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Old 05-22-2022, 12:57 PM   #1
bigdog
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Default Trailer side lights not working ?

My boat trailer 'side' lights and number plate light are not working ?
(Running lights)
The directional and stop lights are working ok, no problems !
I did notice however, that these lights do not stay on 'all the time' and I believe they should when trailering boat at night.

My trailer wire connection to truck is your typical 4 wire plug.
(green, yellow, black, white)

Thoughts as to why side lights are not working ?

Thanks !

Last edited by bigdog; 05-22-2022 at 01:02 PM. Reason: additions
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Old 05-22-2022, 01:03 PM   #2
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For $3.99, Harbor Freight-Gilford has a four-way trailer light tester that will test the standard old connector attached to tow vehicle.
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Old 05-22-2022, 03:37 PM   #3
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Circling back with some testing I've done...
I tried testing the trailer connection on the truck with a volt tester, but couldn't get 'any' from any of the four connection connection leads. I would think I would be able to get 12v by connecting my voltmeter lead to any one of those harness pins and voltmeter ground lead to ground, but I got no reading ?
Thoughts?

Next steps is to check fuses in truck pertaining to trailer, there are three.
25 Amp trailer tow park lights
20 Amp trailer tow stop-turn-relay fuse
25 Amp trailer tow lamps module

Not sure one can actually test for a bad fuse ? Continuity check ?

Your feedback would be greatly appreciated !
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Old 05-22-2022, 04:14 PM   #4
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Your trailer connection carries 12V positive to each light as needed AND a negative via a white wire to a screw which grounds (bonds?) the trailer frame.

The trailer frame becomes the carrier for 12V negative and is tapped in several places to accommodate each light.

Side lights typically have a 12V positive wire and another, negative wire that is grounded to the trailer frame.

If the lights don't shine then find the ground wire and ground it to some bare metal.

It that makes them light then sand or file the place where the ground wire goes and reattach with a new screw.

You can also test for 12V at the side light by probing the 12V wire and a bare metal part of the trailer.

To do this you will penetrate the wire and make it more likely to fail in the furure.

Good luck!
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Old 05-22-2022, 04:59 PM   #5
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May sound like a silly question but when you were testing the truck side, did you have at least the parking lights on?

Based on your description I'd still be looking at the trailer.
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Old 05-22-2022, 06:13 PM   #6
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I agree with 8gv, first check the ground connections.

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Old 05-22-2022, 10:27 PM   #7
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Quote:
Originally Posted by Slickcraft View Post
I agree with 8gv, first check the ground connections.

Alan
The ground goes through the hitch to the truck. If the trailer is not connected well to the truck hitch, you won't have a good ground. Some people grease or lubricate the hitch/ball to reduce squealing. OOPS.
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Old 05-22-2022, 11:53 PM   #8
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I have not seen a trailer wiring set up that relies on the trailer ball to provide continuity for the ground.

In my experience there is a white wire on the trailer side of the connector that gets attached to the trailer frame.

The wiring connector on the vehicle side has a wire that goes to the vehicle ground.

When the two connectors are joined the ground has continuity.
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Old 05-23-2022, 06:12 AM   #9
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Quote:
Originally Posted by bigdog View Post
Circling back with some testing I've done...
I tried testing the trailer connection on the truck with a volt tester, but couldn't get 'any' from any of the four connection connection leads. I would think I would be able to get 12v by connecting my voltmeter lead to any one of those harness pins and voltmeter ground lead to ground, but I got no reading ?
Thoughts?

Next steps is to check fuses in truck pertaining to trailer, there are three.
25 Amp trailer tow park lights
20 Amp trailer tow stop-turn-relay fuse
25 Amp trailer tow lamps module

Not sure one can actually test for a bad fuse ? Continuity check ?

Your feedback would be greatly appreciated !
You say the directional and stop signals are working so SOME of the connector leads SHOULD be giving you a reading.

Are your tail lights (not direction/stop) working on the back modules? If so, you have a wiring problem on the trailer to the side and license lights which are on the same feed as the tail lights.

I would make sure your "ground" spot for the voltmeter is good. A lot of metal on cars does NOT make a good ground and you will get zero voltage with a bad ground. You could try the ground connector with on the car side trailer connector however that itself could be the problem. Start with the directionals that you say are working. If they are not showing on your voltmeter test, you are doing the test wrong/have the meter set wrong/have a bad ground.

Next, I suspect the fuses are not specifically for the trailer. They are for the light circuit in the truck. If the truck lights are working the fuses are OK. The trailer lights generally use the same fuses as the truck lights.

Next I would suspect a bad/poor connection between the car and trailer connector. Have you cleaned the leads (on both sides) recently? Once the leads are clean AND you have a signal on the car side connector, if you have a sharp pointed lead on your tester you can poke it through the insulation just past the connectors on the trailer side to see if the signal is going through. AGAIN, a good ground is essential.

If the signal is getting past the connector junction, I would jump to the bulbs themselves. Bad/dirty socket? Burned out bulb? You can use the voltmeter to test the power to the socket. AGAIN, make sure you have a good ground. Test a working light (turn) first. If you can't measure voltage you don't have a good ground or don't have the meter set correctly.

After that, wires between the connector to the socket. Pinched and broken? Gnawed by varmints?

Essentially, start at the car side and verify the power for each light is getting through at each point along the way. BE METICULOUS. If you are sloppy you could spend hours chasing your tail.
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Old 05-23-2022, 06:33 AM   #10
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Quote:
Originally Posted by jeffk View Post
Next, I suspect the fuses are not specifically for the trailer. They are for the light circuit in the truck. If the truck lights are working the fuses are OK. The trailer lights generally use the same fuses as the truck lights
Most trucks built since the mid 2000s that are equipped with the trailering package have separate dedicated fuses for the trailer in the under hood fuse box.
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Old 05-23-2022, 09:00 AM   #11
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Quote:
Originally Posted by Blue Thunder View Post
Most trucks built since the mid 2000s that are equipped with the trailering package have separate dedicated fuses for the trailer in the under hood fuse box.
Thanks. Up to last year my truck was 2002 so I wasn't aware of the change.
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Old 05-23-2022, 09:41 AM   #12
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Quote:
Originally Posted by 8gv View Post
I have not seen a trailer wiring set up that relies on the trailer ball to provide continuity for the ground.

In my experience there is a white wire on the trailer side of the connector that gets attached to the trailer frame.

The wiring connector on the vehicle side has a wire that goes to the vehicle ground.

When the two connectors are joined the ground has continuity.

I think I may have stated my problem a little hastily, and somewhat (VERY) embarrassed When I tested the lights in the beginning, I just connected the wiring harness from trailer to the truck, I never had the trailer hitch connected to the truck hitch ball as if I was towing.

In your Post you stated 'I have seen wiring set up that relies on the trailer ball to provide continuity for the ground', this may be the reason?
Going to actually attach trailer to truck hitch/ball and see if it resolves my issue. If it does, I feel like such a dumby !

Will report back with results....
.
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Old 05-23-2022, 10:34 AM   #13
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Quote:
Originally Posted by bigdog View Post
I think I may have stated my problem a little hastily, and somewhat (VERY) embarrassed When I tested the lights in the beginning, I just connected the wiring harness from trailer to the truck, I never had the trailer hitch connected to the truck hitch ball as if I was towing.

In your Post you stated 'I have seen wiring set up that relies on the trailer ball to provide continuity for the ground', this may be the reason?
Going to actually attach trailer to truck hitch/ball and see if it resolves my issue. If it does, I feel like such a dumby !

Will report back with results....
.
REREAD 8gv's quote. "I have NOT seen seen a trailer wiring set up that relies on the trailer ball to provide continuity for the ground."

The continuity does NOT rely on the hitch connection. The WHITE wire is grounded to the trailer frame on the trailer side and to the truck frame on the truck side. When you connect the two connectors (truck WHITE TO trailer WHITE) it creates continuity between the two frames.

In fact, if hooking up the trailer "fixes" the lights, it simply exposes the real problem, that one of the WHITE grounds are no longer properly connected at the frame (corrosion/wear/broken wire).

HOWEVER, the ground is NOT your problem. If you have a bad ground, NONE of the lights will work. You have some lights working therefore the ground is working. (Unless the lights are working but dim. That would indicate that the current is grounding through a secondary (poor) path)

I suspect you may have a couple problems. One, a broken connection to your TAIL LIGHTS which also supply the side markers and license plate light. If the back tail lights are working, it is the specific feed wire to the side markers and license light OR corrosion/wear at some point in the loop that is blocking the current.

Two, the flickering could be loose connections at various places along the way, including the ground wire. The first place to check would be the actual car to trailer connecters. Are they clean and fitting well/snugly?
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Old 05-23-2022, 11:26 AM   #14
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Quote:
Originally Posted by jeffk View Post
REREAD 8gv's quote. "I have NOT seen seen a trailer wiring set up that relies on the trailer ball to provide continuity for the ground."

The continuity does NOT rely on the hitch connection. The WHITE wire is grounded to the trailer frame on the trailer side and to the truck frame on the truck side. When you connect the two connectors (truck WHITE TO trailer WHITE) it creates continuity between the two frames.

In fact, if hooking up the trailer "fixes" the lights, it simply exposes the real problem, that one of the WHITE grounds are no longer properly connected at the frame (corrosion/wear/broken wire).

HOWEVER, the ground is NOT your problem. If you have a bad ground, NONE of the lights will work. You have some lights working therefore the ground is working. (Unless the lights are working but dim. That would indicate that the current is grounding through a secondary (poor) path)

I suspect you may have a couple problems. One, a broken connection to your TAIL LIGHTS which also supply the side markers and license plate light. If the back tail lights are working, it is the specific feed wire to the side markers and license light OR corrosion/wear at some point in the loop that is blocking the current.

Two, the flickering could be loose connections at various places along the way, including the ground wire. The first place to check would be the actual car to trailer connecters. Are they clean and fitting well/snugly?

I connected the trailer to the hitch and re-tested, but didn't correct my issues, but then you explained that above.

To review...
- All truck tail lights and Directional lights, License plate lights are working.
- Trailer Directional lights are working.
- Cleaned terminal pins on truck side. Clean all terminal pins on trailer harness.
- Checked ground (white wire) on trailer, all look good, clean tight and no rust.
- Truck to trailer connections are all clean and fitting well/snugly !

Going to purchase trailer/light tester at Harbor Freight, it's cheap and I need one anyway. At least testing in this manner, I can eliminate the truck connections.

Will now begin to look at ALL trailer wiring for issues.
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Old 05-24-2022, 11:17 AM   #15
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Quote:
Originally Posted by bigdog View Post
I connected the trailer to the hitch and re-tested, but didn't correct my issues, but then you explained that above.

To review...
- All truck tail lights and Directional lights, License plate lights are working.
- Trailer Directional lights are working.
- Cleaned terminal pins on truck side. Clean all terminal pins on trailer harness.
- Checked ground (white wire) on trailer, all look good, clean tight and no rust.
- Truck to trailer connections are all clean and fitting well/snugly !

Going to purchase trailer/light tester at Harbor Freight, it's cheap and I need one anyway. At least testing in this manner, I can eliminate the truck connections.

Will now begin to look at ALL trailer wiring for issues.

Bought trailer/light tester at Harbor Freight, and tested Truck plug terminals.
Left and right taillights working fine, lights on tester lit up.
The Running lights terminal tested bad, at least the light tester did not light up!

So now I'm thinking it may be a truck fuse, more to trouble-shoot !

Feedback appreciated.
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Old 05-24-2022, 12:27 PM   #16
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I believe you misstated. Left and right brake/turn lights are working. Tail lights
and running/marker lights are not working.

You have to have a good ground or the working lights wouldn't work so it cannot be that. All of the running lights and tail lights would not have burned out at the same time so it is either no power from the truck, a bad connection at the plug, or the main tail light feed in the trailer is compromised.

I don't know what your vehicle is but my Tahoe had a separate fuse in a different place under the hood. I think that is true of a lot of GM vehicles and maybe many others too.
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Old 05-24-2022, 05:04 PM   #17
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I had a similar issue. My trailer had box beams with the wiring inside the beams. Turned out a mouse ate the insulation off of the wire and it was shorting against frame. Had to rewire.


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Old 05-24-2022, 08:00 PM   #18
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Quote:
Originally Posted by TiltonBB View Post
I believe you misstated. Left and right brake/turn lights are working. Tail lights
and running/marker lights are not working.

You have to have a good ground or the working lights wouldn't work so it cannot be that. All of the running lights and tail lights would not have burned out at the same time so it is either no power from the truck, a bad connection at the plug, or the main tail light feed in the trailer is compromised.

I don't know what your vehicle is but my Tahoe had a separate fuse in a different place under the hood. I think that is true of a lot of GM vehicles and maybe many others too.
Just a FYI if it helps....Truck is a 2016 Ford F150
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Old 05-25-2022, 07:11 AM   #19
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See if this helps.

If not, and you are still stumped and you want someone to fix it, call Nick. He is mobile and very good.

Nwmi out of Meredith 100% set up/mobile/full time & insured for this type of work.
603-520-1662.
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Old 05-25-2022, 08:37 AM   #20
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Quote:
Originally Posted by bigdog View Post
My boat trailer 'side' lights and number plate light are not working ?
(Running lights)
The directional and stop lights are working ok, no problems !
I did notice however, that these lights do not stay on 'all the time' and I believe they should when trailering boat at night.

My trailer wire connection to truck is your typical 4 wire plug.
(green, yellow, black, white)

Thoughts as to why side lights are not working ?

Thanks !
Your brown wire is the problem. It's for the side marker and tail lights. I'm assuming it's a standard 4 pin connector.
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Old 05-25-2022, 08:42 AM   #21
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You can test the circuit using one of these...
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Old 05-26-2022, 10:16 AM   #22
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I have to agree with TiltonBB regarding the truck fuse being the issue.
I found same reference to 'Trailer Tow Park Lights' (25Amp), in my owners manual.

Since I tested trailer connection on truck side with 4-Way Connector tester and
received no light on tester for 'running light', I have to assume, it is that 25Amp fuse. The fuse is a 'module type', so bringing to auto parts store
so they can provide exact type I need.

THanks !
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Old 05-26-2022, 11:13 AM   #23
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If the fuse is blown there is likely a wire with compromised insulation that is making contact with the trailer or vehicle frame.

A careful inspection of the wires is in order.
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Old 05-27-2022, 05:41 AM   #24
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Quote:
Originally Posted by 8gv View Post
If the fuse is blown there is likely a wire with compromised insulation that is making contact with the trailer or vehicle frame.

A careful inspection of the wires is in order.
I agree. If a tail light wire is shorting out, a new fuse could just blow out again.

Aren't trailer lights fun?
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Old 05-27-2022, 12:05 PM   #25
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Default Circling back......

Just circling back with another test...

Just to eliminate issues with 'trailer' lights, I had a friend hook-up his truck to my trailer, and are working

All trailer lights, including side marker lights, and side running lights.

This said, my issue is on truck side ! Now to find the cause.
I will replace truck fuse as a start.

Thanks all for your feedback, greatly appreciated !
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Old 05-27-2022, 12:30 PM   #26
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Default Trailer side lights not working ?

Hi
As a follow up, have your friend pull your trailer down a road with a
few bumps. Follow him and see if any lights blink. This could help find
an intermittent problem if there is one. I had a light bracket that was pinching a wire after dealer replaced lights. Problem only showed up once in a while.

Scott
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Old 05-27-2022, 04:42 PM   #27
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After I reworded my trailer my buddy hooked up his relatively new (2 - 3 years old) Chevy pickup and we had light issues. He took it back to the dealer and they replaced something No Charge and all was well. I think the service guy said he had seen this same issue several times recently. You might want to check for recalls for the problem.


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