Charging System Problems Solved

After many months and a lot of research, my electrical system is now very stable and reliable. I thought I would share what I have discovered and I hope this will help many of you with your electrical problems.

Two years ago, I purchased a 1973 337P and before I could get it home it began giving me fits with its charging system. Luckily, Gmas was having one of his seminars in the Sacramento Area and I was able to explain the problems I was experiencing and he gave me a list of items to checkout. Starting with a new battery and contactors the problem still existed with me unable to keep the rear alternator on-line, so as GMas has described, we looked into the drive coupling on the rear alternator and found that the Lord Coupling had given up and I replaced the coupling with the Continental Coupling. While I had everything apart I took the opportunity to have both alternators overhauled and the voltage regulators checked. Keeping my fingers crossed I hoped that this solved the problem, but no such luck.

Next we discovered that the Over/Under Voltage Sensor needed to be replaced and again I was optimistic that the problem would be solved, but we still had an imbalance between the alternators and pulsing voltage in the system. So, it was on to the diode board on the front firewall, and all of the diodes were replaced and the board circuits checked. I won?t go into detail on how to check and replace the diodes as Gmas has written numerous articles on this subject, but I thought we were running out of things to check or replace, so we had to be getting close to the end of the tunnel, but the same problem persisted. Then it was on to new circuit breakers and field switches and this finally got the system stable and working, but that?s not the end of the story.

After about 3 months all of the problems came back, the alternators would not stay on-line and the voltage pulsing was back. I knew I had replaced or repaired every part Cessna had put into the aircraft at the factory, so what was left to fix? I started calling the different manufacturers of the components and getting their feedback on the problem. After explaining all of the parts that were replaced one thing started to become apparent and that was the aircraft wiring. This system is so sensitive to changes in resistance that the ship?s wiring has to be in top shape.

With this in mind, my mechanic and I went though every wire that was associated with the charging system looking for possible areas where the resistance could have changed. We found that in my case, the ship?s wiring was still in excellent shape, so all we replaced was about a two-foot section of the wiring on the rear alternator. The next thing we did was replaced every connector and or their pins in the entire charging system, making sure that every connection was crimped and soldered. The biggest deal was making sure every connection was soldered, as this will keep corrosion out of the joint which in turn will build up resistance. This sounds like a lot of work but it is just time consuming with very little expense in parts. After all of this was done the voltage was stable out to a hundredth of a volt and finally the voltage regulators could be set and forgot about. So what I have learned from the whole episode is that the components in the circuit cannot do their job if the wiring is not up to the job of flowing a pure current.

In conclusion, if you guys just change out parts in the system without any regard to the wiring, I think you will be only putting a ban aid on the problem without really fixing the problem. But in my case, after about a year of work, I finally have a system that is very stable and most of all reliable. I want to thank everyone who stood by me during this ordeal, especially Gmas, because without your help I would have given up on this project.

[Kevin McDole]

Mike,

Glad to hear to finally nailed it.

I've had screwy electrical problems also - but lacked the patience and perseverance that you did. After one pass through the system using the kind guidance of folks on this board, I stopped throwing money down that hole and converted the system to the series 400 paralleling regulators. Problem solved - and the alternators are now balanced. I've been running about 30 hours now with the new system. (Anyone considering this - make sure you work in advance with the FSDO - they'll have lots of questions and reservations. But if we could get this through Oakland, you ought to be able to get any FSDO to come around).

I suspect the lack of a sense line between regulators in the original 337 design is what makes it so sensitive so to any kind of resistance differences between the two circuits. With feedback (as in the 400 regulators), this will adapt for variations. Of course, the ideal situation is to have clean resistance-free connections regardless of which regulators you use.

[Mark Hislop]

Kevin:

There are two separate problems of "flickering lights" here.

The first is when the alternator under voltage lights flash on and off, indicating that the alternators are not sharing the load. The paralleled voltage regulators will solve the problem of load sharing between the alternators.

The second type of "flickering lights" is evidenced by a flicker in your insturment lights. It is caused by rapid (several times per second) fluctuations in the output voltage of the alternators, which is caused by high resistance in the field or VR circuits (in the field switches, in wire connectors, terminal strips, etc.). The amount of resistance that will cause these fluctuations is miniscule. I have replaced alternator field switches which measured "0.0" with a high quality VOM, and all of a sudden my flickering went away.

You can have flickering instrument panel lights even with the paralleled regulators. The only cure for this that I've found is to keep all resistance out of the critical circuits, paying special attention to crimping and soldering connectors like Mike mentioned.

Mark

All of the aircraft are getting up in age and no one really knows what kind of life the plane has had, but Cessna only used crimped on connectors making them acceptable to corrosion. My concern is until you clean up the wiring you will always have gremblins. I thought about and did research the paralleled regulators, but have had discussions with several owners that didn't really feel that it was worth the paper work and expense to go through.

The bottom line is if you work hard on putting the wiring back to an original condition, the system works as advertised.

Mike

[Kevin McDole]

Mark,

I haven't "enjoyed" the flickering panel lights yet - I'll keep your advice in mind.

I was getting the periodic flashing of the under voltage lights (like once every 5 seconds). The biggest problem I was having was the fact that only one alternator would carry the load until it reached 30 AMPs and then the second would come on line. This meant that often while on the ground (at lower RPMs), only one alternator was putting out current - the other sat at 0 AMPs. Under high loads (with lights and/or A/C) this would cause the battery to discharge. I know the alternator is spinning, but the fricking regulators arent' bothering to use the current it's able to produce. Instead, it decides to drain the battery.

With the 400 regulators, both alternators are contributing current and at 1200 RPM they can pretty much any load I throw at them.

Of course, if you don't have an ammeter (I use the EI), you won't see what's really going on. Ignorance is bliss.

[Damon Banks]

kevinmcdo, would you email me at damon@actas.com.au with details on the 400 series regulator installation?

Thanks,
Damon

[Kevin McDole]

Damon (also sent via email),

I got the FAA 337 form from Tom Carr of Cessna Pilots Association. Tom teaches the CPA Skymaster class and covers all of the different changes to the electrical system over the years.

When he was maintaining a Skymaster for a customer (in the 80's?) he got tired of debugging the Skymaster electrical system and got his local FSDO to approve changing over to the 400 series regulators. Since then he's given out his copies of the FAA 337 paperwork to anyone who asked.

I don’t exactly know how much this cost since I had lots of other things done one the plane at the same time – but I’m guessing it was something like 2 to 3 days of labor, plus several hundred dollars for the cost of the regulators and miscellaneous parts. I strongly recommend the EI Ammeter: http://www.buy-ei.com/VA-1A.htm

I'm going to be traveling this week - going to RTC for recurrent training. When I get back, I'll get a copy of the paperwork & schematics from my mechanic and send them to you. If you're a CPA member, you can also communicate with Tom at: tom.carr@cessna.org

[Mark Hislop]

Damon:

The conversion to the 400 series regulators is easy and cheap (at least in aviation terms). The new regulators mount on the firewall on a small piece of aluminum plate that is very easy to fabricate. The wires to the old regulators reach the position of the new regulators. There is only one new wire, about 6" long, to connect the two regulators together. It should only take a few hours to do the work. The new regulators are about $175 each, if I remember correctly.

Mark