Twin Otter fuel qty inop

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Echohotel
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Twin Otter fuel qty inop

#1 Post by Echohotel » Tue Mar 06, 2018 8:28 am

Both forward and aft fuel quantity systems are pegged below zero as though they are not powered.
I have checked for proper power and grounds finding them ok.
I have checked continuity of hi and lo-z from the junction blocks to the instruments.
I have checked for shorts to ground and shield in hi and lo-z.
I have checked that the instruments work in another aircraft.
The probes were just validated.

With no experience in this DC capacitive system I am at a loss.
Any advice is appreciated.

Jon
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Heliian
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Re: Twin Otter fuel qty inop

#2 Post by Heliian » Tue Mar 06, 2018 9:45 am

Check your wiring again, check to make sure you have a good ground. No corrosion. Wires connected to correct terminals.

Unplug one probe to see if it changes.

Check fuel quality or dirty probe.

A bad pfm box could too.

Do you have the standard fuel system?
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digits_
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Re: Twin Otter fuel qty inop

#3 Post by digits_ » Tue Mar 06, 2018 10:15 am

Is there fuel in the plane?

Not familiar with the twin otter system, but it's common to be calibrated to show zero with zero usable fuel. So if your tanks are completely empty for maintenance, a below zero reading might be "normal"?
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groncher
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Re: Twin Otter fuel qty inop

#4 Post by groncher » Tue Mar 06, 2018 10:43 am

You say both gauges are below zero? I wouldn't think there are too many things could cause that.

It's been a long time, but if i recall correctly there is a fuel quantity test switch that drives both gauges to zero. Is it possible that is stuck in test position? CB pulled?
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Echohotel
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Re: Twin Otter fuel qty inop

#5 Post by Echohotel » Tue Mar 06, 2018 10:48 am

Continuity and the ability for a circuit to carry a signal are two different animals...

Found two cold solder joints and a hi-z that flicked apart at a touch.
Aft tank needed calibrated and fwd fine with repaired connections.

Whew!

I was looking forward to all your responses tho.
By the way... The test switch opens the lo-z circuit sending the gage to zero or less.

Jon
I’ll be back
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Heliian
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Re: Twin Otter fuel qty inop

#6 Post by Heliian » Tue Mar 06, 2018 11:04 am

Nice!

No doubt working on those ballerinas. Do you have an interior in yours to make it look pretty too, pita.
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Echohotel
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Re: Twin Otter fuel qty inop

#7 Post by Echohotel » Tue Mar 06, 2018 6:50 pm

Jumpship. Utility interior. Gotta be! Nice and clean tho.

Jon
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