Dave, you will probally call me a coont but I will give my opinion anyway
What is wrong with 13.4 volts at the ecu - Nothing IMO.

I agree if you had more than that previously then there may be
an alternator issue.




All ecus (even pectel ) correct the injector time based on
battery voltage. 13.0 to 13.5 is the nominal volatge used as an industry
standard in voltage correction software in nearly every ecu ever made.

More voltage makes the injectors deliver more fuel slightly (to a limit)
and less voltage reduces the fueling but the software adjusts for this
automatically (to a point if set up correctly) so teh AFR should be correct
all of the time at all battery voltages.


BUT too much voltage will kill/reduce the life of the electronics
in the car (stereos/ecu bulbs etc).

Sort the wiring first then check the alternator..