1965 C337 "Alternator not charging" light
 

Hello ALL.
I have a 1965 C337, and the "Alternator not charging" light did not illuminate when BOTH alternator went off line!! I was flying last week, getting my CLT rating. When I looked down at the circuit breaker panel, because my G4 insight engine monitor showed voltage was down to about 20 volts, BOTH C/B popped! I had both landing and taxi lights ON, for about 15 minute, which I guess was too much load for the old girl's electrical system? SO checking the electrical schematics for the 1965 and newer Skymasters and found that the 1965-1967 do not have an alternator failure sensor. SO I ASK- how does the "alternator not charging" light get it power to show that one or both alternator are off-line? I've asked the Cessna Pilot Association Tech help but so far no word from them? Any assistance with this would be appreciated!!
Many thanks. BILLS

Greetings:

The alternator out light get's it's "power" from a differential between alternator and bus voltage. When the alternator voltage is lower than bus voltage the light will come on. There is not a true negative or ground in the circuit, it is the difference in voltage that forms the electrical circuit. Beginning with serial number 337-0756 there is a transistorized over/undervolt sensing unit. This unit has both a low volt light and a high volt light. From 337-526 through 755 the light is between the battery and field terminal of the OVR.

Landing lights should not take out both circuit breakers. Something else is going on. If you dig through the system you will see plenty of places for electrical gremlins to hang out. I rewired my charging system including the circuit breaker panel. I have much more confidence in the system now. I found a few poor connections that had clearly been causing problems for years.

I'll look for a picture of my new panel.

Dave

This is the panel I built. I used the Klixon 2TC2 breakers. The 35's are the 3TC2 breakers. It worked out really well. This was a time-consuming project. This will solve myriad of prior electrical deficiencies.

Dave

1965 C337 "Alternator not charging" light
 

Thanks Dr. Dave. I'll check over the system-as soon as the temperature is a bit higher. My hangar is NOT heated and currently the temp is about 20 with about the same wind speeds. I didn't see the wiring diagram, in the MX manual, showing the "Alternator not charging" light? Do you have it or if not could you draw me the schematic? I checked out your panel VERY NICE! Are all the wires coming from breakers original aircraft wires or are you replacing the wiring as well?
Many thanks, BILLS

Hi BILLS:

The circuit wires on my panel are mostly original. The eight-place circuit bus is for the avionics. I'm doing the Approach Fast-Stack system. All of the avionics power wires will go to that bus and the grounds will go to a dedicated single-point ground bus for the avionics. I ran parallel #8's to feed the battery, these are replacement/upgrades. The original charge wire had a poorly crimped end that was burnt from a poor connection. The alternator feed wires are new all of the way to the alternators.

What is the serial number of your plane? I will look and see if I have the wiring schematic for the alternator system.

Hope it warms up there soon. We can certainly make a list of things to check so you don't spend too much time in a freezing hanger.

Dave

Dave

1965 C337 "Alternator not charging" light
 

Hey Doc. My serial number is 337-200. A 1965 model. You know that when I was looking to buy this airplane, I did the PRE-BUY, I'm an A&P. I noticed the wiring from the regulators, looks like lamp cord. Insulators around the wires where all cracked. Last year I had a diode problem that took about three weeks to track down. I was thinking of installing Zeftronics regulators and possibly an updated alternator sensor that is installed on the 1968 models :)and beyond? I have a C337 parts bird, but it's a 1965 as well. Any suggestions in reference to the alternators going off line? Many thanks-in advance!
BILLS

Okay, here we go. I have the electrical manual with all the schematics. I have the "Alternator System", drawing number 1570000, effective serial number 337-0001 thru 337-0268. I believe we have you identified.

There is nothing too special going on here. This will be some basic electrical troubleshooting and repairs. There's two sides to the charging system that have to be addressed. First, let's take on the control circuit. We have to get a good solid signal from the bus bar to the regulator and then to the alternator. While we're here let's clear up a common misconception. Your airplane has two regulators but it only uses one of them. The other regulator is a backup unit. So if you're buying a new Zeftronics unit you need to only buy one. Where were we? We are talking about the field circuit that controls the alternator output. It's important to remember that the alternator only does what it"s told to do. The field wire has two jobs, it is actually the shared field supply and bus sense wire all in one. The field wire tells the regulator what the bus voltage is. The regulator takes that information and tells the alternator how hard to work. The regulator does this by varying the pule width of the field signal to the alternator. Let's consider this one circuit for a moment. The field circuit goes from the bus through the field breaker, then to the master switch. The master switch is the most used switch in the plane. From the master switch we go to the run-standby switch then the regulator switch. From there it's off to one of the regulators and then the often forgotten OVR (over-volt relay). Now we finally get to head to the alternator. In that little pathway we've passed the signal through 25 connections, not counting one breaker and three switches.

A voltage drop on this circuit greater than 500mv can cause the regulator to become unstable.

In aircraft electrical we need to take a lesson from the orthopedic surgeons. Some time in the mid-1980's they stopped doing knee repairs, they did reconstructions. Doing repairs or patch jobs never really fixed the problems. We can learn from that. To properly repair these 50+ year old circuits we need to replace the wiring and any faulty switches. The breakers aren't going to escape either. The field breaker is a five amp breaker. The lower amperage breakers have the highest internal resistance.

The problem is compounded in the Skymaster since we are trying to balance the overall resistance of the fiend circuits. By far the easiest way to do this is to fix the problem correctly. In my airplane I replaced the entire field circuit. We have a regulator mounted to the front firewall with a four foot field circuit. In the case of the rear engine the field circuit is 16' long. With just a little corrosion there is no way we're going to have balanced alternator outputs.

In my airplane I ran a 14ga wire from the generator switch to the field terminal of each alternator. I used 14ga simple for the low internal resistance. I rewired the entire field circuit and obviously replaced the breaker. Your plane did not come with pull breakers. I really like being able to pull the field breaker.

Now let's talk diagnosis and testing. Field circuits are tested with the master on and engine off. This configuration put's the biggest load on the field circuit. The regulator detects the low voltage of the non-running motor. The regulator calls for heavy charging. It does that by putting a big load on the field circuit. So the test is simple. You are going to measure the voltage drop of the field circuit. We do this in two section since we have a one volt drop across the regulator. Take your Fluke digital volt meter, I like the 87 series V for this work and put the meter on mv DC. One probe goes on the bus bar the other goes to the battery terminal (red) of the regulator. Let's hope for a number <500mv. If it's not that low, divide and conquer. go back and measure at each point along the way, cleaning and replacing as you go. You will find problems along the way. There will be lose connections and poor contacting switches. The master switch is a common source of field resistance.

Once this side of the charging system is under control you have a good solid basis for a great charging system. Mine charges at a happy 28.8 volts.

There's two more areas to talk about, the alternator output and the ground system. I need to do some work on another project for a bit. We can revisit this tomorrow evening.

Dave

1965 C337 "Alternator not charging" light
 

Thanks Dave. I'll have to re-read this information a couple of times to better understand it and I'll start looking/purchasing wire, switches and breakers. So I'll be ready for a major battle with my charging system. Quick question, why only one Zeftronics regulator? Will I keep one of the original regulators installed??
Thanks again!! BILLS

Greetings:

You only need to buy one regulator since the system only uses one regulator. The other regulator is a backup. This seems to be a point of common misunderstanding. Based on the wiring diagram for your airplane (mine too) it only uses one regulator for the system to operate. You said that the wiring to the alternator looks cracked. Check the picture below for comparison. This was removed from a 172 charging system.

I think we pretty well covered the basics of the field circuit. There's only three other areas of the charging system to address. I will be brief.

Now we need to deal with the alternator output leads. According to your diagram you have 30 amp breakers in the panel. The output wires appear to be 12ga according to the schematic. They used a shielded wire originally. The alternator output lead does not need to be shielded. That wire certainly needs to be updated. I'm guessing you have 30 or 35 amp alternators? That entire circuit needs to be rewired. Let's start with fresh 8ga wiring all the way from the alternators to the shunts. Look closely at the connectors at the shunts. Those can come loose and get hot and create all sorts of problems. Then run new #8's from the shunts to the circuit breakers. I like the Klixon 35 amp breakers. I believe the part number is 3TC2-35. These breakers are the most expensive component in the system at ~$165 each. 8ga. wire is rated for 40amps. That gives you plenty of load carrying capacity and low resistance. 8ga is still easy to work with. According to the diagram wire K-PA1 from the bus to the shunt is a 6ga wire. This is the wire that goes back to feed the battery. This wire also carries the load when the alternators are offline.

The only item left is the ground circuit for both the starter and alternator. I like running a piece of #8 from the base of the rear starter to the base of the front starter. This bonding provides a sold ground path for both starters and alternators. Run the new ground wire from the front motor to a common ground point for the front battery.

Send plenty of pictures of your current setup configuration. We will solve the charging problem with logical steps and system upgrades and repairs.

If you get all energetic and decide to do your entire circuit panel I have the templates for my panel that should fit yours. We could just have another back-plate cut. Oh yeah, when you pull out your circuit panel you will see the breakers are really close to the skin. I lined that entire area in polycarbonate. I made paper templates and took them to the plastic store to cut them out of thin scraps of polycarbonate. Total cost $7.

I've included two pictures of Cessna's idea of how to make a circuit panel...

Let me know if I can offer any clarification or assistance.

Dave

1965 C337 "Alternator not charging" light
 

Dave. Great hearing back from you. Well that information is intense but I'm up to the challenge. Give me some time to get the needed parts and for warmer weather! Also I don't want to get too deep in this project right now as I'm finishing my CLT rating. As usual I appreciate all this information and technical expertise! As I get started I'll be sure to send pictures and comments along the way.
Regards. BILLS:eek:

Thanks for expertise
 

Dave,
Just want to thank you for your expertise as well - I am reading your posts with great interest. You have a great way of explaining things.

One question I have is when you said "I like running a piece of #8 from the base of the rear starter to the base of the front starter.", did you mean exactly what is written - running once continuous wire from the front starter to the rear starter? I think I understand the concept - but could you help me understand the reason for this clearly?

Thanks again for spending the time in going through the older Skymaster electrical system. I am not having any problems, but since I am doing a major refurb on my aircraft right now and cleaning up wiring, any upgrades now is worth the time.

Thanks,
Jeff

Hi Jeff:

I hope the info is of some benefit to you. Let me try to give you an illustrative way of visualizing the aux ground/bonding wire. Let's assume that the wiring from the positive terminal of the battery through the master solenoid and rear engine starter solenoid, back to the rear engine starter is all happy.

Now let's try and work our way back to the negative terminal of the battery. The stream of electrons leave the starter housing which is bolted to the case go through the ground strap from the motor mount into the engine frame and into the airframe. Now we have to get all the way back up to the battery through the heart of enemy territory. It's helpful to think of this part as fish going up a fish ladder at a dam. We need to go across all of the overlapping skin pieces that have been vibrating and moving from less than soft landings for the last fifty years. Once this stream of electrons get to the front firewall they have to travel into the battery tray and go through the little bolt, up the wire, and into the negative terminal of the battery. That was a long journey with numerous pathways and connections. We have no control over the quality of those airframe connections that were meant to be structural connections. We are now using them as electrical connections. That point is worth thinking about for a few minutes.

You can test the quality of this path by putting a volt meter between the mounting bolt of the starter and the negative terminal of the battery and cranking the engine. I've seen over 2.0 volt drops between these two points. That's 2 volts that's not going to the start circuit. To do this test you need some long test leads for your meter.

As an aside, the alternator has to use this same path back to the alternator housing and negative side of the battery. It's a little bit easier for the alternator since it is generally down in the low amperage range. But the starter is looking at about a 100 amp draw. That 100 amp draw on the positive wire is the same 100 amps on the ground circuit. I don't want to dismiss that this path for the alternator return path as insignificant. It can certainly make a difference in the charging system operation.

So let's look at the remedy. If we run a piece of #8 Tefzel covered wire from the rear engine starter mounting bolt to the front engine mounting bolt we've got a good connection to get the negative path back up front, to the point of the front alternator and starter. This provides an alternate path that parallels all of the overlapping skin pieces and funky ground straps. Since electricity takes the path of least resistance this wire could carry a majority of the ground circuit load. Now all you need to do is take a piece of #8 from the front starter mounting bolt to the point where the negative cable mounts to the battery box frame. This gives us a good ground path for the both front and rear starters and alternators.

I've spec'd this out as a piece of #8. On a 182 I saw this auxillary ground wire carry 70 amps during starting. On that install I wish I had used a piece of #6.

I hope this answers your question and gives you some background as to the rationale.

* I have heard of mounting a marine grade terminal block where the negative terminal attaches to the battery box frame. This would make a nice place to bring these wires together. Again, I've only heard of this being done.

Dave

Awesome info
 

Dr. Dave,
That info is immensely helpful and now I get it!!! Thank you for the taking the time on this electrical info - it is great to have this type of data to be proactive in eliminating potential electrical issues. I have never thought of the ground plain like you have explained, but that makes complete sense. I will be making some of these wiring upgrades to my bird for sure.
Jeff

Thank you for the kind words Jeff. I fixed a couple of typos and omissions from my previous post. It is quite gratifying to see these very capable electrical systems work properly. Mine does a very happy 28.8 volts under all loads. My battery voltage drops less than .1 between no load and everything turned on. I'm happy with that.

Dave

1965 C337 "Alternator not charging" light
 

Dr. Dave. Question. I was revisiting your reply about your circuit breaker panel re-wiring. I noticed that there is a narrow jumper strap between the lower row and the second? Do you not have a Avionics master switch? Or is that jumper connected to a switch instead of a breaker? I started ordering my electrical parts, but waiting for Zeftronics to verify their part number for my C337's ACU?
When you installed the new ground cable from starter to starter did you need a 1/4" or 5/16" hole in the terminals? I'm think/ordering 5/16"??
Many thanks-again.
Regards, BILLS