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SDS CPI breaker/fuse question ?

Larry DeCamp

Well Known Member
Install docs and recent threads refer to breakers and fuses. I could consult Ross, but for the benefit of all who might care, are fuses as specified by SDS the bottom line for the coils and ECU?s ?
 
You may use breakers or fuses as you prefer. They are really there to protect the wiring in most cases.

The ECUs, individual injectors and coils draw less than about 2 amps nominally, though short term surge current is higher on the coils and injectors for a millisecond or two, hence our cautions about setting breaker thresholds when using a VPX.

The fuel pumps draw the most current at around 4.5-5 depending on fuel pressure and surge current will exceed 10 amps on these.
 
Combining pump, coil, and ECU on one fuse

Ross, the SDS EFI wiring diagram for a 4 cyl Lycoming shows a 2A fuse for an ECU channel, 10A fuse for a coilpack, and 15A fuse for a pump. If I were to combine these together (to reduce the number of switches in the panel), would a 20A fuse be big enough or would the combined inrush of the coil, pump, and ECU coming on together possibly blow a 20A fuse even though the steady state current for those three things combined is less than 6A?
 
25 amp would probably be fine with one pump on but it's not standard practice to place several critical components on the same breaker or fuse especially where redundancy is concerned.

I'm a big fan of ATO type fuses. A fuse block to hold these behind the panel can be added if you were installing EFI in an existing airframe where you had no more breaker space or even in a new airframe.

I've used these for 30 years in race cars and aircraft, never had any issues with fuses coming loose or losing contact. Have had 3 of these blocks in my RV for 15 years now.

In the case of the CPI, where we're just driving the module and coils, you could use one fuse for a module and its coil pack since if either one goes down, you lose spark on that coil pack anyway. You'd want the 2nd ignition system to be on a 2nd fuse or breaker of course.
 
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CPI question for Ross ?

Thanks for the "one fuse for CPU & coil " comment. Less is more. That said, I wondered if the CPU tolerates intermittent cycling ? I was planning to power the cpu from the buss and have a switch for the coil only to " mag Check".
 
Thanks for the "one fuse for CPU & coil " comment. Less is more. That said, I wondered if the CPU tolerates intermittent cycling ? I was planning to power the cpu from the buss and have a switch for the coil only to " mag Check".

The CPI has a runup feature to stop sparks at the CPU output level while keeping the CPU powered and running.

If you want to cut power to check coils, I'd cut coil power rather than CPI module power to do this.
 
Ron

Not ready because time spent putting CPI on the -3 would simply be time not finishing the -4. Don?t wait for me to get your Pmags. My 0320 with Catto and EartHX are just a joy to work with.
 
If you want to cut power to check coils, I'd cut coil power rather than CPI module power to do this.

Does this advice apply to the EFI ECUs as well? I was planning a switching scheme where I could shut off one channel at a time while also switching the injector relays in order to do a full check of all redundant components in the system during run-up. Based on a previous email, it sounded like the EFI ECUs take a second or two to power up and start commanding spark & injectors, so I would expect the engine to sputter for a moment during this procedure (only done on the ground). But as long as the ECU isn't commanding any spark or injectors until it is fully booted up and come to its senses, there shouldn't be any backfire, kickback, or other bad things that happen...it would just miss a few revolutions and then pick up running normally again?
 
You don't really want to shut down ECUs to check spark. Currently there is no runup check on the EM-5 so we recommend having coil power switches to check each ignition with dual ECUs.

The ECUs boot up in about .15 seconds and don't command any spark events until 1 crank rev establishes a time interval between magnets. Software changes about 5 years ago allow a seamless transition even turning the ECU off and then on at full rpm.

On the EM-6, we're likely to add a runup feature like the CPI has.
 
I'm a big fan of ATO type fuses. A fuse block to hold these behind the panel can be added if you were installing EFI in an existing airframe where you had no more breaker space or even in a new airframe.

Do you happen to know of any with a second power input terminal? I'd rather not have both power feeds coming into the same stud.

Otherwise, that's my plan. I'm trying to find one with a removable cover that will protect that power in lug. It looks like they make for a really neat and simple power distribution system.
 
Do you happen to know of any with a second power input terminal? I'd rather not have both power feeds coming into the same stud.

Otherwise, that's my plan. I'm trying to find one with a removable cover that will protect that power in lug. It looks like they make for a really neat and simple power distribution system.

I'd just use two 6 fuse ones maybe. I have 3 in my plane of various sizes.
 
You don't really want to shut down ECUs to check spark. Currently there is no runup check on the EM-5 so we recommend having coil power switches to check each ignition with dual ECUs.

The ECUs boot up in about .15 seconds and don't command any spark events until 1 crank rev establishes a time interval between magnets. Software changes about 5 years ago allow a seamless transition even turning the ECU off and then on at full rpm.

So if there's no harm done, why not shut off the ECU during spark check? I understand you don't have to if you install coil switches, but it sounds like it doesn't hurt anything if you do...? From my perspective adding separate coil switches is just an unnecessary proliferation of switches that aren't needed for any safety or operational reason that I can see. If there's real value in putting the coils and ECU on separate fuses, they can still be controlled together on one double pole switch.
 
So if there's no harm done, why not shut off the ECU during spark check? I understand you don't have to if you install coil switches, but it sounds like it doesn't hurt anything if you do...? From my perspective adding separate coil switches is just an unnecessary proliferation of switches that aren't needed for any safety or operational reason that I can see. If there's real value in putting the coils and ECU on separate fuses, they can still be controlled together on one double pole switch.

For the EM-5, we think it's easier to understand what's happening if coil power is cut to check the ignition operation since with dual ECUs (4 cyl.), each ECU is firing one set of plugs and either ECU is running the injectors so you have to remember to switch injectors as well when doing an ignition check.

On 6 cylinder, dual setups we have normal (both), primary and backup positions and depending on that selection, one ECU may be running either all 6 injectors or only 3.

If you want to eliminate some switches, I'd wire the ECUs right to the essential bus with just the breaker or fuse in between. Retain the coil power switches.
 
For the EM-5, we think it's easier to understand what's happening if coil power is cut to check the ignition operation since with dual ECUs (4 cyl.), each ECU is firing one set of plugs and either ECU is running the injectors so you have to remember to switch injectors as well when doing an ignition check.

On 6 cylinder, dual setups we have normal (both), primary and backup positions and depending on that selection, one ECU may be running either all 6 injectors or only 3.

If you want to eliminate some switches, I'd wire the ECUs right to the essential bus with just the breaker or fuse in between. Retain the coil power switches.

Sorry this is in a CPI thread but for dual EM-5, 4 cyl at least, if you don't shut down the ECUs how will you verify the injector relays are working?
 
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