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Discussion Starter #1
Hello everyone, I've come here for help with our 04 Pilot. The problem is with the headlights. The right side lo beam is not working. The high beam works fine. I installed a new bulb, no fix. I then swapped the bulbs from side to side, no fix. I then swapped the relays, no fix. Swapped the fuses, no fix. Bought a new headlight switch, no fix. Shook down the harness, no faults found. Going back over my troubleshooting I noticed something weird. When I pull out just the fuse for the left headlight, both bulbs come on but very dim. When I pull out just the fuse for right headlight. When I pull out both fuses, both bulbs come on, but very dim. This was when fuse pulling with the switch on. I have pulled out the fuse box from the car, but I can't figure out how to separate the box itself to look at the backside of the fuse receptacles. Also to add to the mix to make this even better, my toolbox is in storage and not available to me. So I'm working with a small set of emergency tools. I don't have my meter to check wires. Thank you for any help in this matter.

Herb.
 

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Definitely sounds like there is a short in the wiring since lights should not come on at any level when the fuses have been removed.
Do you know if your Pilot has been in for the headlight harness recall?
 

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And if it has already had the recall performed it’s going to be tough to troubleshoot without your multimeter. Unfortunately you might have to bite the bullet and buy a cheap meter if you can’t get yours out of storage.
 

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Discussion Starter #4
Hello, I had the recall done about two or three years ago. I did not have a problem at the time so they just did the upgrade required by the recall. I looked at the connector when I slaved it the new light switch. It looked perfect, no burning or discoloration at all. So I agree that it probably is a short somewhere. I don't have a good wiring diagram either. I was hoping that someone might know a common area where there is harness problems or possibly a component I missed in my troubleshooting. Thank you very much for your time.
Herb.
 

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Download service manual
Find electrical diagram. Stare at the diagram until you understand everything, then stare some more.
buy a multimeter from Harbor Freight ($5)
Do not replace parts until you diagnose a problem.
Post diagram here, we will tell you something smart (or stupid :))
 

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Without a meter the only other thing I can think of looking for is start at the fuse box and trace the wiring to the headlights. See if there are any signs of rodent damage or where the harness could have rubbed against something.
The list of components you checked seems complete to me.
 

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Put the volt meter back in your tool box and grab a test light instead. A meter will lead you to the wrong conclusions in a high amp circuit like the head lights. I would use at least an incandescent test light. Often times I use an old headlight bulb pigtail to use as my test light. Remember, your wiring has to carry voltage but also current. You can have a bad connection somewhere that will allow 12 volts to be on the wire but won't carry the current. That's why the volt meter will lie to you and tell you it's good when it's not. That's why I use a high amp test light on a high amp circuit, if at all possible.



Go straight to the bulb socket. Use your test light with the clip connected to ground to check for power at the low beam terminal. If the light lights up then it's good. Now connect the test light clip to battery positive and check the ground. Determine which one is missing and troubleshoot from there with a diagram.


I was looking up the diagrams but it looks like there are two different systems, likely between trim levels. One has a low beam cut out relay and one doesn't and then some are controlled through the MICU and some are not. Once the head lights are switched on they work pretty much the same except for the low beam cut out relay.



Do the checks above with a test light and figure out which side of the bulb is missing its feed. Then we know which direction go for troubleshooting...power or ground side.
 

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What is your rationale that a light meter is the better test tool for debugging this compared to a multimeter which will measures voltage and resistance (and lots of other parameters)? I understand that open circuit voltage at the headlight harness will be higher than the voltage measured at the same point (technically you might need to pierce the wiring with a very sharp probe when the headlight is plugged in) during full load condition. Once I determine which lead is had the issue then I’d start probing various points to determine when the issue is.
I guess I don’t like the idea of using the light meter or incandescent bulb (I’m guessing you’ll have to judge if things are good based on light intensity) when I can read a quantifiable value on the multimeter.
I’m not trying to start an argument. Just want to determine/understand if there is a better method that I can add to my troubleshooting arsenal.
 

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I explained this already. I used to believe as you, and thought a meter was the best tool (I even thought test lights were for those who couldn't afford, didn't want to buy, or didn't know how to use, a meter) but with experience, and after going through electrical diagnostic courses, I have learned the error of my ways. It is possible to have 12v on the power wire and test just fine but the circuit won't carry enough current due to resistance (i.e. corrosion or loose connection) to light the headlight. You connect your volt meter and all looks good but when you connect a test light, the light won't light, or is very dim. You always want to simulate the circuit's normal load when testing it. A small incandescent test light is better than a meter in this case, but a higher amp test light is best.



It's true that with a test light you don't know what the voltage is, but if the light is lighting then that's the goal anyway. Most small standard incandescent test lights draw around 250mA so they won't tell you if the circuit will carry the full load for a 5-6A headlight either. That's why I keep old headlight and tail light harnesses to use as test lights for troubleshooting.



Your goal here is to determine which side of the low beam bulb circuit is not working. You want to make sure the ground side will carry current and that the power side will carry current. The problem may be that no voltage is getting to the bulb at all and that's easier to figure out, and a volt meter will tell you. If you simply have corrosion or a loose connection somewhere then the volt meter can/will lie to you and give you a false positive. So, if you show 12 volts with a volt meter just make sure that 12 volts can carry the current needed to light a head light bulb.
 

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Thanks for the info. I think the main point of emphasis is whatever debug tool is used that the measurement should be made at full load conditions. Either a light meter or multimeter will work as long as the user understands the limits of the tool being used.
 

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Yes, you can use a meter as long as you leave the load connected, in this case the headlight bulb. It's a bit more difficult as you may need backprobes to insert into the connectors. It's just easier in this case to remove the bulb and use a test light. I just diagnosed a 2008 F250 with a trailer light issue this evening. Trailer lights are controlled by three fuses and four different relays in the underhood fuse box, Ford calls it a smart junction box. With a wiring diagram and my trusty test light (never even pulled the volt meter from the tool box) it took me all of 5 minutes to track down the problem after the owner spent a couple hours trying to figure it out and finally got frustrated and brought it to me.
 

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Mechanic with multi tester looks much smarter than with test light!:)
You are so right and there is a lot of truth to that statement...but...man with test light fixes cars. Man with multi-meter gets duped. There's a time and a place for both but in this case the test light is the best tool.
 
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