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Low volts/high amps

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The CTSW is showing low voltage (~12.5) and high amps (~7-8) during normal operation. The voltage rises (~12.9) and the amps drop (~2-3) when all accessories are switched off. Also, and it may or may not be related, the fuel flow is zero, occasionally flickering to a higher value with the engine running. And the ammeter stays at 2 amps even with the engine switched off, and the fuel pressure stays ~2 psi with the engine shut down.

The battery and voltage regulator have been changed, and the Dynon checked to rule it out.

Any ideas?

Thanks!

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Loosen up and clean the Shunt, lube with Vaseline and retighten

shunt located behind left side of panel.

Worked for me 2006 CTSW 

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Thanks for the replies!

Roger has already Scotch-brited and greased the shunt. As far as we know, all grounds were checked. My theory is that the higher-than-normal current draw is related to the possibly faulty fuel flow sensor.

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All done. The VR was changed, the battery changed and new ground added, wire ends checked, each electrical circuit instrument or power user checked,  the D120 checked for accuracy, AC voltage to the VR checked, DC voltage checked coming out of the VR checked. battery voltage during a run is confirmed with a meter at 13.5 - 13.7 DCV. Voltages across the VR confirmed at 13.7 DCV output.

Dynon had no suggestions.

Any electrical Wiz out there we could use some help.

 

p.s.

The fuel flow reading on the D120 quit this A.M. to. Related or maybe not.

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Check for voltage drop from the dynon unit ground to battery negative with all instrumentation on and someone transmitting on the radio. This should be in the tens of milivolts or less. The case is grounded, you can just test it from there. Anything higher than a few tens of milivolts will be substantially more sensitive to electrical noise and grounds must be further improved. You can find the problem spot by taking your voltmeter and placing the leads along each segment of wire until you find the troublemaker.

There is also one, and ONLY ONE ground coming from the dynon main ground pin to the ground bus, right?

I have other suggestions to pursue after verifying the above.

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Did you do the voltage drop test though with the single ground?

Don't run extra grounds to the dynon case, you'll be providing alternative ground paths, and you risk backfeeding into the dynon unit if there's a problem with the ground from another electronic device, resulting in interference at best, a damaged unit at worst. That's why I was asking, is there one, and only one, ground?

Electronics need a ground reference voltage that is stable. If the ground floats or is unstable, it becomes a fluctuating ground reference voltage. For example, you have a 12v system, and the ground voltage fluctuates between .5 and 1.5 volts, then you effectively have a fluctuating system between 11.5 to 10.5v system and the transistors and amplifiers will not be able to give you the correct readings. This isn't a problem as much on the power side because the units have internal voltage regulation circuits that stabilize the voltage coming in.... but there is NOTHING stabilizing ground (it's difficult to do).

Please do the voltage drop test from the dynon case to the battery negative terminal. This will provide a huge amount of data and eliminate a large number of assumptions.

 

Also: flight design is taking power for the dynon unit AFTER the ammeter. This is known to cause odd problems in some installations. I don't want to have you run a new wire just yet (it didn't solve the problem in a plane I worked on, so let's check other things first).

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Thanks, Corey! We'll try that.

What do you think of my theory that a faulty fuel flow sensor is causing excessive current draw on the system? Is that even a possibility?

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The fuel flow sensor picks power off of the +5v excitation pin of the dynon unit, along with several other units too. If it was faulty, you would also see other sensors and devices malfunction. I won't rule that out, but it's being filed under the "last resort" things that we'll try later.

 

I am curious though. You're saying your ammeter always reads positive (or negative)? It's not supposed to.

The ammeter is situated in the circuit between the battery and the power bus. The idea is that when the battery is discharging, power will flow through the shunt to your electrical bus and you will read a negative amperage.

The regulator power though, flows from the regulator, to a junction on the capacitor (but not through the capacitor, just a common terminal, this is by design and strongly recommended by rotax), into the airplane to the alternator breaker, and finally directly to the electrical bus... In this way, when the regulator pushes the electrical bus power higher, electricity will flow backwards across the ammeter bus and back to the battery, showing a POSITIVE reading until the battery is charged.... then it should settle around 0 when they are equal.

The fact that you -always- read something on the ammeter means something is not hooked up right or shorting.

Is your capacitor fried? This can cause all sorts of electrical havoc. To test it, remove it from the aircraft. First short out the terminals and keep them shorted. Put an OHMMETER on it, with the correct polarity (there's a blue stripe, or a polarity indicator near one of the terminals, make sure you hook up the NEGATIVE lead to that), and when you remove the short, you should see the resistance rise over time. Anything else, you have a bad cap, replace it.

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I did look at the capacitor and it was hooked up. I can try a new one, I can try a new main ground to the Dynon, I cleaned the shunt, but if memory serves there are two fuses back there I didn't check.

This has been a frustrating issue.

Corey, Kevin and I had this fuel pump talk for hours the other day.

Kevin,

I'm going to torture that fuel pump idea out of you. LOL

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There's still a serious question about why the ammeter never settles down. A lower system voltage and a high ammeter reading makes me think there's a short. Let me know about the cap.

Also, those fuses protect the wiring from a short. It's unlikely that the ammeter would work with a blown fuse but do check it out, and make sure there's no continuity to ground either.

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