What say we tar and feather the guy that designed the electrical system...

[kevin]

Hi folks,

I am having an interesting experience with my electrical system. You know, like the chinese curse: "May you live an interesting life."

Aircraft is a '73 P337. Electrical system is all stock, plus an EI volt/amp meter wired so that current can be measured for each alternator individually, or for the system. Voltage cannot be measured individually, except by turning off one alternator or the other. Oh, and I have an EI Superclock with voice annunciator that monitors bus voltage all the time.

Last week, I started to see the rear alternator drop off line after landing. This is visible ONLY on the EI gauge, the stock Cessna stuff was happy, but the EI gauge would show only .1 or .2 amps and after a bit the whole system would go into discharge (again, only on EI gauge (yellow discharge light), the Cessna idiot lights were all out. I checked the idiot lights by turning off one, then the other, then both alternators, and all three lights (left, right and discharge) work as expected. Anyway, I can't remember all the symptoms, but everything just seemed sick and wrong.

So my mechanic and I attempted to rebalance the regulators, which is the first thing I look for when the system is sick.

BTW, I have one really old regulator (three adjusting screws) and one relatively (1 year) new one (one screw).

Anyway, in the course of troubleshooting, we discovered that the rear alternator was in fact going off line, as evidenced by an external voltmeter. After it went off line, whacking the regulator (the new one) would get it to start working again. We decided to replace the regulator (and balance the system). After doing that, all seemed well for two or three flight hours.

On my flight to Canada and back today, I noticed the rear alternator was only putting out 0.1 amps. I did not screw with it, as I was IFR, and needed my electrical system. But I did determine that it seemed unaffected by load. The front alternator was taking up the slack just fine, putting out about 14 amps.

I shut down when I arrived, to drop some pax at the terminal. When I started the airplane again, everything was suddenly working correctly. The rear was not putting out a lot of current, maybe 2 amps, but it was contributing. Mumbling under my breath (because I had already called my mechanic and he was on his way to the airport to work with me on it), I shut off the avionics master in preparation for shutting down. The FRONT alternator went completely offline, zero amps, and a front alternator idiot light. Turn the avoinics master back on, the idiot light goes out.

My first theory was that the regulators were unbalanced again. The voltage on the front alternator was 27.4, the rear 28.2. So we started to balance them again, by changing the adjustment on the new regulator. (We have been trying to avoid touching the old one, as they are harder to adjust.) We reduced the voltage on the new regulator to 27.4 or so.

Starting both engines, now on the ground the airplane behaves exactly as it did in the air. Here is the sequence:

1.) Both engines running 1500 RPM. Front alternator putting out 6 amps or so, rear 0.1 amps. Turn off front alternator. Rear alternator does NOT pick up the load, rather, it rises to 0.9 amps for a few seconds, then drops to a 1 amp discharge. Turn on the front alternator, the system returns to original condition, 6 amps on front, 0.1 on rear.
2.) Whack the brand new regulator. Rear comes up, starts putting out expected amperage (4 amps or so), front is putting out 2 amps.

We tried some more things which I don't remember, but basically it always came back to 1.) and 2.) above.

I am dubious, but we decide that we suppose we could have gotten two bad regulators in a row. We decide to swap the front and rear regulators. We do that.

Again, both engines running 1500 RPM, at first, the regulators just look way out of balance. The rear (now the old regulator) is putting out about 27.8 volts, the front (new) 28.9. The front alternator is completely offline (no amps). So we adjust the front (new) regulator down to 28 volts.

Now, at least on the ground at 1500 RPM, everything works perfectly. (No whacks on either regulator required, nearly even current draw, correct system voltage.)

Why? I don't get it. I have not really looked at the circuit, and I don't know more than the basics about electricity and electronics. I don't understand why it works now, and I don't understand what whacking the regulator was about. Neither does my A&P. I pulled the regulator we replaced out of the garbase, as it may be good after all.

I will fly it tomorrow (oh good, an excuse to go flying!), and we'll see how it does.

Any thoughts anyone?

Kevin

[GMAs]

Ahhh... seems that we said to check the drive coupling some time back.... and you may be losing one of yours... either that or the brushes are going south.... check for coupler by doing the fan moving thing... check for brushes by using a Muohm meter.... by placing a volt meter on the brush line too you can tell... if it is up and the alternator is still not puting out... you got bad brushes... if it turns with the engine off... when you put your fingers in the fan... smile...

Ask Mike Swanson about the fan and coupler thing... he had two of them that went bad... one right after the other.... replace with the cont style.... let us know what other problems you come up with.. GMAS

[kevin]

Gmas,

Thanks for the reply. I searched this board, can't find any messages about alternator drive couplings or "the fan moving thing". If you or someone that knows could enlighten me, I would appreciate it.

Not sure why swapping regulators would make everything work though.

I test flew it today for a half hour. Everything is perfect now.

I don't get it.

Kevin

[kevin]

As you may have read in my Oshkosh trip report message, my charging system is still giving me problems. Both alternators stopped charging in the air on the way to Hibbing, MN, and any combination of alternator switches, plus turning of the master several times (along with the alternators) would not make it work for about five minutes. Then, with the front alternator switch off, the rear started charging, so I left it alone until landing at Hibbing. When I reduced power in Hibbing during the approach, the rear alternator went off line. I turned on the front alternator switch, and it started charging just fine.

My first theory was that the regulators were too far apart in voltage again. I found a mechanic, and we started the front engine, intending to start the rear, then balance the two regulators. But the front alternator would not put out anything at all, even with the rear engine shut down. So we shut down the front.

If you read the previous messages in this thread, you will note that at this time I had one new regulator, and one museum piece. I had brought the newer "bad" regulator that we removed with me, and I decided we would swap out the museum piece with the newer regulator, because that would at least make it easier to balance the two regulators (one adjusting screw on each). We changed the front regulator to the newer "bad" regulator, put the museum piece in the baggage compartment. Started both engines, but no matter how far we turned the adjusting screws on either regulator, we could not get any output out of either alternator.

Through a long series of turning things on and off carefully, I determined that if I turned the master and both alternators ON, then turned the master OFF, I would get balanced, correct power (voltage and amperage) from both alternators. But if I turned the master on, both alternators went to zero.

This made me think that there is a problem in the overvoltage protection circuit somewhere (even though I am not getting a light). I decided to give up and move to Minnesota.

After shutting down the engines, the mechanic inspected all the wires that go to the overvoltage/control circuit board. He did not find anything, although there were a couple of noise filter capacitors (large), one of which MAY have been in contact with something wrong. There are two 14A 32V fuses visible on this board, and I asked the A&P to ohm them. He inspected them visually by removing them, but ohmed them *in circuit*, which is not what I wanted, but he was holding the meter. We found nothing wrong with them, but put them back in swapped - one where the other was and vice versa.

Now I was SURE I was going to be in MN a while. We cowled the airplane again, and I decided to run it one more time, to make sure that moving the wires and fuses around had not changed the behavior.

Everything worked perfectly. I mean *really* perfectly, nearly balanced voltage and amperage out of both alternators, even a low RPM.

Hurray, I don't have to move to MN, but boo, I still don't know what is wrong with it.

The next morning, it still worked perfectly. On the trip back, I kept checking the amperage output from each alternator. During runup in Hibbing, it worked great. Early in the leg from Hibbing to Dickinson, the front alternator output went to zero, but the rear was fine at 20 amps. Later in that leg, the front started putting out 17 amps, the rear 3 amps. For the remainder of the trip, both alternators continued to work, with the share between each varying - sometimes high front/low rear, sometimes high rear/low front, sometimes (less often) balanced.

It got me home, for which I am very greatful. But now I am not sure how to proceed.

Anybody have suggestions?

Kevin

[Bob Cook]

Kevin
You just don't live right.......<G>
I suggest you sell the aircraft and buy another one, however, now everyone knows your problem you probably won't be able to sell it until the problem is fixed..........

here are my suggestions;

You are an intermittent problem.... worst kind. You need an exorsist or medicine man!

1) make sure both regulators are the SAME!. There are two types of alternators. make sure the regulator is for the type of alternators you have.

2) sometimes the diodes and caps are not soldered to lugs and corrosion occurs on the lugs. Make sure they are soldered. For the cost i would replace them all.

3) Check all connections with ohm meter including field breakers and main fuses. Make sure the fuses make good contact.

4) Check grounds on alternators and straps from engines.

6) Make sure all connections are CLEAN and TIGHT.

7) load test your battery. If you have bad connections or bad internal connection to plates the battery will open and overvolt COULD POSSIBLY occur.

I do not believe it is the alternators, it is connections. Also check the main isolation diodes (big ones) on the firewall. Good connections. If all fails I would replace these two as well with 70 amp 400 piv versions. GMAS can give u the generic part numbers.

Glad to see you made it back safely.


Bob

[GMAs]

Way back when we discussed this on the other site... I spent 7 long articles on how to check the system and how it worked... and showed that it worked well... some of the oldies might remember... but, a good mechanic should be able to go to the area of the problem...

If as you say you pulled the fuses and checked them and they were OK... then the system worked for a while... I would have done it again.. except that this time I would have cleaned up the fuse holders.. hint...hint... which have a tendency to get tarnished and can cause you not to get any voltage back to the alternators to make them take off....

Yep that is a problem area... best to take the fuses and throw them away.. put new in because sometimes the lead connection inside gets bad too... check the wires on the fuse blocks... check the screws for rust and corrosion... dont just spray it with magic cleaner... remove the parts and clean with something like a pad...

Now if the alternators were not taking off.. it means that no voltage from the system is getting back to them... here their are only a couple of things... as they are related to the field...

The main master to the ckt breaker... to the field switch on the dash... thru the overvoltage limiters...thru the fuses thru the connectors on the regulator plugs... to the regulators then on to the fields themselves...

The return ckt is....

from the alternator thru the cable to the blocking diodes to the master to the battery...

so what part is not working...

First start with the alternators themselves... look for a mechanical problem.. they have to turn... so the coupler is first to check... then the next thing is the brushes.. for the field... these can be tested with a meter... then the wires that go to the alternator.. look for broken wires at terminals...

At the other end of the plane.... the overvoltage regulator is only going to trip out the relay inside it when the voltage gets too high... ya the contacts could be dirty... and as we have said before.. the planes that goe way up their... have a world all their own that they go thru... with moisture and pressures being the bad guy....

The field switch is another problem area.. with P and T planes... again because of the pressure... but, these can be tested... I again wrote an article after talking to the cessna person I found who used to design the systems.. and test them... he had lots of helpful info.. that I passed along... on how they trouble shot the system... just using a volt meter...

But, when everyone keeps putting more on the system and adding instruments that can present problems to the system... it sure isn't going to work the same as it was designed....

So that leaves the stuff out in the engine compartment.. the regulators and the diode/relay board... with the fuse holders and connectors being the biggest problems... (fuse holder being the biggest problem...) as it will corrode..... and it only has 24 volts at 5 amps max that it has to handle... that being 100% output from the system.... from their the amps go down to about 1 each... so if their is any resistance... the sense would say no voltage but, the alternator is not getting its field voltage... kinda thing...

First stop.. the shop manual...

Second stop... voltage checks... around the system...

Third stop.. back here with some data that we can make a determination about... (look at the fuse blocks... hint hint...)

Forth is not to go fiddling in the system (and that means put the adjusting screwdriver back in your pocket... 99% of the time its not the regulators... its other parts of the system that they can't handle...

So you need voltage to make it happen... sort of thing... if you are not getting voltage from the buss... then your not going to generate any from the alternators... kinda thing... so go out to the plane and get the mechanic to take readings... without the enignes runing... (again you can shoot the system without having the engines on... till the last Part... mechanical problems)

Let us know how you do... I have been asked to hold a meeting out here in the west for just that thing... electrical charging system... and I think we need to have one so that you all can get caught up on the right way to trouble shoot it... after all Harry Bowdish is going to be hairless if he gets into a bind too... and then his wife will have to send him to the hair club for men... ever notice how they have good looking young gals in their adds... with these old bald men... smile... GMAs

[kevin]

My avionics guy, who is also an A&P/AI, helped me troubleshoot this problem (first one, then the other alternator going off line, intermittantly). After examining the schematic and listening to my description of the symptoms, he initially suspected the overvoltage protection unit, but we removed that and jumpered past it (on the ground), and the problem remained. He examined the voltages at that point in the circuit and decided the main isolation diodes (on the firewall center in the front, behind the warning light circuit board) were probably the culprit. We found that both diodes were loose in their bases, and one diode had a loose connection on the bottom of the diode as well. We tightened all these connections (easy to say, took five hours to disassemble, tighten, and reinstall), and everything seems to work great now. Since this is an intermittant problem, I won't really know if we fixed it for a while, but I am hopeful because it matches all the facts. "Whacking" the voltage regulator was also vibrating the diodes (right next to regulator), jiggling into better contact. The "laying on of hands" repair in Minnesota was the A&P feeling each wire in the bundle below the diodes, so in fact, he was applying force to the diodes.

Thanks Bob and Gmas for your suggestions, you both included the actual problem in your lists of areas for investigation.

Kevin

I just bought N9CV in Sacramento. It is a pressurized T337G. The previous owner went through virtually the same problem you are having. I will contact him and suggest he contact you to see if he can be of help.

[GMAs]

Ahhhh.. the man from texas.. yes we know N9CV well.. Mike took good care of his plane.. your lucky to have gotten such a good one...

Welcome to the web... and glad to hear that it has a new home and that your one the board too...
Keep us informed about how it goes... be looking forward to seeing it make the meetings.. G.M> GMAs