So a new issue has arrived in my 98 12valve, only while cold it doesn't charge on a high idle, charges fine at idle but if I don't let it warm up for 5-10 mins as soon as I bring up the idle or drive it will stop charging, quite strange to me. Batteries test good as well as the alternator, no codes, any ideas?

 

are u sure its not just ur grid heater comin on? that thing draws an awful lot n it'll pull my guage right down to 10 or 11 volts. when i drop down to idle it kicks off n the guage goes back up. im bettin thats what ur experiencing. no issue there at all, they're just doin their job

 

I had an issue similar to this. For me it was actually my serpentine belt slipping just the slightest on the harmonic balencer. You might look at that.