Electrical Failure on Take Off

[Dale Campbell]

I just flew up to Albany yesterday and had a power failure. I
had not flown for a number of weeks and the battery was low when I started engines at home base. I flew to Lake George and landed. On restart battery was back up to full charge. I then flew on to Albany. When I started engines again all was fine. I returned to Lake George and landed. On starting engines all was fine. I took off, pulled up landing gear. Gear went in fine, doors closed and I got a green light. Still in climb mode with full power on. All instruments went dead. Lost all electrical power, and both 30 Amp breakers poped. I turned off master switch and Alternator switches and pushed both circuit breakers in. I turned master and alternator switches back on and both breakers poped again. I leveled off at 3,500 feet and reduced power to 25 inches and props to 2,500 rpm. I once again checked all other breakers and I pulled out landing gear breaker since it happened when gear went up. I turned off master and alt. switches again. I waited about 3 minutes. I reset both 30 amp alt.breakers, turned on master and alt. switches. I had all the charge lights on. Front,Rear,Disc, and low voltage all on. I pressed alternator restart switch nothing happened. I was getting close to Albany,so I let them know I was having electrical problems I was working on. I flew north of their control area to stay clear and continued for another 20 minutes. Atc told me they were losing transponder. I started losing total battery power. At that time I tried again and this time I was able to get the alternators back on line. All the alternator error lights went out accept the alt.disc. light. The ammeter was showing a good charge all the way home.
This was a day flight with 50 mile visibility which is why I kept going. I also turned on my portable Garmin 295 and my handheld nav/com just in case I needed them. The mechanic did not look at problem yet. Any idea as to what happened.

[Pete Somers]

Hi Dale
Sounds like you had fun and games?

1.Check behind the circuit breaker panel, make sure there is no signs of corrosion or damp.

2. Check the warning panel in the front engine bay, top firewall center and the diodes to make sure there is no corrosion.

3. I have had a HI/LO voltage sensor box fail, which put a Hi volts light on for a second or two, but this will automatically disable both alternators, we did find a circuit breaker poped, but put this down to FOD thru the rear alternator.
A replacement HI/LO sensor cured this problem.

If the gear motor caused the problem you would have noticed the high discharge on the ampmeter.

See how you get on and let me know.

Regards
Pete

[big al 08]

three years ago mind did the same thing. after t/o the starter cylnoid stuck on and burned up the starter, boiled the battery and a complete electrical failure, had to pump the gear down. replaced the starter, battery and the cylnoid. mine is a 73p and none of the cylnoids have ever been replaced, after that i changes all three and have never had a problem with them sence. these cylnoids as they get old start to stick, they are only 30.00 and very easy to chang out. also sometimes the alt circuit board get old, corrorded and can even crack with will cause intermittent elect. problems, wear down the batt give gear problems and finally break the alt coulp, had that happen too. this fleet is really starting to age so don't be supprised what can crop up.

[Dale Campbell]

In the last 2 years, I have replaced all the solinoids on fire wall, new battery, all the diodes on fire wall, replaced the old voltage regulators to the new parallel system, replaced both alternator couplings, installed both new alternators and one starter also new motor on landing gear power pack. along with many other parts. This plane has always been hangered, so shows no corrosion anywhere. Total time airframe 2800hr., total time re-built engines 950hr. Dale

[Pete Somers]

Sounds like you have covered most items.

Have you still got a DIS light on even though both alternators are ok now?

If you have the the HI/LO sensor is u/s

Pete

[Dale Campbell]

Yes, I still have the red disc. light on. Dale

[WebMaster]

Where are the Diodes, and what do they do?
Mine is overcharging, I suspected that the regulators are gone, the rear one was, and now the front one seems to be.
Could it be diodes?

[Mark Hislop]

Dale:

On my aircraft, the red light is "over voltage", not discharge. If you are getting an overvoltage condtion, it will trip the alternators off line. I would first check the output of each alternator with a voltmeter, one at a time. I would also check the diodes (both the little signal diodes and the large power diodes) on the circuit board on the firewall. This is very easy to do with a volt meter as well. If any of the diodes are bad, you can replace them with identical components from Newark Electronics for a fraction of the price compared to Cessna.

Mark

[SkyKing]

Dale,

When is the last time you thoroughly checked the battery, the cables and the connections at the studs? AND water level?

SkyKing

[SkyKing]

Larry,

Diodes are rectifiers consisting of a semi-conductor crystal material... they convert alternating current (AC) into direct current (DC). Your airplane only uses DC, unless it has an inverter for supplying AC voltage to the electroluminescent panel, if so equipped.

There are different diodes for different things... and your 1969 bird is a whole lot different than my 77 P-model, so I won't go there. Maybe look into your Parts & Service Manual for the electrical stuff would be a good place to start.

SkyKing

[Mark Hislop]

Larry:

There are two types of diodes on the circuit board, located on the top center of the forward side of the front firewall. There are two large power diodes mounted on heat sinks in the center of the circuit board. There is one for each alternator. They act like check valves, making sure that current only flows out of the alternator. That way, if one alternator fails, the current from the other alternator will not feed back to the failed unit. There are also some small signal diodes on the circuit board. They act in such a way so as to determine whether the battery voltage is higher or lower than the voltage put out by the alternators. It is this circuitry which controls the illumination of the alternator discharge and battery discharge lights on the instrument panel. There are also two large capacitors mounted under the circuit board, and they tend to dry out and fail. All of these are components that are easy and cheap to replace.

Mark

[Guy Paris]

Dale,
My POH for a 77 say's the red light is an over voltage sensor any time electrical system voltage exceeds 32 volts. (section 7 page 7-41) states to turn the alternator switches on separately to see if one or the other (front or rear) is the faulty system. guy

[Pete Somers]

Hi all
OK, if the RED HI light is on and you had an overvoltage, you would notice as some electrical equipment would fail, and a high indication on the ampmeter would be a clue.

However if the RED HI light is on all the time even when the engines are shut down, the HI/LO sensor is inop.

This unit is mounted on the port side of the center console, top above the HYD Power pack.

The big diodes-
They are wired into both alternator output and a reed relay is connected across the diode.
In normal operation the diode is forward conducting and the relay sees about a 1 volt drop across the diode and remains de-energized.
Should an alternator fail the votage drop across will be approximately equal to the buss voltage and the relay will energize and put a ALT OUT light on.

The HI/LO sensor is field repairable however, they do not cost all that much.

Hope this helps

Pete

[Dale Campbell]

HI GUY AND OTHERS,
I just ran up to hanger to check plane on lunch hour. Guy, you are right the red light is the over voltage light. The one that is now staying on is the DIS amber light. I started both engines on ramp and did some testing at 1500 rpm. I tried first one and then the other alternator switch on and both give the same output on ammeter. I turned everything off while keeping engines at 1500rpm and ammeter shows a good charge but the DIS light stays on all the time. I will go back saturday and pull cowling and check all diodes and solinoids with a VOM meter. I have found many problems in the past myself. Dale

[Pete Somers]

Dale

If the DIS lite is on all the time, it still sounds like you have a HI/LO sensor u/s.

Unless there is a problem behind the warning panel.

Let us know if you find the problem.

Regards
Pete