When I turn my heater fan up my lights go nuts

[SixtyEight-F100]

So my heat works great....on low. Thankfully usually this is more than enough but if I turn the fan speed up it makes a few noises and my headlights dim, etc. It still physically work and blows out at the higher speed but there is definitely something funky electrically going on. The truck has a strong battery and a new alternator so my guess is the prob lies within the fan control switch itself, but who knows. Electrics are not my thing so I usually just start replacing things and hope the problem goes away. How does the heater fan speed selector function. Could the higher speed contacts just be bad and the slower speed not explaining why low speed works fine??? who knows.

[Dragon]

The fan is drawing power and it needs to ground. If the full current cannot return through the ground at it's closest point current will find other places. It must form a complete loop. Look at the technical wiring diagrams on http://www.fordification.com/tech/schematics_h.htm for your year truck. Find all the grounds and clean them of corrosion at all the attachment points and clean them.

The heater speed selector is a simple 3 position switch. On top of the heater is a piece that has the low and middle speed wires go to it. It is a resistor wiring network. By sending the low speed through a large resistance the speed is slowed down. The wire gets real hot. The middle speed goes through a lower resistance and the speed is slows down less. High speed goes direct to the fan. The switch fails like once every 50 years except in real dusty/sandy areas.

[fordman]

there is a ground right on the heater motor. check that first. another ground i would check is the headlight to coresupport ground wire. then the engine to firewall ground. that is famous for not being hooked back up when soemone ahs done soem work to the cab or the linkage. the last groudn i can think of is the battery to block ground. there are other grounds such as behind the instrument cluster to dash support. there may be others.

[SixtyEight-F100]

thanks, that explanation really helps to know how the system works to start with. I will try to find all my grounds and clean up the connections and see what I find. Thanks.