96 grand Cherokee Volt meter issue

A customer of mine has a 96 Grand cherokee w the voltage reg in the PCM....Well its gone bad and I've done the external voltage reg trick and installed a Field replacement module in the old field wires...The truck runs at 14.6 volts But the Volt meter on the dash only reads right for the 1st 30 seconds or so after the truck is started....it reads 14.6 then after 30 secs or so something clicks in the dash and the volt meter goes full scale 18+ volts and the trucks message center complains....

What going on? Anyone have any ideas on making the volt meter happy?
Thanks.