HeadLights Dimming / Fade during Idle RPM

[tcramstrek]

91 Roadster - L98 with the engine at idle, without the use of headlites the instrument lighting dimms from 100% bright to 75%. With headlights on they also fade in and out, as does the headlights . I just replaced/ & swapped the alternator, and the battery with a known good unit, NO Help!

 

[WhalePirot]

The voltage drops commensurately?

RU using underdrive pulleys?

 

[tcramstrek]

1) yes

2) No

3) I am preparing to install a Pro-Charger, I wanted to rectify this condition first