can a bad ecm cause weak spark i have just done a rebuild and i am getting weak spark when trying to crank over. i have all new wires pulgs cap and rotors tried 2 distributors 3 coils
does the ecm have have anything to do with giving spark?

 

no. It just triggers the spark. The ignition control module and the coil are responsible for the actual spark.

 

and yet it could be the ecm, if you have replaced the coil and the ign. module, possibly the wires from the ecm to the ign. module. idk exactly what you have done, but if you have replace the ignition components then it could very well be the ecm, my car starts, but stumbles oftenly, and even stalls, i am suspect of a bad ecm, as i have checked the wires many times over, and have had 3 ign. modules, only thing left would be the ecm for me, but i have a code 42, gotta hate the code 42!

 

Try unpluging the tan with black trace wire ( EST bypass) it is the same wire used to check timming , and is located in the main engine harness near dist. this will make ignition system stand alone.Check to make sure you have 12v or more at the ignition system and a good ground.Also make sure you are checking spark a coil, it should jump 1 to 2 inches.If its good there but not at plug check cap and rotor for carbon tracking or holes.Good luck.

 

+1.
Although IIRC the ECM does not control cranking timing.

 

{edit: note, bad advice follows, it will prevent the ECM from firing the injectors. This is from the DRP's also being disconnected from the ECM.}


To completely take the ECM out of the ignition picture, disconnect the 4-wire connector at the distributor. The distributor will still fire the coil to crank, start, and run the engine.

But with that connector disconnected even a bad ECM can not have an affect on the ignition system.

Note: leave the 2-wire connector between the coil & distributor connected. This advice is for the small cap remote coil setup. I am not sure which connector to disconnect on a large cap distributor.

RBob.

 

are you sure your battery has a good charge? A low battery can result in a weak spark.. I know it's kinda obvious but just thought I'd ask

 

Rbob, With that wire disconnected the car wont run if it is EFI.

 

Brain fade on my part. Yes, you are correct. No DRP, no injector pulses. Sorry about that, it will take the ECM so far out of the picture that it won't even fire the injectors (unless it is TBI in async mode (a rare calibration setup)).

RBob.