View Full Version : C402C Voltage Reg / Alternator Mystery Fault

16th Oct 2009, 15:34
I'm writing this on behalf of a 71 year old engineer who has a total aversion to computers.

We are preparing a C402C for a ferry flight from Indonesian to Australia , the aircraft hadn't flown for 18 months due to a number of electrical problems that the local engineers couldn't figure out.

The Australian engineer has spent 3 days resolving the following problems

- mismatched alternators on either side of the aircraft , 100A and 60A , now both 100A

- faulty wiring including the LH alternator being wired directly into the field of the RH alternator, it took about 2 days to go over all of the wiring and return it to spec in accordance with the wiring diagram. There were a number of incorrect connections which had burnt out the diodes on both of the original alternators (now replaced).

The current fault that has him a bit perplexed is this. When testing the LH alternator and setting the voltage regulator to 28 volts at 1,200rpm and then loading it with 15 amps for 2 mins as per the maintenance manual and then engaging the RH alternator with the engine off everything is fine and both voltage regulators show 28 volts. As soon as engine rpm goes to 1,500rpm the voltage goes to 31 volts and trips the over voltage switch taking the alternators off line , we have tried 4 different voltage regulators including a brand new unit.

Has anyone run across this before ? I don't rule out the possibility of a further wire being hooked up incorrectly (even though he's been over it a number of times).

Any suggestions of what else we can look at would be appreciated.

16th Oct 2009, 15:45
Sounds like plane still has a number of gremlins residing in the airframe and engines.

First and foremost... be safe, and don't take any chances.

You may want to look up the Cessna Rep. in the area.

Contact Us (http://www.cessna.com/contact-us.html)

An alternative would be to look up a Teledyne Rep. too, as it is their engine on the airframe.

Welcome to Teledyne Continental Motors - Contact Us (http://www.genuinecontinental.aero/contact.html)

You should be able to make some progress there.

Good luck with the Utilitwin... great plane to fly:ok:.

16th Oct 2009, 16:12
I have personally noticed this only one time, however...

Baring any further mis-wired electrical connections, it is entirely possible that one alternator (although 'new') has a partially shorted field, and in so doing draws excessive field current, forcing the voltage produced by the faulty alternator to trip the overvoltage relay.


The load paralleling wiring between the two voltage regulators is either incorrectly attached, or has a short to ground.

I would check the second option, first.
I personally own one of these 400-series twin Cessna aircraft, and they are a fine piece of flying machinery...if maintained properly (by someone who knows what they are doing).

16th Oct 2009, 19:02
My background is electronics. I know zip about aircraft regulators but if they are anything like the regulators I have used..

Check any earth connections/wires are ok. Bad ground reference at the regulator can cause over voltage sometimes.

displaced gangster
17th Oct 2009, 03:44
In an earlier life I was operating a C310R in a remote location, in Northern Australia. Every 5-6 weeks the aircraft was flown to a maintanence facility for a 100hrly service/inspection.During one particular visit the the E&I tech (cone head) elected to balance the load sharing of the two alternators,from memory there was a 5-10 amp bias. Five days and $3500 later, the issue was resolved, however the tech had no idea what the problem was he simply replaced components. Interestingly the 5-10 amp bias remained.:*

18th Oct 2009, 02:24
Yesterday he checked the ground connections and that was all ok and then he disconnected the RH alternator voltage reg from the LH voltage reg and guess what ? The LH side is working like advertised , holding 27.5-28 volts output and as rpm increasing the VR was sending between 11 to 15 volts to the alternator field depending on rpm so the problem is on the RH side.

Same test with the RH side showed 31 volts , suspect there is unregulated 28 volts somehow getting to the RH alternator field circuit but the connection is not at the alternator. Probably a further incorrect connection somewhere within the bowels of the airframe.

To be continued ....

18th Oct 2009, 14:58
411A, my late neighbor had a 411 he sold repeatedly to "informal importers", then would buy it back at a sheriff's auction after they were caught. It was a much better built plane than the Cessna singles.

He didn't make a fortune at it, but lived an exciting life - on the edge sometimes. Heck, one time the perps came back into our local airport with law enforcement in hot pursuit. This place had just 1600 feet after the displaced threshold. They landed and ran on foot, only to be found soon, hiding in a Kentucky Fried Chicken.


19th Oct 2009, 05:39
He found the problem , it was a corroded connection to the RH alternator shunt in the area of CB panel in the cockpit.

Now both alternators working as they should ! :ok: