What is the regulated voltage coming from an alternator (presumably the voltage the battery sees)? I also presume this is the same voltage to all other accessories (fuel pump, electric fans, etc.).
IIRC for years the Delco alternators had a segment of circuitry in the regulator that varied the output voltage depending on ambient temperature. This directed/allowed the alternator to charge at higher voltage (about 14.4 -14.6) during colder battery temps in winter times, but ramped down the charging voltage in the hot summer (to about 13.2v) to keep from cooking the battery.
I believe the later alternators (starting sometime in the nineties) had additional inputs from the ECM to modulate/soften the alternator output to keep from causing excessive instantaneous alternator loads (such as when turning on the rear window defroster or heated seats) on an idling engine, causing idle instability.
Which alternator model are you using?
I would think all the EFI wants from the chassis harness is 12v, Key-on and ground... The rest is self contained (sensors, injectors, etc... ) No? The one thing that I would think matters is to not hook up your 12v source at the alternator output post because of potential electrical noise
The "default" voltage output on the newer (DR44) GM alternators is supposedly 13.8v. When hooked up to an ECU they are controlled with a PWM signal to vary the voltage for vehicle/environment changes.
So would you suggest running all added accessories (that were not original to the car) directly from key on power?
If they have their own power switch, I'd go for it. I'd be sure and in-line fuse each one though. Like a Radio, Ham Radio, Spot Light, Fog Lights, etc. If there is no separate circuit (On-Off Switch) - or you forget to turn off a high load - Radiator Fans for example, when you shut the car down, you'll have a higher demand when cranking, more than just the starter. You'd be trying to crank with all that stuff on, and on a COLD morning, or after a long sit without charging, you might bog the battery down. Just thinking...
Cheers - Jim
So would you suggest running all added accessories (that were not original to the car) directly from key on power?
Good stuff. Thanks.
On my C4, I can hear the fuel pump as soon as the key is turned to "ON" before the key is turned to "Start". Is that what is happening with the two "IGN" relays in your diagram?