I've notice when my heater is on it drops a bar on the voltage guage is this normal?

 

what year is your truck Andrew?
is this the stock or aftermarket gauge?

 

87' f350 6.9 stock 1g alternator ang guage

 

I believe its normal due to the small size of the stock wiring. Mine does the same thing. I'll bet if you measured the voltage at the battery it wouldn't change much.

 

I have an 87 and can tell you mine does the same with the A/C fan on. The higher I turn the fan on, the more the gage goes down. I think my fan motor is getting tired and plan on replacing it soon. Also I have put the 3g alternator on my truck and it made a world of differance.

 

Does it still drop with the 3g update?

 

3g upgrade will make it charge better, but will do absolutely nothing to change this. The problem, as I understand it, is that the wires feeding the fuse panel, gauge, and heater are all small wires. Since they are small, the load from the fan causes a noticeable voltage drop on the gauge.

 

should the small alt. wire be change to a bigger one. I know you said it want help with the voltage drop but it look's a lil small to me. Just was going to upgrade the wire if i needed too.