headlight switch testing

[fordman]

i was going to do a headlight switch test out of the truck. but i couldnt get power to do anything to a headlight at all. i just put one hot wire direct to one prong. and the neg to the switch. i did make a complete ciruct. anyway. i couldnt get anything. i know the switch and the headlight are good. so i looked at the schematics and it seems the dimmer switch gets involved when the wires leave the switch and go to the headlights. do i have to wire in a dimmer switch too? or do i need a second wire (ground) directly to the headlights? do i have to use all three prongs on the headlights to make them work. i thought only two would give me light. do i have to do the dimmer switch in the circut to make this work and hook up all three wires to make it light up? i want to do as little as possible to show the switch works.

[m-mman]

The three prongs on the back of the headlight BULB.
1 is for the low beam filiment
1 is for the high beam filiment
1 is to be grounded

Power comes to EITHER of the filiment prongs then flows through the bulb to the third prong which is wired to a screw in the fender/core support. Which acts as the ground.

So to make a bulb work ya gotta have power at one prong and ground one of the others.

This can be tested by trial & error with two wires. One coming from the + positive battery post and the other test wire clipped to the - neg post. two outta three prongs connected and the light should come on (unless the bulb is bad)

Check for power at the headlight bulb plug - turn switch on - connnect test light to - Neg post and then test for power at each plug space. ONE should have 12 Volts. (the 2 others should have no power)

IF the little ground wire is not screwed into the fender (AND CLEAN) there will be no light.

HL switches are tough to bench test. because they do not 'ground'. The power comes from the battery, flows THROUGH the switch (sending power to each HL bulb and Pkg light and the dash lights and taillights)
You COULD put power to the single "power into the switch" tang and then check for 12v at EACH power out tang . . . . but plugging it into the dash plug and checking at each bulb is almost always easier.

[rjewkes]

So the question is how to show some one your selling it to on ebay that it works. withought wiring it into the truck.

[fordman]

basicly. i didnt have it wired right. i was in a hurry. i might mess with it later tonight.