View Full Version : Trailer light connections???

11-12-2005, 03:11 AM
I've got the boat trailer wiring figured out, but it's what my meter shows at the four pin plug on the vehicle that has me confused. After I'd metered to find the ground pin and the tail light pin I had my friend click on the turn signal. Now what I had thought was the ground reversed polarity with a small varying voltage. After exposing the connections on the back of the plug I found some factory wires had been clipped. More probing and noodling just before sunset left me in the twilight zone!

Does anyone have a diagram showing how a 4 pin plug should be wired? From reference material I "googled" off the net I know a white wire should be ground and brown the tail lights and green (right) and yellow (left)for combined turn signals and stop lights.

Guess I'm confused on how the stop and turn lights work together. What should I "see" with my voltmeter connected across ground and a turn/stop pin? It shouldn't show a polarity change should it?

11-12-2005, 05:03 AM
Honestly, I quit trying to figure light wiring out using a voltmeter after having the same "deer in the headlights" feeling you're having.

The easiest thing I have found is the "trial and error" method. I make four little jumpers using alligator clips and short pieces of wire (12-18"). Set the trailer lights up per the directions (if new) or just leave them as is on the trailer. Clip the "jumpers" (all four) to your trailer plug making sure they don't touch eachother and cause a short. Now, clip the other end of the jumpers to each of the wires at the vehical and test them out. Keep moving the jumpers around until you get the right combo. Mark the wires and install the plug.

Easy as that! ;) 8) ;D

11-12-2005, 06:48 AM
You are right on track with color combo that you listed. The brown wire provides steady voltage when the lights are on, but only to the first element of the bulb. The right/stop/signal works off of green, and left/stop/signal works off of the yellow. The same bulb element is used for the stop lighting as the turn signal. Basically, when the brakes are on, the voltage is full and steady to that bright (second) element. when the signal is on, the voltage "pulses" letting that element flash on and off. This way, you only have to have a single dual element bulb on the trailer. It's much simpler than having to have two bulbs and more wiring. That's why they do it that way. The only time it gets confusing is if someone has hacked the wiring on the vehicle. Then you have to go under the truck and reverse the damage to get things right.
As far as the meter reversing polarity. Make sure the ground wire on the system is connected well and not corroded. You should have a good 0 ohm reading on the wire with the frame of the truck. Also do the same for the ground wire on the trailer. I have seen trailer ground wires have a poor ground and test meters read really goofy. It can be deceptive because once you hook up the trailer ball, it creates the ground for the system via the truck!

11-12-2005, 06:50 AM
One more thing. What kind of meter are you useing. Sometime a digital meter is not the best kind for this type of testing. For instance, the turn signal pulse. A simple test light will flash on and off for you, just like the trailer light, but a digital meter take a few mili seconds to gather a proper reading. By the time it reads the pulse, its already gone, therefore giving false or inaccurate readings.

11-12-2005, 07:16 AM
Another thing is that if you have amber turn signals (mostly Japanese) you need a converter box. I about went nuts the first time I tried to wire a trailer to a Japanese truck without one. They share different elements for the brakes, tail lights and turn signals and without the converter you can only get two of them to work. I got one for my Toyota that plugs into the stock harness at the taillights. Most parts places carry them.
Good luck

11-12-2005, 09:07 AM
First of all, what kind of truck is it? Many newer model trucks have a 4-way connector plug already in the wiring harness somewhere in the rear wiring.

As Trout Chaser said, a test light is by far the best way to go doing trailer lights. Just clip it to a good ground and probe.

Regarding your question about the ground reversing polarity: No, the ground does not reverse polarity. Depending on how you were connecting your meter, it is possible for your meter you have shown you that though.

Finding the right source voltage on your truck shouldn't really be that hard. Probe till you find the wire that gets hot when you turn on the lights. Once you find it, step on the brakes, turn on the signals, etc, and make sure that wire stays constant hot. Now do the same for each side with the brake lights. Find the wire that gets hot when you step on the brakes. That same wire should pulse with the appropriate turn signal. Sounds like you already got your colors for the connector, so you know which wire color goes to which source from the truck.

Once you get it figured out, remember to make very good connections, and secure all the wiring. Especially the ground. If you don't, everything will work good for a year or so, then you will go crazy chasing electrical problems. You ever follow a trailer that all the lights blink when it goes over a bump? That's a pain in the neck to find in the driveway.

Hope this helps.

11-12-2005, 09:42 AM
it sounds li ke you do not have a good ground to the trailer . bad ground you will go nuts as current goes to delta ground and make tail lights come on when you hit brakes . otherwise follow tips with jumper wires

11-12-2005, 10:03 AM
I got tired of bad grounds to the trailer, so I just ran a solid ground from the truck to all of the lights. Problem solved.


11-12-2005, 12:07 PM
`After sleeping on it and reading these helpful posts I'll go back to troubleshooting my neighbors truck & trailer wiring problems better prepared to not only solve but to improve the situation.

Your comments reflect other info I dug up in that poor ground causes lots of headaches. So I plan to confirm the vehicle, a early 80's Ford 3/4 ton pick-up, has a good zero resistance ground pin connection and even though the trailer seems fine with it's chassis ground I'll run a new wire to insure it stays good.

Erreck - Yeah, I think your temporary jumper connection idea is good but it assumes the wiring is good to the plugs and in this situation it seems there's problems with both the truck and the trailer. My neighbor's other neighbor was first to go fishing in the newly purchased boat and he got it "fixed" first ... blowing both the truck's stop and turn signal 15 amp fuses. Rather than risking fuses in the truck I decided to use the trolling motor battery as a power source in deciphering the trailer wires. Under one meaty cocoon of black electrical tape near the front of the trailer I discovered the turn signal wires were connected together. I've been using a long length of speaker wire with alligator clips to help with continunity checks between the plug pins up front and the actual lamp socket contacts in back. I've been doodling reference sketches of each connection area ... the cocoon, a area of splices in the rear, the round 4 pin, and the flat 4 pin adapter etc, but I'll start taging the wires to help keep things straight even better.

Trout_Chaser - Yeah, I'm using a digital as it's all I've got now. I'll rig up a test bulb for figuring out what's up with the truck plug which I removed from it's mount and found the four unused clipped wires behind it. The 4 pin there now was not what came from the factory. While that plug is apart I'll use some fine sandpaper to clean the age glaze off the brass pins.

Off the Hook - It was supposed to be easy just metering the pins on the truck but the reverse polarity reading really threw me. I'll try it again with the test light.

Thanks fellows for the suggestions and hopefully if you're following two grey haired guys in a Ford pick-up with a small boat that has the dome light flashing on instead of the brakes I won't be in it!

11-13-2005, 03:12 AM

Again using the digital meter I confirmed the wierd reverse polarity readings on the truck's 4 pin didn't emanate from the twilight zone. I took off a tail light thinking I could learn out something there and just got more ohmeter readings that didn't make sense. When I used the test light instead of the meter on the truck's 4 pin connector and everythiing checked out great!

Back to the trailer. I followed color code best I could, but the ribbon cable had purple instead of green. After soldering the new splices I jumpered to the battery for a test. "See neighbor how it works now." ARGH! I showed off too soon! Under the boat in the rail I found another tape bundle of the previous troubleshooter's splices . Once those were resoldered "my way" the test went fine.

With the trailer close to the truck I jumpered for a final test again and then redid the connections on the back of the truck's 4 pin. The screws that clamped the wires were rusty and corroded so I soldered directly to the pins. Put the plugs together and it lit up like Christmas.

Neighbor says I earned a second trip to Amador. Won't argue with that.

11-13-2005, 05:57 AM
;D ;D ;D

Great story! Glad to hear you got the problem resolved.

11-13-2005, 07:02 PM

Now go fishing :D :D

11-30-2005, 11:38 AM
Trailor parts section of wal-mart sells a 4way flat circuit tester. plugs directly into tow vehicle's flat 4 pin plug and little green led's light up as you check turn signals,brakes...etc. pretty sure it was $3.29.../gary