allright so recently my headlights have stopped working on my '74 620.  i checked the wiring within the engine bay, and it is fine.  i tried to replace the headlight relay, but when i was removing it, all the wires came off on accident and i dont know how they go back in.  first of all, i need some help in getting the wiring correct on the relay.  on the relay there are 2 prongs that say "NO" (or maybe its ON), 2 prongs that say Coil, and one that says Com.  i have 4 wires that get connected to the relay.  so hopefully getting that back correctly, my headlights should work.  i tested the 4 wires that plug into the relay by connecting them directly to the battery.  2 dont seem to do anything.  1 makes the low beams turn on, and the other makes the low and high beams turn on.  i know its not the headlights because of this.  oh, and all 4 wires that go into the relay are red.  i dont remember if they have any stripes or anything.  oh, and one more thing.  i used a spare piece of wire and tried connecting the relay directly to the battery.  when i connected either of the wires that were for the light, they would turn on and the relay would click.  however the lights would not turn off when the switch was off because they were wired directly.   any help would be greatly appreciated.

    I have a diagram that is labels as for a 521, but it is not, I think it is for a 620.  The headlight relay wiring looks the same as a 521.   The four wires on the suspected 620 are this, a solid thick red one, this wire supplies power to the relay when the lights switch is on.  There is a thick red wire with a black stripe, this runs power from the relay to the two low beam headlights.  There is a wire, Red with a white stripe, this supplies power to all four high beam headlight filaments.  The fourth wire goes back to the dim/high beam switch.
    Lets identify the wires. on your truck.  The colors may differ.  Look for the thinnest wire, this wire is the dim/high beam switch.  This wire will go to ground with the dim/high beam switch in one position, will not go to ground with the switch in the other position.
    The power in to the relay wire will have 12 volts on it when the headlight switch is turned on.  It will be a thick wire.
    Unplug the headlight lamp, and check continuity from the headlight plug back to the relay wires to identify the other two wires.
    NO stands for Normally Open.  This terminal connects to nothing in the relay, until the coil has power applied to it. 
    NC stands for normally closed, this terminal connects to another terminal without power on the relay's coil, and connects to nothing when the relay's coil is energized.
    Com, I am not 100% sure what it means.
    Coil is the coil. 
    This is how the relay works, on a 521.  There is power supplied to one terminal, and internally this terminal supplies power to both the coil, and one set of contacts on the relay.  Power then goes out to one set of headlights.          When the dim/high beam switch is moved, it grounds the coil, the relay then switches the power to the other set of headlamps.
     Hope this helps you.

thanks, that helps me identify which wires are which.  but i still dont know how theyre connected to the relay.  i actually forgot that there is a wire harness that plugs into the relay, and the 4 wires have spade connectors that plug into the wires.  one is red, one is white, one is blue, and one is blue with a yellow/white stripe.  im assuming the red is the 12v source from the switch.  im not sure about the other 3.  

 

[img]http://www.geocities.com/g--i--r--o@sbc ... relay3.jpg[/img]

    What electrical tools do you have?  A test light?  A multimeter of some type?  That will help me tell you what to do to figure this thing out.
    A guess, the red wire is the power into the relay, one of the blue wires goes to one set of headlights, and the other blue wire goes to the other set of headlamps.
    Grounding the white wire probably is what switches the relay.

i got the lowbeams to work, but i havent got the high beams working yet.  i wired without the harness, so hopefully when i use the harness, the high beams with work also.  i thought it might be a problem with the 12v source wire, or a problem with the switch.  so i took off the switch, and took it apart and saw the contacts were filthy.  cleaned all the contacts and thats what caused it to work fine.  thanks for all your help.

I hate working on electrical. Have you looked into this web site yet?

http://www.the620.com/620tech.htm

Lots of good info there.