I have a 91 dodge van b250 i have replaced the head light switch and the dimmer switch and checked the fuses but the high and low beams do not work

wow very old van.
so engine runs car drives ok , right, just head lamp problems.
HL flasher work or not, (hi beam flash switch)??????
in HIGH beam mode is cluster hi lamp glowing?
no? fuse is blown.
well a volt meter finds the cause most easiest.
and schematic from the service manual.
telling what engine or VIN first, is best
or the wrong fuse boxes are seen,, ok>?
you checked fuse but forgot this van has lots of fuesible links. like this super simple schematic shows.
nothing can be more simple, no BCM computer mess.
it can only be 5things (engine runs)

1. fuse link
2. dimmer
3. HL switch
4. both lamps burned out, nah
5. grounds to both lamps cut/rusty.

if both lamps lugs are at 12vdc, the ground is dead

I would check the fuses first. Headlights are typically on a separate fuse then the parking lights. If the fuse is fine check and see if the Bulb is burnt out but it is rare for both lights to go out at the same time.

Replace any blown fuses, and check or replace the head light relay.

Sounds like a wiring problem. You may need a repair book to track it all down. Could be a ground connection too.
The headlight switch sends power to the dimmer, then thru the body control module, then the high and low beam relays, then the four fuses, then the actual headlights. So you should have power on the low or high beam fuses depending on which way the dimmer switch is positioned.

Well the high and low beam lamps use the same fuses, and the dimmer switch handles the ground circuit. So I would check the dimmer switch if both high beam bulbs are ok.

The headlight switch gets power for the headlights from a 50amp fuse.
The fuse powers the headlight switch then the dimmer switch breaks for high and low beams. So if the high beams work, the problem may be with the dimmer switch.
The dimmer switch is made into the turn signal switch on the column. You would need to check the low and high beam circuits at the switch for power.

check relays and fuses located with owners manual if you dont have
look in fuse box may be under hood not familiar with astro vans

suspect low beam head light fuse, check fuse if ok suspect that both low beam head lights are burned out ( it happens) ,if ok suspect headlight dimmer switch.

On the van you have a high beam fuse and relay. On the low beam you also have a fuse and relay. Check the fuse/relay panel for a blown fuse controlling the low beam lights. If you do not find a blown fuse have some one to turn the key on and turn on the high beam lights> During this time locate the low beam headlight relay and place your finger on top of relay, have the other person to switch from high beam to low beam lights. You should feel a click in the relay when they make the switch from high to low beam. If not replace the relay and the problem should go away. Hope this will help you.

Need copy of wiring schematic for 1991 chevy corsica