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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
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