Alternators off line

[hharney]

Aircraft is a P337 1977 G Riley conversion

The alternators would go off line if the cowl flaps were selected but was intermittent. Some connections were checked and terminals replaced on the cowl flap switches and everything appeared fixed. After a second flight the landing gear was selected down and the alternators went off line again. Now in the shop the gear selection takes the alternators off line even with the CB pulled for the gear pump. The alt CB does not trip just alt go off line. By selecting the master off and back on brings everything back.

Any thoughts out there?

[Mark Hislop]

I would guess that the OV relay is causing the alternators to go off line. On my aircraft, it is located on the right side of the center pedestal, up high near the firewall.

[edasmus]

I love electrical issues.....(kidding)

I have had similar issues although not having anything to do with the cowl flaps. Herb, you need to ask more questions. You say the alternators drop off line. Is the over-voltage sensor tripping the alternators off? In other words, is the over voltage annunciator illuminated after the event? It sounds like it is because as you say, turning the master switch off then on resets the over voltage sensor.

I do know that I cannot solve this problem but I can tell you what my IA uncovered when I had this problem. The first thing he checked was the voltage across the entire system. We went flying to do this and he had me running all sorts of appliances. The gear went up and down many many times. The voltage should be darn close to 28.4 if my memory serves. Please verify this. Apparently the voltage regulators can wander and should periodically be checked to make sure they are keeping the voltage at the proper value. My voltage had crept up to near 31 volts which is way too high. The over voltage protection should trip the alternators off line about 32 volts (again please verify). As you can see, any momentary spike in voltage such as operating the power pack for the gear, or maybe even the cowl flap motor for all I know, can exceed the parameter for the over voltage protection therefor tripping off the alternators. Once the regulators were "dialed down" a bit to get the voltage correct, the problem went away.

In addition to accomplishing this task, my IA replaced my master and alternator switches as they showed signs of arcing. He also installed a diode somewhere in the landing gear system to dampen the inevitable voltage spike that occurs when the landing gear power pack is called upon or I believe more accurately, when stopped being called upon. In fact, I could hear a "pop" in my headset when the power pack stopped. After this corrective action, I have had no further issues or popping in the headset and it has been about 100 or more hours now.

As I write this, my IA is installing a digital buss voltage readout in my panel. I want to know exactly what the voltage across the buss is at all times. I hope to be able to see when the regulators need an adjustment before they get too far out of whack as well as knowing that the battery is getting charged at the proper voltage so as to not induce any boiling or insufficient charging.

These issues are never fun. I would suggest finding a tech who is extremely proficient in electrical issues on the SkyMaster. It will be worth the savings in grief alone. In the meantime, be as precise as you possibly can in describing the symptoms.

Good Luck,

Ed

[Red Air Rambo]

I had the same problem and we think we got it fixed by replacing both voltage regulators with new solid state ones. I say I think because I've only flown it a couple of hours since then but no issues yet. I talked to Don Niesser and he said that he's experienced it too and you need to replace both because one goes bad and starts sending stray voltage causing other things to fail. He had some in stock I think for $155 which was cheaper than Cessna which I think was $180.

[Cloud9sa]

We received the replacement voltage regulators from Don Niesser and installed them, but the problem remains. We get a high voltage warning indication, kicking out the alternators, occasionally when we cycle the gear or cowl flap motors on the ground (with a battery charger attached to the battery), but the problem happens most of the time when they are cycled in flight. We checked all the switches and relays in the landing gear and flap motor circuits, and they seem OK. In addition to getting an intermittent high voltage indication when the cowl flaps or landing gear (down) switches are actuated, sometimes the gear doors don't close when the gear legs lock into place, suggesting low voltage. We have a digital system voltage readout, and it does not indicate high or low voltage, just normally 27.7 v. Any ideas? Faulty overvoltage sensor? Something causing instantaneous voltage spikes and dips when switches are actuated? Thanks, Tracy 1977 P337G

[jhickam]

I have had the same problem and I am thinking it is a bad master relay. My problem is very intermittent maybe three time in the last two years. The problem appears to go away with more use. I will changing mine out at annual the 1st week of March. I will let you know the results.

[Jerry De Santis]

Hi Tracy

A battery not up to par will do some strange things on a skymaster. Before you go wild checking out everything, A new battery is the best thing to do since in general, they should be replaced more often than you think. On average, I get about 18 months of useful battery performance then I get a new one to avoid things you are experiencing.

The voltage reg could be checked out very quickly. Just keep one alt off line. If the problem continues when you test each that way, then it is not the volt. Reg.

I suggest a new battery

Good luck
Jerry
BTL

[Ernie Martin]

I agree with Jerry that the battery is often the problem. I get more like 3 years on mine, but I "recondition" them every 9 months or so. Take it out, fill as specified with distilled water (you will find several cells way low), then trickle charge overnight at the 2 amp rate. Most 24V chargers have an automatic 2 amp rate.

I actually go further. I downloaded the online battery charging directions from Gill, read that there is an optimum voltage that should be maintained during the trickle charge, and (after some calculations and testing) devised a line of resistors (bought from Radio Shack) that I put in line between the battery and the charger. I do it during the day, rather than overnight, and as the battery charges I measure voltage at the battery posts and adjust the resistance to provide the optimum voltage. In the first 4 hours or so, I measure and change resistance roughly every hour, then make a final resistance adjustment about 3 - 4 hours later, then there is no change needed thru the final 6 hours or so.

Gill and others sell constant-voltage chargers that do this automatically, but they are expensive. With $5 of resistors and a little TLC I accomplish the same thing.

Ernie