Topic: Headlights only work when applied to direct power.

[SO Teen]

When I turn the headlight switch in my cab my headlights do not turn on. Everything does including running lights and tail lights. But I took a piece of wire and connected it to the headlight fuse to another fuse that has power and the headlights came in and then I pushed the switch to turn on the brights and the brights came on. What does this mean? What is happening between the switch and fuse box and how do I problem solve this?
Thank you for your time
SO TEEN


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[Snowtoy]

But I took a piece of wire and connected it to the headlight fuse to another fuse that has power and the headlights came in and then I pushed the switch to turn on the brights and the brights came on. What does this mean? What is happening between the switch and fuse box and how do I problem solve this?
Thank you for your time
SO TEEN

This means you have either a headlight relay issue, power issue to or from the headlight relay, or headlight switch to relay issue.

Check the relay port and see if you have constant voltage at #1&#4), and at #2 w/headlights turned on.


If you do, you either have a bad headlight relay, or an issue between the port and the headlight fuses, trace/check the wires on the back of the fuse block.


If you have constant power to #1&#4, but no voltage at #2 w/the headlights turned on, it means you have an issue between the relay and the switch, you will need to test the continuity of the switch(will need multi-meter to test it)

[SO Teen [OP]]

This means you have either a headlight relay issue, power issue to or from the headlight relay, or headlight switch to relay issue.

Check the relay port and see if you have constant voltage at #1&#4), and at #2 w/headlights turned on.


If you do, you either have a bad headlight relay, or an issue between the port and the headlight fuses, trace/check the wires on the back of the fuse block.


If you have constant power to #1&#4, but no voltage at #2 w/the headlights turned on, it means you have an issue between the relay and the switch, you will need to test the continuity of the switch(will need multi-meter to test it)

Okay so I have constant power in 1, 2 and 4 w/ the headlights turned off. Does #3 do nothing in the relay or should there be something there? I have a red wire with green stripe there that goes to my head fuses. When I give that wire power my headlights will turn on.  I also have no wire coming out of #2 spot but it has constant power.


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[Snowtoy]

Okay so I have constant power in 1, 2 and 4 w/ the headlights turned off.

Only #1 & #4 should have constant voltage, whatever the battery's is, #2 should only have voltage, milli amps when the headlight switch is on.

Does #3 do nothing in the relay or should there be something there? I have a red wire with green stripe there that goes to my head fuses. When I give that wire power my headlights will turn on.

#3 is the connector where the relay sends power to the headlights.  The relay set-up is #1 & #4 feed power from the battery to the relay, #2 is the trigger wire, it turns the relay on which allows the voltage from #1 & #4 to be directed to #3, and is why it only has voltage when the headlight switch is turned on.

I also have no wire coming out of #2 spot but it has constant power.

#2 being the trigger from the headlight switch to the relay, should have a 16 gauge red wire w/white stripe going into it from the wiring harness.

[SO Teen [OP]]

Only #1 & #4 should have constant voltage, whatever the battery's is, #2 should only have voltage, milli amps when the headlight switch is on.
#3 is the connector where the relay sends power to the headlights.  The relay set-up is #1 & #4 feed power from the battery to the relay, #2 is the trigger wire, it turns the relay on which allows the voltage from #1 & #4 to be directed to #3, and is why it only has voltage when the headlight switch is turned on.

#2 being the trigger from the headlight switch to the relay, should have a 16 gauge red wire w/white stripe going into it from the wiring harness.

Oh my okay so now I've got something going. That's exactly what I thought that wire was. Now how do I connect it to the relay? It's got a little tab on the end that the person before me must have put on. What do you suggest I do?


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[SO Teen [OP]]

After getting home from work and putting that little blue tab to the missing connection in the relay I can flash my brights. But the switch still does not turn on my dims and cannot use brights normally.


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[Snowtoy]

With that wire plugged in, can you hear the headlight relay in the engine bay click on and off?  If not, the relay could be bad, or the splice is causing too much of a drop in voltage to turn the relay on.  When I tested that connector in my '91, the voltage was about .1-.2 volts.  Might try hitting the solderless connector with some sand paper and remove any oxidation.

[SO Teen [OP]]

With that wire plugged in, can you hear the headlight relay in the engine bay click on and off?  If not, the relay could be bad, or the splice is causing too much of a drop in voltage to turn the relay on.  When I tested that connector in my '91, the voltage was about .1-.2 volts.  Might try hitting the solderless connector with some sand paper and remove any oxidation.

The relay is brand new so it shouldn't be bad. I replaced everything electrical to do with the lights. I will let you know if I hear the relay click when I get home from dinner. I also have a multimeter but I have no idea how to use it


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[SO Teen [OP]]

Okay so now that I am home. The relay only clicks when I "flash" my brights. That's the only time that my lights come on.


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[Snowtoy]

With you being able to make the highs flash, it is looking like an issue with the headlight switch.  You will need to check the continuity when the headlight switch is turned on to rule it out.

[SO Teen [OP]]

With you being able to make the highs flash, it is looking like an issue with the headlight switch.  You will need to check the continuity when the headlight switch is turned on to rule it out.

Okay so I've been messing with the plugs from the switch under the dash and I can get headlights or I can get running lights but not both. Why is this. How do I check continuity with a multimeter I don't know how to use it.


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[Snowtoy]

To check the continuity, set the multi meter to the ohm (Ω), should be the lowest or the one with a )))) symbol,
 


take one test lead(black or red) and put it on B4, and the other on B11.



If there is a chime when both are touched at the same time with the headlight switch on, it means that there is continuity, if no sound the switch is bad.

If your meter doesn't have the ))))) symbol, set it to the lowest ohm setting, and touch the two test probe ends together, that number and the one from the combination switch harness(headlight switch turned on) should be the same/similar.  You are testing to make sure that this connection is being made.

[SO Teen [OP]]

Here is what the meter reads with the probes together.
Here is what the probes read when put on the switch plugs
This switch is brand new though so what's wrong?


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[Snowtoy]

The negative tester is plugged into the wrong port, should be plugged into the Com port.

[SO Teen [OP]]

Probes touching

Probes in plugs



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[SO Teen [OP]]

Here is what I'm looking at with the plugs. Mine looks a little different then what you're showing me


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[Snowtoy]

The drawing is looking at the switch's harness unplugged.

Do you get a "1", when testing those ports with the headlight switch turned off?  If you do the switch is fine, and it is either the relay, it the source of power for the relay trigger wire.  Have you tested the relay or voltage of the trigger wire you found before and after the splice?