REFLECTOR: Intercom Noise

[Brian Michalk]

You mention that 1, 8 and 9 show 11.8 volts with the mag on., and 
voltage goes up with mag off.  That difference is because V=IR.  The 
higher the current, the larger the voltage drop.  It's either a lot f 
current, or a lot of resistance on the wire from the battery to that 
terminal strip.  I would not expect an EI to pull that much.  Now, the 
coil packs do draw a lot of power, but I'm not familiar if your EI has 
them integrated.

Also, what is your voltage relative to?  Ground, yes, but which ground?  
Just like you have the forward voltage drop, you'll also have a voltage 
rise on the ground before it gets back to the battery.  Get a long piece 
of wire for your voltmeter.  Any small gauge will do, because we are 
only needing a milliamp or so for the voltmeter.  Attach to the battery 
for your ground reference.  Now check your voltages.

1) battery -- probably 12.4 from your data
2) at positive terminal -- probably going to be above 11.8
3) at negative terminal --- probably going to be about .2 volts or so.

For test 2), you want to be as close to 12.4 as possible.
For test 3), you want to be as close to 0v as possible.

See if you can clean some contacts to get those numbers better.

On 03/08/2011 11:55 AM, aminetech at bluefrog.com wrote:
> A while back I reported that I found a bad ground and eliminated the intercom noise. After 20 hours of quiet operation, the noise is starting to return. It's infrequent and not to annoying, but I don't want it to get worse. I've attahced a pic of the barrier strip where I found the bad ground. (I'm sending the pic of the left side separately). My plane was built with a second battery and I believe this barrier strip was part of the backup system. The backup battery and alternator were removed before I bought the plane, so I'm not sure.  Here is what I know.
>
> Terminal B is the EI mag wire.  It goes to the mag switch and the circuit returns through the shield and terminates at terminal C which appears to be grounded to the firewall.
>
> Terminal 1 is the power wire going to the EI.  With the EI mag switch up, B is grounded and 1 has 11.8 v.  With the mag switch down, B is not grounded and 1 has no voltage.  Is this how EI s are normally wired?  I figured the mag switch would simply cut off the power.
>
> Terminal 8 has 2 wires.  One is power from the master.  I don’t know where the other wire goes.  It reads 12.4 v with the master on.
>
> The terminal 2 wire goes to the EI ground.  I need to check where it is grounded to.
>
> Terminals 8 and 9 are connected and read 11.8 v when the EI mag is on and the voltage goes up to 12.4 v with the EI mag off.
>
>
>
> Terminal 3 is the bad ground, and 4 is a mounting screw that is also grounded to the #3 wire. I didn't replace anything, just removed #3 and 4 and put back on. Before I removed them, the #3 screw was reading 5-900 ohms, now it is 0. With both 3 and 4 removed, the EI did not operate. With 4 connected the EI ran, but rough at low rpm. With 3 also connected it ran fine.
>
> I would appreciate anything anyone can tell me about all this.  If there is something that I can remove, I would like to do so.
>
> John
>
>
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