Trailer lights
Thread Starter
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Senior User
Joined: Aug 2005
Posts: 336
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From: Leander, TX
Trailer lights
need some help on some trailer lights. all lights work fine on the truck. only my parking lights work on the trailer. turn signals and brake lights won't work. any ideas? thanks in advance.
FTE Leadership Emeritus
Joined: Sep 2000
Posts: 23,174
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From: Stephensville WI
Do the trailer lights work with another truck? If not, the trailer is at fault. If they do, then it's the truck wiring.....but it is usually the trailer.
One of the first things to check is the trailer ground. An easy way to check this is to put a jumper cable from a ground point on the trailer to a ground point on the truck, then recheck your lights.
One of the first things to check is the trailer ground. An easy way to check this is to put a jumper cable from a ground point on the trailer to a ground point on the truck, then recheck your lights.
Junior User
Joined: Dec 2005
Posts: 50
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From: Flushing MI
check fuses under the dash, i have to pull every fuse to find out which one or i would tell you excatly which it was. (dont have a owners manual) this happens to me all the time on my older flat bed when a ground is bad. hope this helps
Thread Starter
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Senior User
Joined: Aug 2005
Posts: 336
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From: Leander, TX
i was going to check it on another truck today. i guess i just figured the ground was ok due to the parking lights working flawlessly. thanks for the input guys. i'll let you know what i find. if anything else comes to mind, let me know.
New User
Joined: Mar 2008
Posts: 7
Likes: 0
From: Cumming, GA
Sometimes the contacts in the truck's socket get bent in so that they don't connect properly with the trailer plug. You can bend them slightly outwards with your fingers. This solved my problem.
You can also try a voltmeter on the socket contacts to verify the truck wiring/fuses.
And of course check the bulbs in the trailer.
You can also try a voltmeter on the socket contacts to verify the truck wiring/fuses.
And of course check the bulbs in the trailer.
Lead Driver
Joined: Jul 2003
Posts: 7,406
Likes: 52
From: Wheatland, Mo
Check under the hood as well. there should be a "power box" that has alot of larger fuses. Usually if something like that happens, you don't have the fuses/relays installed in there. I don't have a diagram for you but you should be able to find one in here from soneone. I know with both of my trucks, F150 and F350, I had to add a couple of the under hood fuses/relays to get everything I needed on my trailers to work...back up lights, 12V charge feed, trailer brakes etc. All this this info is only good if you are using the factory harness to the connector to the back.
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New User
Joined: Jul 2007
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Finding trailer wiring problems: a subject known to make grown men weep and drive others to drink. Get a multimeter, flip on your flashers and check to see the voltage is being applied at your truck's connector. The flashers, brake lights and turn signals are all the same pins, one for the right and one for the left -- this is true for the 4, 6 and 7 pin connectors. A diagram for your conector is readily available on the internet. If the voltages at the truck connector are good, open up the trailer connector. plug it into the truck and check the voltages inside the trailer connector. I have had more instances where the power never got to the trailer plug because of corrosion, dead bugs, dirt, or whatever. Drove me nuts trying to figure out why the truck was good, but voltage was not showing up back at the lamps. Now, I check the trailer plug first. Do a resistance test to make sure the trailer ground and truck ground are connected. Then, if voltage is getting to the trailer plug, pull a trailer lamp and see if the voltage is getting to the trailer lamp socket. If not, the problem is in the trailer wiring. Take a stiff drink before trying to find the break, you'll need it 
. If you have voltage at the lamp socket, it's a bad or corroded bulb.
. If you have voltage at the lamp socket, it's a bad or corroded bulb.
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Cargo Master
Joined: Jan 2007
Posts: 2,243
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From: Roman Forest, TX
Sounds just like what is happening on mine. My boat trailer has a bare wire I can't find yet, and keeps blowing my fuse under the dash. The running lights will still work, but no turn or stop lights. It is fuse #6, a 20 amp fuse, for the "Trailer tow turn/stop relay". I've changed a half a dozen of those stupid things and still can't find my bare wire that's shorting it out! Grrrr! Time to replace the whole wiring harness I reckon!
Senior User
Joined: Feb 2008
Posts: 355
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From: Happy Valley Or
Check under the hood in the "power" box I have towed several "other" trailers and their wiring 100% of the time was suspect. I dont remember the number of the fuse but I believe it was 20 amp under the hood with all the other trailer fuses. I would start there first. Then on to the multimeter and ground checking. Or a test light and test the truck first. I would venture to say it is the trailer wiring.
Lead Driver
Joined: Sep 2007
Posts: 8,180
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From: Scio, OR
The trailer connector turn signals are fused in the Central Junction box with 20 AMP fuse #6. This is the case on my 2004 anyway. I'd be surprised if they changed that in 2005.
Also, the right and left turn/brake wires each go through a relay in an auxiliary relay box under the hood. It will be on the drivers side firewall towards the rear.
I'd check these first, then use a test light at the trailer connector to see if their is power there. The pin diagram is on the connector cover. Hook the ground clamp of the test light to the ground pin (bottom left if I recall correctly) and then touch the probe to the pin you want to test. Left turn/brake is the leftmost pin, right turn/brake is the rightmost pin.
If those test properly, the problem is in the trailer.
Also, the right and left turn/brake wires each go through a relay in an auxiliary relay box under the hood. It will be on the drivers side firewall towards the rear.
I'd check these first, then use a test light at the trailer connector to see if their is power there. The pin diagram is on the connector cover. Hook the ground clamp of the test light to the ground pin (bottom left if I recall correctly) and then touch the probe to the pin you want to test. Left turn/brake is the leftmost pin, right turn/brake is the rightmost pin.
If those test properly, the problem is in the trailer.