will a bad alt. give me weak spark from the ecu?

 

The ecu doesn't drive the coil directly, and it runs on 5V internally so it doesn't get trouble until the voltage drops below 7V, at which point the coil in itself is no longer giving a hot spark.
After all, the engine is supposed to jump to life when cranking on a cold winter morning, and the alt. is certainly not charging when cranking.. The voltage can even drop to 9V but it's still supposed to start.
So no, not like you present it, but yes, a low voltage does give a weaker spark, but that lies inherent in the laws of physics concerning induction in coils.
What are your actual problems and conditions?

 

well,i put on this car a new dist.and coil and plugs and wires yet i only produce yellow spark. and it wont start now.and its getting fuel.

 

Is it a coil-in-cap HEI or is the coil separate? What was the reason for replacement? Does it crank over with normal speed?

 

its a seperate coil ,and yes it turns over very easily. i replaced all the stuff bcuz the car sat for a while.

 

Hmm, ok, but did it start after sitting, before you replaced it? If the old stuff isn't too damaged or discarded I'd suggest swapping it back to see what happens.

 

yes it started ,but it looked more like a flame thrower out of the tbi when i tried giving it gas.i replaced all the wiring harnesses ,and everything electrical for that matter. after all that it fired right up and ran strong. until the night i tried putting it back in the garage.

 

Hmm.. I guess it could have been a number of causes for that flame throwing, like stuck valves, clogged injectors, bad tps/ gas/ pressure. Now it could be a bad contact or ignition module. I don't know, too much has taken place at the same time so this seems to be a bit too complex issue for me to be able to offer any help over the net. Hope someone else can come up with any suggestions. Good luck anyway.