#7 fuse replacement

[gluaisrothaii]

Howdy,
Just wanted to throw out an idea in case anyone else felt the need. My #7 fuse was burnt to a crisp when I received my DMC. I replaced the entire fuel panel but wasn't very happy with the idea of a marginal circuit capacity on #7 possibly damaging the new fuse board. I have mitigated the risk elsewhere by converting to LEDs and low current fans etc. I also liked the idea of an easy means to interrupt the fuel circuit, either for troubleshooting or immobilization purposes. Longer term I would like to replace the entire fuse panel with a CB panel but space is a constraint. Would probably have to create a remote harness and move the CB panel to the rear bulkhead for an aircraft style aesthetic.

Anyway at the risk of irritating the purists LOL here's what I did. Klixon pull-to-interrupt 20 amp breaker from Aircraft Spruce. Tefzel MS22759/16-16 wiring (would have used brown but that's not available ). Aircraft grade crimp terminals. Made a small L-bracket from some stainless stock on the bench-top brake, and used a 90-degree pneumatic drill to make mounting holes as shown. Riveted the bracket in place, and used silicone boots to insulate the terminals. It will clear the lock modules once I replace them.

In case anyone wants to replicate here's the BOM

Klixon 20 amp circuit breaker $20.80
16g Tefzel wire, about 4 feet $2.68
#8 crimp terminals 2 ea $0.60
Window butt splice 14-16g 2ea $1.50
Silicone boots, 1S 2ea $2.30
Sheet metal L bracket- whatever you have lying around
2 x 1/8" rivets

[Bitsyncmaster]

Just an FYI:

Teflon and Tefzel insulated wire is a great space saver and weight saver because the insulation is so thin. But that also makes it more susceptible to damage if rubbing on sharp metal. Last time I used it you need a thermal stripper to remove the insulation.

[David T]

Very neat installation with very high quality materials. Kind of overkill but what the heck. You are only dealing with the symptom though. The circuit becomes overloaded because the fuel pump goes bad, the wiring connections to the fuel pump get corroded, and the heater in the CPR short out. Continuous loads are only supposed to draw up to 75% of the capacity of the wiring and you fuse to 100%. So in a 20 amp circuit (that's what the wire is rated for) you are not supposed to draw over 15 amps continuously. Continuous loads are loads that run without stopping for more than 15 minutes. The fuel pump is a continuous load (obviously). When they start going bad they start drawing a LOT of current. Not enough to blow the fuse but more than 15 amps. That overheats the wires and the fuse, melting the fuse out of the fuse block. Add in some bad, corroded connections to lower the voltage on the fuel pump and it draws even more current so it can consume the necessary power it requires (P (Power)= I X E, Reduce E (Voltage) and you need more I (Current) and now you can see why the circuit gets overloaded. While moving Fuse #7 out of the fuse block will prevent any further damage to the fuse block, it doesn't solve the underlying problem.

[gluaisrothaii]

Very neat installation with very high quality materials. Kind of overkill but what the heck. You are only dealing with the symptom though. The circuit becomes overloaded because the fuel pump goes bad, the wiring connections to the fuel pump get corroded, and the heater in the CPR short out. Continuous loads are only supposed to draw up to 75% of the capacity of the wiring and you fuse to 100%. So in a 20 amp circuit (that's what the wire is rated for) you are not supposed to draw over 15 amps continuously. Continuous loads are loads that run without stopping for more than 15 minutes. The fuel pump is a continuous load (obviously). When they start going bad they start drawing a LOT of current. Not enough to blow the fuse but more than 15 amps. That overheats the wires and the fuse, melting the fuse out of the fuse block. Add in some bad, corroded connections to lower the voltage on the fuel pump and it draws even more current so it can consume the necessary power it requires (P (Power)= I X E, Reduce E (Voltage) and you need more I (Current) and now you can see why the circuit gets overloaded. While moving Fuse #7 out of the fuse block will prevent any further damage to the fuse block, it doesn't solve the underlying problem.

Agreed (almost*). I also replaced the fuel pump, cleaned the connections, and verified all the other loads are within specification. The fuse block damage occurred prior to my ownership.

* Incremental additional series resistances like a corroded conductor will always lower current in a circuit. A failing alternator with low output voltage could potentially cause the pump to draw more current, as could a seizing or otherwise restricted pump motor.

Ken

[gluaisrothaii]

Just an FYI:

Teflon and Tefzel insulated wire is a great space saver and weight saver because the insulation is so thin. But that also makes it more susceptible to damage if rubbing on sharp metal. Last time I used it you need a thermal stripper to remove the insulation.

I would respectfully disagree. MIL-W-22579/16 has become the de facto standard for aircraft wiring because of the improved mechanical and thermal resistance over standard PVC. It can be stripped using conventional mechanical strippers. I went through a lot of work on my airplane during the overhaul with Tefzel wire; on just about every gauge from 2 down to 24.

Just to differentiate- Teflon is not Tefzel. Teflon (PTFE) is a softer insulation which is processed and handled very differently than Tefzel (ETFE).

Best,
Ken

[Rich_NYS]

Looks great Ken.....nice work!

[David T]

Make sure you check the plug hidden up inside the washer bucket. The fuel pump circuit passes through that plug and on some cars it gets so corroded that the pump won't run. The fuel pump circuit also passes through the inertia switch. Check that the connections are good there too and that you have a white splotch on it. If you have an early car a recall moved it to the side of the footwell along with recalling the switch. What kind of plane do you fly?

[gluaisrothaii]

Make sure you check the plug hidden up inside the washer bucket. The fuel pump circuit passes through that plug and on some cars it gets so corroded that the pump won't run. The fuel pump circuit also passes through the inertia switch. Check that the connections are good there too and that you have a white splotch on it. If you have an early car a recall moved it to the side of the footwell along with recalling the switch.

Roger. Connectors are all good. I don't have the recall but I have rewired the fuel pump ground.

What kind of plane do you fly?

1974 Rockwell Commander 112A. I had an engine failure in 2017 where one of the cylinders decided to separate from the block...landed successfully but that triggered a 17 month overhaul. Zero-timed engine, new prop, panel, GPS, audio panel, LED lights, main battery wiring, engine mount, exhaust, carpet. Removed the entire vacuum system along with pumps, regulator, filter etc. Went to solid state ADAHRS. ADS-B out/in. I did the work, my IA signed off the logs.



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[dtavres]

Great photos! Thank you!

Here's my (much worse) write up - www.DMC10515.com/no-rundisney-marathon-show