I have a 1991 D150 that I just purchased and the headlights don't work.I can pull the dimmer and the brights come on. I checked all the fuses and found nothing blown. I had a switch that came out of a working truck and swapped it in and still no lights. The horn and the cruise control is not working either. I am going to check and see if it has power to the switch this weekend.Is there a relay that might be bad all I have is a Haynes manual and it doesn't show much about a 1991 truck. I had a 1991 W250 diesel a few years ago that the radio,cruise, and horn stopped and it was some bad fuse links under the hood. Is there anything to check first? Thanks in advance.

 

Do the parking and dash lights come on? When you switch to high beams do they stay on or only when the lever is pulled back?

Check your fuses, fuseable links, and grounds

 

The park lights work fine and so do the dash lights.The high beams only come on when pulling the turn signal/dimmer back. I checked all the fuses and all are fine.

 

Not to suggest the obvious, but I would pull each bulb and test each one with a couple of jumper wires. If they work on high & low the next stepis to follow with wires backand see where the break is. Headlights are not fused as best as I can recall..

 

The headlight circuit is protected by a fusible link but the link is OK because the high beams are getting power. There is also a circuit breaker in the headlight switch but that is also OK or you wouldn't have any headlights at all.

It sounds right now like the problem is in the dimmer but these voltage checks will tell.

On the headlight switch. Do you have power on the 14 gauge LG wire and the 14 gauge LG/BK wire when the headlights are on. The LG wire feeds the low beams and the LG/BK feeds the high beams on the dimmer switch.

There is a 2 wire connector down by the bulkhead connector. It has a 14 gauge RD/OR wire and a 14 gauge VT wire from the dimmer switch. The RD/OR feeds the high beam filament and the VT feeds the low beam. Check for voltage at both wires in high and low beam.

 

I will check it this weekend and see what I find thanks!

 

Well,I just got the truck back from the body shop and am now trying to figure the headlight problem.I had a problem with the ign. lock cylinder so i put a new ign. switch and lock cyl. in and while I had it apart I bought a new dimmer switch and I still have no headlights. What should I try next?

 

if your high beams work your low beams should also work. sounds like a wire between the dimmer switch and the headlight broke.

 

Check the actual headlamp bulb ground. At the headlamp there should be two hot wires (one for high and one for low circuits) and a third wire that should be to a ground.

 

Try to replace the headlight pig tails the may be bad had the same problem drove me nuts try napa only place I found them