ok my 91 f350 crew cabcummins 12v is off to paint so im trying to figure out an issue. all my guages work even the tack but when i turn the running lights on it drops 200RPM the the headlights another 200RPM. when im at 2k rpm it doesnt do it only at low RPM and at idle if i hit the grid heater it goes back to normal

 

have you checked your ground wires yet to make sure your getting a good ground

 

You know , I have a 95 dodge ram 5.9 gasser also the 83 chevy diesel , But the dodge ram , I have exactly the same problem as you well almost, you can test the charging system and its fine, you can test the batteries and there perfect, but as soon as you turn on accessories I lose volts in everything , dims down and everything at idle.. Now something I know, there is a capacitor or something inside the brain of the computer that monitors the system and voltage and tells the truck to kick up idle or whatever for the charging system or regulates the voltage in the charging system , and from what I hear that is the main cause of my problems, but I am not buying a new computer for it , I live with it . But if that is the case with you, and you do find that out, a new computer is the only way to fix it ..

Thats worse case scenario , you might have a short? a bad ground , a power drain, check the grounds in your headlights, I had a problem there before as well, Check your belt, it might not be spinning the alt like it should because its a lil lose , dont forget when you turn accessories on your truck has to make up for the use in voltage thats why I say belt , or like the regulator in the computer that tells the system to help out ...

Just some things I am familiar with , may not be your cause , but there things to look at ..

 

well i know for a fact it aint the belt never took it off durring motor swap and its only 3 months old anyway, as for the CPU scratch that 12v dont need one i did have to add a external voltage regulator which the directions explained to ad a capacirtor which i never did yet, headlights may have to ground them directly to the crossmember