Muscle Car Retrofit - ECU Power in Start/Run

[goodysgotacuda]

I am going through wiring [from scratch, factory ignition switch] and am wondering if this circuit will work to provide power to the ECU/Injectors/Coils.


My "run circuit" 12v is powering part of my fuse box so I do not want to backfeed it with power from the "start" circuit while cranking. Otherwise things like the radio, blower motor, etc. will turn on/stay on while cranking.

Will this work?

Events:
Key On: ECU/Inj/Coils have power, Fuel Pump primes
Key Start: Momentary hiccup of power to the ECU within the ignition switch
Engine Runs, Let Go Of Key
Momentary hiccup of power to the ECU within the ignition switch

Will that short period without power to the ECU matter? How else can I do this?

[billr]

Generally, you do not want the injection or spark enabled directly from the key; that (along with fuel-pump) should be turned on by the ECU, via the fuel-pump output, only when engine rotation is detected.

Power should never be interrupted to the ECU during the start cycle.

Can you post a diagram of the ignition switch you want to use? The switching you described doesn't sound normal for any I have experienced.

That relay config you show doesn't make any sense to me. Are "run12V" and "start12V" outputs from the ignition switch? If so, eliminate that relay and power the ECU (only, not spark and fuel) from the "run12V" and send the "start12V" directly to the starter solenoid/relay. I think you will find the "run12V" remains powered in both "run" and "start".

[goodysgotacuda]

Generally, you do not want the injection or spark enabled directly from the key; that (along with fuel-pump) should be turned on by the ECU, via the fuel-pump output, only when engine rotation is detected.

Power should never be interrupted to the ECU during the start cycle.

Can you post a diagram of the ignition switch you want to use? The switching you described doesn't sound normal for any I have experienced.

That relay config you show doesn't make any sense to me. Are "run12V" and "start12V" outputs from the ignition switch? If so, eliminate that relay and power the ECU (only, not spark and fuel) from the "run12V" and send the "start12V" directly to the starter solenoid/relay. I think you will find the "run12V" remains powered in both "run" and "start".

Thanks for the input. The ignition switch is not necessarily the problem, it is how the chassis harness is working out and powering the rest of the fuse bus. I am trying to avoid the blower/radio/etc. being powered during cranking while not using a significant relay to handle the work. I could go into detail, but it's a bit more than I could explain with just text.

What is the thought behind not having the coil/injectors see power from the key? In the event of an accident things don't run on. I am trying to do all I can to avoid long cranking times like I seem to see with many aftermarket EFI swaps. If I wait until there is rotation/sync/injection/spark, I feel that it will encourage long cranking. [I consider "long" more than 1 second of cranking]

I think I can have the ECU give a "pump shot" of fuel during key-on/initial cranking to help it get running ASAP. [?]

[billr]

This is a '72 switch and harness? I think you will find the radio/heater stuff is controlled by a separate wire to the switch that is not powered when the key is in the "start" position. That was common practice back in those days, maybe still is.

Yes, it is simply a matter of safety to turn off the spark/inj/pump when the engine isn't actually running. It's your choice there, in fact I may not have my coils powered like that (too long ago to remember!).

MS has a built-in "priming function" that powers the pump for a programmable period at key-on and will pulse the injectors for a programmable period during this "pump on" time; all the priming control you could want. Again, I don't have priming turned on at all. Fuel pressure comes up from dead-zero to 44 psi in about 1/2 sec from the time cranking is detected, and the engine will fire in about two revolutions. At 200 rpm cranking speed, that is a whole 6/10 sec. Since my install isn't in a hybrid, where you want "seamless" transition to engine power, I really don't care if it takes that long, or even quite longer.

[goodysgotacuda]

This is a '72 switch and harness? I think you will find the radio/heater stuff is controlled by a separate wire to the switch that is not powered when the key is in the "start" position. That was common practice back in those days, maybe still is.

Yes, it is simply a matter of safety to turn off the spark/inj/pump when the engine isn't actually running. It's your choice there, in fact I may not have my coils powered like that (too long ago to remember!).

MS has a built-in "priming function" that powers the pump for a programmable period at key-on and will pulse the injectors for a programmable period during this "pump on" time; all the priming control you could want. Again, I don't have priming turned on at all. Fuel pressure comes up from dead-zero to 44 psi in about 1/2 sec from the time cranking is detected, and the engine will fire in about two revolutions. At 200 rpm cranking speed, that is a whole 6/10 sec. Since my install isn't in a hybrid, where you want "seamless" transition to engine power, I really don't care if it takes that long, or even quite longer.

Thanks. Yes, 1972 switch, but that's about it. The only electrical components that are original lie within the steering column. I think have sorted out my issue with the use of a diode within the new fuse box on the relay control side. The fuel pump priming will be handled by the MS, but on a different relay with an inertia switch in series. I'm not too concerned with 12v being applied to the inj/coils/ECU with the key on w/o the engine running.

[billr]

I want to be sure, do you understand the coils and injectors may fire unexpectedly as the MS powers up, if all are powered together? That can result in backfiring into the intake, fire there or even damage to intake components. It's not *just* your personal safety at issue, also that of the engine/car.

[Rick Finsta]

With all due respect, follow the wiring diagram in the manuals and stop trying to reinvent the wheel by squaring it off. It is inadvisable to have the injectors and ignition powered when the ECU isn't commanding it. When your key is in the "on" and "cranking" positions, the ECU needs to have power, and you need to have 12V power supplied to the relays controlling the fuel pump and "fuel pump on" accessories like ignition, injectors, maybe a water pump, fans, etc. That allows the grounding signal from the ECU to turn these things on or off and avoids issues with the nature of the control transistors themselves (which as just pointed out, can do things like flood your engine or even pop an ignition coil).

I'd suggest following the manual's wiring diagram.