The International Cessna 170 Association

I’m fighting an issue where my Zeftronics generator controller shuts off when I transmit on my radio. I’ve had this problem in the past and was able to address it by moving my com coax away from and orienting it 90 degrees from the field wire. I replaced my master switch over winter and now I’m dealing with this issue again, but it has become intermittent. I’ll be checking all of my grounds, but while I’m at it I plan to replace my old field wire with new shielded wire.

Here is my question... how much current is in the field wire? I want to use the correct size wire and have found copying the old isn’t always the right answer. In case it matters, I have an original 12A generator.

Also, when I install the shielded wire, can you confirm that I only ground one end of the shielding?

Thanks,
Doug

Doug,
As you can see by comparing the wiring diagrams for the 170A and the 170B the field wires, wires 7 and 69, use 18 gauge shielded wire. The field wire carries very little current. This wire could be smaller but how much weight are you going to save by going to 22 or 24 gauge shielded wire? The FAA says,

As a general practice, wires smaller than #20 should be provided with additional clamps and be grouped with at least three other wires. They also should have additional support at terminations, such as connector grommets, strain relief clamps, shrinkable sleeving, or telescoping bushings. They should not be used in applications where they will be subjected to excessive vibration, repeated bending, or frequent disconnection from screw termination.

Page 41 Electrical Wiring Interconnect Systems (EWIS) Best Practices Job Aid

I'd talk to Zeftronics about your regulator shutting off when transmitting. I've heard nothing but good reports about their customer service. Pinpoint Harnessing has a lot of useful information on wiring. Here's what he has to say about grounding shielding at both ends: "Audio panel manufacturer knowledge about ground loops and RF noise is considerably advanced today compared to the 1960's. This is why my harness shields are daisy chained at the V/R connector and terminate to a single point ground under the regulator mount bolt."

In researching this I discovered why the '48 C-170 is so much louder than the later models. Check out the '48 wiring diagram.

Thanks, Gary. I never remember to look at the wiring diagram in the parts manual since my 170A has almost all new wiring (except for what goes through the firewall) and the 170A manual doesn’t have the wire size. I do see the wire size in the 170B manual, though.

I’ve got extra 20 ga shielded wire I was hoping I could use. Bummer.

I talked to Zeftronics last year when I had a similar problem. That is when I learned that it has internal circuit protection that shuts off the generator output when there is a spike to the regulator. He said then he wouldn’t be much help finding the cause of the spike, but that there must be a spike coming from somewhere.

I’ve got it in my plan to have a new radio installed later this year. I may pull that trigger sooner than planned if there is any chance that may help. I’ll be talking to the avionics shop tomorrow.

Doug

Have you checked the shielding of the coax from end to end including all connections? The antenna is a common point of failure due to moisture on the exterior of the aircraft. I would suggest removing the antenna, carefully checking the mounting screws, gasket and surface of the fuselage and even nutplates. Some antenna require mesh gaskets for proper grounding. Not sure what antenna you have. Any coils in the coax or field wires due to excessive length? Coils will generate a magnetic field and induce voltage into circuits where you don't want it.

FredL wrote:Have you checked the shielding of the coax from end to end including all connections? The antenna is a common point of failure due to moisture on the exterior of the aircraft. I would suggest removing the antenna, carefully checking the mounting screws, gasket and surface of the fuselage and even nutplates. Some antenna require mesh gaskets for proper grounding. Not sure what antenna you have. Any coils in the coax or field wires due to excessive length? Coils will generate a magnetic field and induce voltage into circuits where you don't want it.

Fred,

I installed new RG-400 coax last year and did check it end to end at that time. I have an old antenna with the ceramic base. With my next upgrade being a new nav/com radio I plan on installing a new com antenna at that time as well. I believe there may be a coil at the antenna end of the coax which is behind the cabin so it is many feet from the field wires. I'm fairly confident the issue is in the panel area since I can make the problem worse by moving the com coax at the back of the radio closer to the field wires.

Last night I routed some new shielded wire from the master switch to the regulator and generator field terminals. I'm still waiting to hear back from Zeftronics on if they recommend grounding the shielding at each end or just one end. I figure I'll follow their suggestion and see if this new wire helps the cause. If that doesn't work I'll need to look deeper into the radio end of things (I'm slightly suspicious of the coax pigtail that is soldered to the radio tray).


Doug

The shielded field wire should help but the coax shielding issue should be addressed also. What type of connectors does your coax have? solder or crimp on type? Since the antenna end doesn't seem to be grounded and the radio rack seems to have a questionable ground I would temporarily connect a jumper from the connectors to a suitable nearby gtounding point on both ends of the coax and see if that makes a noticeable improvement, That would confirm the need for better grounding of your coax shielding and shouldn't take much effort.

Theoretically speaking... grounding shield at ONE end is all that is necessary and sufficient to shield the cable... and not necessary to ground both ends, which in some circumstances can create a ground-loop which can introduce interferemce.

GAHorn wrote:Theoretically speaking... grounding shield at ONE end is all that is necessary and sufficient to shield the cable... and not necessary to ground both ends, which in some circumstances can create a ground-loop which can introduce interferemce.

This is exactly how I was taught at work to handle shields to avoid interference and minimize the probability of ground loops. The theory is that grounding only one end will shunt external noise to ground, yet not allow the shield to carry current, preventing it from generating more noise.

Ground Loops question, are they more, less prevalent in AC or DC circuits?

johneeb wrote:Ground Loops question, are they more, less prevalent in AC or DC circuits?

AC systems are most sensitive but ground loops can occur in either if there is a difference in potential of the various ground points. This is why a single ground point is preferred. Remember that audio circuits inherently have variable current fluctuations, and in a predominately DC system such as an airplane, there are strobes, rotating beacons, generators (with commutators which vary current each time the brush passes to the next commutator-bar) etc etc. If any computer is nearby...(and can you imagine how many micro-computers are in each avionic we have on board these days?)... the possibility of interference can occur.

In the case of an alternator...we have AC current. In the case of a generator we have not only the commutator issue, we also have vibrating point regulators OR we have solid-state regulators (can you say microchip/micro-computer?) which vary current by design.. After all that’s what they do. So that creates variable potential all the time and we don’t want that to occur across a ground or shield.

That’s the way I understand it anyway.

Thanks George,

A little more light has been shed on this part of electrical theory. I was able to find a lot, on line, about ground loops in AC circuits and nothing about DC circuits.

I also came across an EAA article that recommended grounding the shielding at both ends of the Magneto P leads.

I called Zeftronics and asked them specifically if they recommend grounding both ends of the shielding on the filed wire and he said YES. He also said to double check my battery ground to the frame as it can act as a buffer of sorts. That is what I’ve done and plane to go for a flight today. We’ll see if it helps. I’ll report back.

Doug

Dr. Ibitayo at Zeftronics is an electrical engineer and certainly knows his stuff... but he’s incorrect about ground loops (if that is who you actually talked to.) The shield should be grounded at only one end of a shielded cable....the one electrically closest to the negative battery terminal.

I went with the recommendation from Zeftronics because of his knowledge and so they don’t have anything to question in my setup.

I flew for 1.3 hours today and didn’t have any issues. I can’t say that the new wire fixed my issue, but it didn’t happen this flight and it had happened the six flights before that. It will take a number of flights with no issues before I can be confident, but it was nice to have a good flight today.

well that is good so far. Have you duplicated the problem on the ground previously by keying the mike or does it only happen in flight?