I hope when you splice you put a seperate "in-line" fuse for the ecu. Alsop make sure the ground connection is also good (maybe even consider wire back to battery ). Check also main gearbox to chassis ground (usually aroun the frount gearbox mount - it is on my 64 ) ECU's are strange beasts i used to work in the trade of fixing ecu and have seen many an ecu "blown" up due to starter motor current going up a sensor ground.
Don't like giving horror stories but its always good to have info. when doing a car
To be honest it depends on the ecu , what the ecu is driving (injectors , coils ) What it car it is out of ? If your going to just do sytems type checking , sensor checking ect.. start with a few amps. and work up.
A typical EFI car has a main relay powering injectors, ignition and ECU through separate fuses. The coil of the relay is powered via the ignition-on contact on the key switch, and input power comes from the battery. A plain old automotive relay will do the trick. Small fuse panels are available with 4 to 8 positions for automotive blade fuses to make the distribution of power from the relay robust and to look professional.
Injectors are usually "driven" by pulling their bottom end to ground. The supply is off the same source that provides power to the ECU, typically NOT through the ECU. The ECU instead "sinks" the injector current through its ground, so make sure that that is secure and heavy enough to take all the juice. Same goes for the ignition primary coil current if it's driven directly by the ECU.
If the main ECU ground fails, the current will go through the path of least resistance; and that is one of the things that has the potential to damage ECUs.
Separate fuses will allow a low-amp one for the ECU, and larger ones for the injectors and ignition coil/drivers.
MotorsForum website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.