5 min. Shut down. Grounded the field terminal at the regulator. Ground ran again. The voltage climbs with RPM increase and is not regulated. Turned on every electrical component and set RPM to maintain 14.5V for approx. 1 min. Shut down and removed ground from field. Ran again and set regulator at 14.5V. Flew airplane for 45 min. and varied the electrical load. All seems to work fine now. Hard exercising of generator with grounded field ?????