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Ron, ASE Certified Technician
Category: Ford
Satisfied Customers: 35858
Experience:  23 years with Ford specializing in drivability and electrical and AC. Ford certs and ASE Certs
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I have a 2004 Ford F150 and on the truck the turn signals and

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I have a 2004 Ford F150 and on the truck the turn signals and hazard warning lights work, but on the trailer the left turn signal don't work. I have checked the wiring on the harness and it is ok on both the 7 pin and 4 pin connectors annd they have continuity. The fuse #42 is good and shows 12 volts constant when the key is on and turn signal in left position. When in the right position, the fuse #36 shows pulsing like the flasher should. Is there another relay or something?
Hello and Welcome to Justanswer, My name is XXXXX XXXXX I am here to try and assist your with your question.

If your getting 12 volts at the fuse #42 then you have power from the turn signal switch to the CJB. Are you getting voltage out of the CJB on the yellow wire (pin 2) out to the left trailer turn signal ? See the diagram for reference HERE
Customer: replied 4 years ago.

I don't kn ow what CJB is. I don't get any voltage out at the rear connectors, eithere the 7 pin or 4 pin straight connector. If I use the right turn position, the corresponding fuse shows the voltage pulsing as it should when the flasher pulses. On the left position, the fuse shows a constant voltage and does not pulse.

Sorry Dan, the CJB is the interior fuse panel where your checking your #42 fuse. The constant voltage at the fuse means you have signal in and now we need to see if your getting signal out, the wire to check is the yellow wire out of it to the trailer connector.
Customer: replied 4 years ago.

How can you check the yellow wire out of the fuse panel? I have checked the yellow wire back at the rear harness and nothing there. How does it get constant voltage and not pulse? Does it count on the load to cause the pulsing? Since the truck left turn signal flashes, is there a second flasher or circuit for just the left turn signal to the trailer?

Yes it needs a load , if your not getting it at the trailer connector then you need to pop the fuse panel off and check the wire coming out of it, the 2004s have had know issues with bad CJB, testing the wire out will tell you either way. There are no second flashers and if the flasher on the truck works its likely okay, there also are no relay in this circuit beside the flasher.
Customer: replied 4 years ago.

How does the truck left turn signal work and how is it connected differently than the trailer circuit. Does the truck left turn signal also go through the CJB or straight to the truck light directly. If the truck left turn signal works then the flasher is working so how does it require a differently load on the trailer wiring?

No, both the right and left signals come right out the turn signal switch thru the CJB. If is going to flash at the trailer it needs a complete circuit, if its open in the CJB, it will not flash like the RH side does.
Customer: replied 4 years ago.

If it flashes on the truck and not on the trailer, how does a trailer load cause any different flashing since the load from the truck light is causing the flasher to work or is there a second section in the flasher for trailer only?

No, there is on the LH and RH circuits, . The RH side is flashing as you have stated but your getting nothing at the trailer connector on the LH side but you say you have it at the fuse, that would indicate your open from the fuse to the trailer connector, the next steps is to see if you have the LH signal coming out of the CJB to the connector.
Customer: replied 4 years ago.

Is there any way to talk to you on the phone instead of this method?

if there is only one circuit for both the truck and trailer lights, i don't see how the truck can be flashing and the voltage at the fuse for the trailer not flashing since you say it is all the same circuit.

I am sorry, I am not allowed to speak to you in any other forum except this one due to site policy, Weather it flashes or not, if you have power in and out of the fuse, you should have power at the connector if the circuit is complete, I suspect when we find out where you loosing the power you will have found the problem. We need to see if the yellow wire out of the CJB has power like the fuse does.
Customer: replied 4 years ago.

is there any way to show the wiring diagram for this circuit? i still don't understand how the load can cause any flashing since the truck is already flashing and it is the same circuit from what you say.

Yes, CLICK HERE and HERE, this is the complete circuit.
Customer: replied 4 years ago.

the drawing you sent shows the output wiring. i don't see where it comes from to get into the cjb from the flasher wiring. on pins 13 and 15. if i had that i may be able to understand how it can only be a constant voltage and not flashing at the input to fuse 42, since the truck is still flashing.

The circuit comes from the Turn signal switch, if you have power on both fuse, the switch is sending the signal. The signal from the TS switch comes into the CJB on Pins #15 and #18, #15 being the left side. Click HERE for the turn signal switch and HERE for the harness out.
Customer: replied 4 years ago.

since it looks like it comes directly from the flasher, it should not require a load at the trailer. is it possible it could be the multifunction switch and is so is that the switch on the steering column?

Its not likely if the other signals are working, even if the turn signal switch was bad you would still have power on the LH side at the trailer connector if you have it in and out of the CLB like you do on the right side. If you have power at the fuse then the signal is there and you should have it at the connector .
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