The alternator regulator will make the alternator charge more when the engine first starts due to the drain on the battery from the starter turning the engine over and making it start, with the belt tight the alternator will put out over 12.75 VDC idling, it may only put out less than the minimum voltage the ECU has as it cut off voltage at idle with the motor warmed up with a loose belt work shruggy
A quick check with a decent voltmeter will tell you if that is the case or not shruggy scope

Last edited by Cab_Burge; 10/22/22 10:24 PM.
Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)