strange problem with CURT Custom Wiring Harness #56106

OK..this is going to sound whacko, but it is true.
Vehicle 2014 Toyota Sienna Van
Boat trailer. This trailer has side running lights, so I can see if the Running Lights are on or not while driving.

The RUNNING Lights ( ONLY the running lights) work intermittently.

#1. It isn't the Ground wire (all other lights flash and work)
#2. It affects only the Running Lights (Brown wire)
#3. I've had the trailer installer check things out and they couldn't come up with the answer.
#4 I've had a local auto mechanic, who is pretty sharp, check things out.

First off, I REALLY don't know if this is a new occurrence or has been going on for some time. I recently noticed it shortly after I purchased a New-to-me 2007 Lowe 1756 tinny. For the first few times that I towed the trailer, the trailer running lights worked just fine. Then, the TRAILER RUNNING LIGHTS stopped working, but the directional lights all worked and the brake lights all worked.The van's Running Lights work just fine.

After leaving it with the aforementioned mechanic, he deduced that the issue was on the VEHICLE side, not the trailer side. A voltmeter, applied to the Brown wire output showed only (.6 volts) when the outage was occurring.

There is a "black box" connected between the van's taillights and the extension that runs to the outlet connector near the hitch receiver.

I decided that I'd simply replace the wiring harness, which included the "black box" with the exact recommended one, which matched the wiring harness that we discarded. The original trailer wiring harness came with the installation of the hitch approximately two years ago.

After installation of the new wiring harness, the same issues kept occurring sporadically. The guys who put the wiring harness in said that they would replace it...but they couldn't find out what the issue really was.

As things happened, and after the hitch installer worked on the wiring harness with his 12-volt tester, the darn system worked... it worked well, for about three weeks.

However, today it has reverted back to sporadically functioning and then not functioning.

Some additional weird observations....

A. When the Running Lights are NOT working, I can, sometimes, rotate the Light Control on my van, and watch the Running Lights flash on and then off.

B. Other times, rotating the Light Control on my van, causes the Trailer Running lights to start working again and stay on.

C. Today, I noticed a new phenomenon...IF I STEP ON THE BRAKE PEDAL, and at the same time Rotate the Light Control in my van, the Trailer Running Lights come on and stay on???

Now, earlier, I said that this might have been going on for two years. The reason that I cannot verify that is that the original Trailer Harness was used on this same van, but on my other boat. That boat is now in TN. It did not have Side Running Lights, so I had no way of noticing if the Running Lights on that trailer were on when they should have been on, or not. I didn't drive that rig often in the dark, so Running Lights were not required. They may have been working, or they might not have been working.

Wheew.. Long story for such a "simple" problem. I did send a message to Curt Company a week or two ago...(The manufacturer) but never received a response.

This isn't my first rodeo with trailer lights, but it sure has me, and two mechanics stumped.

I believe that it has a lot to do with the "black box" which, as I understand it, is there to "fool" the van's lighting system from shutting off when additional trailer lights are added.

Curt's wiring harnesses even provide for an additional heavy wire to be connected directly to the van's battery so that the new lights don't overload the present van's lighting system

Has anyone else experienced anything like this?

regards, richg99

Not a mechanic or a electrician either but I’ll try.

If only your running lights are affected it’s likely not your ground.


When it’s not working are you getting power from the brown wire after the black box before the trailer.

If you are it’s either a problem with the trailer wiring or the connection trailer/car.

If your not, are you getting power between your cars running light and the black box.

If you are getting power from a running light to the black box but not past in on the brow wire then it’s your black box.

If your not getting any power between running lights and black box then it’s a problem with the car. Start looking for a loose or poor ground.

Also just to be safe before doing any of this, when your trailer running lights are not working check your car battery.

Likely somebody already tried all of this but it’s really the only way to isolate the problem.

Hope you figure it out. I hate electrical problems.


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Thanks , but all of those things have been looked at. The real killer was... when stepping on the brake pedal, the van's light control would turn on the trailer lights????

Weird.

I an considering bypassing the brown wire going to the black box, but would rather have an expert tell me what is really going on.

I bet your converter is internally burning out ...

For those Curt trailer light converter ‘black boxes’ that convert the vehicle lights (separate lights) into a logic the trailer can display - be aware they sell 2 versions! One, costing a few dollars more, has an external red power lead where you must run a new independent wire down from the battery to add ‘power’ to the unit.

This is to avoid power loss issues like you’ve experienced. This model is $33 or so from Del City. I’d say that and a new independent 12V lead should resolve all your issues.

Recheck all of your grounds. Bad grounds can cause strange problems just like you mention. Everything can work fine and have one item that can be wacky. It doesn't even have to be on the same circuit. If your left turn signal comes on when you turn on the radio - bad ground. If your horn blows when you turn on the windshield wipers - bad ground. If your right headlight comes on when you turn the blower fan on - bad ground. I have seen some things like this with bad grounds. You will actually have to disconnect each ground and check it with a fine tooth comb. Just because it is hooked up doesn't mean it is properly grounded.

Exact same Van and exact same Converter (56106)- Related problem:

In my case the side marker lights work fine, except:
They flash with turn signals only when the van headlights are on, and they go off any time the brake is used.

Go figure.

DaleH said:

I bet your converter is internally burning out ...

For those Curt trailer light converter ‘black boxes’ that convert the vehicle lights (separate lights) into a logic the trailer can display - be aware they sell 2 versions! One, costing a few dollars more, has an external red power lead where you must run a new independent wire down from the battery to add ‘power’ to the unit.

This is to avoid power loss issues like you’ve experienced. This model is $33 or so from Del City. I’d say that and a new independent 12V lead should resolve all your issues.

Click to expand...

Mine has the separate power lead, and it, along with the rest of the unit, is wired precisely as directed in the instructions (logical thought though).

I have the kind of CURT connector that already has the separate power line.

Today, they worked on the way to the lake and failed on my way back. I tried the "step on the brake" trick, and it didn't work. All of the way home (85 miles) I tried to turn on the lights numerous times. They never came on....UNTIL...I was one block from my boat barn.

I was hoping that they would stay off so I could have a better chance of tracing what the problem might be.

I, too, think the problem is inside of the black box. What about just bypassing that one wire?

p.s. (just added late)...I am going to check the FUSE on the separate power wire. Wouldn't it be nice if the fuse blew somewhere along the way, and all of the troubles run from that!

Darn....the fuse is OK.

Another issue that I have is that one of the 4-pin connectors is not that good. Sometimes my lights go off, but if I push the connectors together, and bend the union slightly at the same time, the lights are ok for a while. Maybe it is the Curt end.

Thanks, that is certainly another thing to try.

However, I was having the trouble before I put the NEW Curt harness on. Hard to believe that both of them were faulty, but anything is possible.

Lot's of people trying to help on this one. I'll try to try, too.

You stated that according to someone else: A voltmeter, applied to the Brown wire output showed only (.6 volts) when the outage was occurring.

You need to measure the voltages yourself. When the problem is occurring, measure both the output, and the input immediately proceeding the black box. You might want to use a variety of ground points-- the one the black box is using, a metal part of the van, and a bare spot on the trailer.

If the input is 12v and the output is 0.6v, the problem is a defective box or an improper installation. If the input is 0.6 and the output is 0.6, the problem is with the vehicle.

If you determine the problem is related to the box, and you have tested two separate boxes with the same failures, then KMixson posted the likely solution:

KMixson said:

Recheck all of your grounds. Bad grounds can cause strange problems just like you mention. Everything can work fine and have one item that can be wacky. It doesn't even have to be on the same circuit. If your left turn signal comes on when you turn on the radio - bad ground. If your horn blows when you turn on the windshield wipers - bad ground. If your right headlight comes on when you turn the blower fan on - bad ground. I have seen some things like this with bad grounds. You will actually have to disconnect each ground and check it with a fine tooth comb. Just because it is hooked up doesn't mean it is properly grounded.

Click to expand...

Back when all this started, about a month ago, I measured the .6 volts. At the time, I had no idea that there was a "black box" hidden between the rear light assembly and the output connector.

After I turned it over to our local mechanic, all that I got from him was that he agreed that the issue was in the van, not in the trailer. What brought him to that conclusion, I don't know.

Next step was to go buy a whole new wiring harness, believing foolishly that replacing everything at once.... would be the most expeditious way to solve the issue. Obviously, that was WRONG.

The installer of the new wiring harness (my choice to buy, not his ) was surprised when the new harness worked intermittently. He thought he found the problem by squeezing the connector fittings. That solution failed on the very first bump that I hit. I brought it back ten minutes after leaving.

Next, he dug around with his 12-volt tester and seemed unhappy with what he was reading. Since he was Hispanic and almost completely non-verbal, I have no idea what his tester was telling him. However, since we were running out of their business hours, I agreed to bring it back in the morning.

On the way home, the d@#$ system worked flawlessly. I chose to not bring it back and have them mess with it again. It has continued to work flawlessly for 3 weeks, until yesterday.

Personally, I think that, by digging around with his tester, he pressed on some loose connection and temporarily fixed it. I hope to find that hot spot tomorrow. The biggest problem is that the issue is sporadic/intermittent.

Sounds like a faulty converter to me ???? Foreign vehicles have 5 wires that need to be reduced to 4 (converter), reason being they have 2 individual tail light bulbs with individual functions that need to be combined so that a single 1157 bulb on the trailer will perform both functions. I recently replaced a converter on my Tacoma and I think it was also the brown wire that was causing the problem, all lights work fine since I replaced it. I've had to replace several trailer wiring harnesses with converters on other Toyotas that stopped functioning properly, pretty easy now that they are plug and play.
Mine had power in all wires to the converter, nothing coming out of 1 of the corresponding color wire on the opposite side, bad converter!
Before I start poking any holes in any wires, I would check for power at the plug first.

That's the way I am leaning.

Since this is the second converter harness, I'm not excited about spending another $60. for a third.

If it appears to be the converter, what about jumpering the brown wire?

richg99 said:

That's the way I am leaning.

Since this is the second converter harness, I'm not excited about spending another $60. for a third.

If it appears to be the converter, what about jumpering the brown wire?

Click to expand...

Nobody has offered a single suggestion that explains how a bad black box could cause the interaction you are experiencing between light dimmers and your trailer lights. An intermittent ground (in the black box, or elsewhere in your vehicle), or even ground potential, explains it all day long. So does interference, which a 2 minute web search reveals can be an issue with Sienna vans.

5 minutes with a voltage tester, following the suggestion I made, will allow you to determine if you need to spend the extra $60. Randomly replacing components is guessing, not troubleshooting. You've already guessed once for $60. Do you really expect that another $60 guess will produce a different result?

What is the harm in following a reasonable troubleshooting procedure? If you aren't measuring the voltage going in and out of the black box, you aren't troubleshooting. If you aren't securing the grounds and testing for resistance or ground potential, you aren't troubleshooting.

.Mike said:

richg99 said:

That's the way I am leaning.

Since this is the second converter harness, I'm not excited about spending another $60. for a third.

If it appears to be the converter, what about jumpering the brown wire?

Click to expand...

Nobody has offered a single suggestion that explains how a bad black box could cause the interaction you are experiencing between light dimmers and your trailer lights. An intermittent ground (in the black box, or elsewhere in your vehicle), or even ground potential, explains it all day long. So does interference, which a 2 minute web search reveals can be an issue with Sienna vans.

5 minutes with a voltage tester, following the suggestion I made, will allow you to determine if you need to spend the extra $60. Randomly replacing components is guessing, not troubleshooting. You've already guessed once for $60. Do you really expect that another $60 guess will produce a different result?

What is the harm in following a reasonable troubleshooting procedure? If you aren't measuring the voltage going in and out of the black box, you aren't troubleshooting. If you aren't securing the grounds and testing for resistance or ground potential, you aren't troubleshooting.

Click to expand...

Great suggestion here. You need to logically troubleshoot, not hunt and peck.

Well, she is fixed ( I HOPE). I actually did pretty much everything that various people suggested on this and another site or two. CatFan on the BBC site suggested I make a long wire tester so that I could see when she was ON or OFF. That helped immeasurably.

On this site, Renn90 said that he had the exact same wiring harness, and his troubles disappeared after he fixed the end connector by squeezing the "tubes". Many have encouraged me to re-do the ground connections. I did all of the above.

I could have stopped and checked for a solution after each and every "fix". i.e. The Scientific Method. I chose to do all of them at once and take her for a ride.

When I was done, and with my extension sitting next to me, I drove all over and stopped, bumped, and turned the lights on and off many times. She never went off. I then drove to the boat barn and hooked up the trailer. We went for another couple of miles trek over every railroad track and bumpy road nearby. Once again, the lights stayed on.

I don't have a real solution for the next guy, other than to follow the excellent suggestions from this and other groups until you cure the problem.

thanks for all of the help and regards, richg99

p.s. Of course, the last time I thought I had the problem fixed, it re-occurred in 3 weeks. I won't call it DONE until I take the boat and trailer out for a half dozen additional runs.

I'm with Mike. Ya gotta prove the input is good or bad by poking wires. If you determine its the vehicle then put in a new car battery next. Swap with the wife's car. That fixes weird computer problems and is simple to try.

Since this is not 100% conclusively resolved, I’ll add two very simple suggestions that you probably tried in the beginning, mostly for the benefit of some future sufferer.

First, make sure your plugs are clean using electric contact cleaner and dielectric grease. This fixed an intermittent light problem on my boat trailer.

Second, always check your grounds with the wires plugged in but with the hitch unhooked. Intermittent grounding through the hitch concealed wiring problems on my utility trailer that advanced my hair loss considerably.


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