Automotive Trailer Wiring - Something weird is happening here.

Thread Starter

PaulCalif

Joined Oct 14, 2018
8
My buddy has a new to him RV. He tried to connect wiring for the lights on a Jeep he wants to tow, so trailer wiring. I told him how, and he was getting strange results.

The RV shares the turn signal filaments and brake filaments. The Jeep also shares the brake and turn signal filaments, he had installed dedicated bulbs used solely for towing, he did not use the Jeep's normal lights.

Skip ahead to purchasing magnetic external trailer lights for troubleshooting. These lights have the same weird problem as the Jeep lights.

The Problem:
When either of the blinkers is turned on while either the brakes and or the tail lights are on the other side bulb dims a little. It looks like a blinker is turned on, on one side, while the opposite side also blinks (dims) just a little. So bright on one side while dim on the other.

The lights on the RV work normally.

I figured my friend had wired something incorrectly, or that it had a bad ground, or that the wires couldn't carry enough current.

I had him purchase a four relay circuit board and he ran the wires from the blinkers and the tail lights to the trigger of the board.
Then I had him run a dedicated 12 gauge wire from the RVs battery to the circuit board's 12V+
I also had him run a second 12 gauge wire from the battery + to the common side of each relay.
I also had him run a dedicated negative/ground wire both to the circuit board and to the trailer wiring harness

I had him do all of this as the last step since nothing else we tried made a difference.

I really thought that it was going to be a bad ground, a bad connection, or too small of wire.
But it doesn't appear to be any of the above.

It just struck me that maybe the ribbon cable used to connect the 4 way connector to the relay may have under sized wires???

Anyway, I could use some brain power on this from you guys that know electronics - any idea what else might be causing this?
Thanks in advance
Paul
 

drc_567

Joined Dec 29, 2008
729
... Try cycling the RV Hazard Light switch a few times. That switch passes the turn signal wiring through, but likely doesn't see much use. Maybe it has accumulated oxide or dust or something. Electrical contact cleaner may do some good if it can get to any contact surfaces.
 
Last edited:

Thread Starter

PaulCalif

Joined Oct 14, 2018
8
... Try cycling the RV Hazard Light switch a few times. That switch passes the turn signal wiring through, but likely doesn't see much use. Maybe it has accumulated oxide or dust or something. Electrical contact cleaner may do some good if it can get to any contact surfaces.
Thank you for the suggestion. I'll try cycling the hazard switch, but if that was the problem wouldn't the coach lights act the same as the external trailer lights?
 

Thread Starter

PaulCalif

Joined Oct 14, 2018
8
Or you wired it wrong, which is more likely by my opinion, you should have some specification for that trailer and jeep.
Thanks for that. It does seem obvious that it is wired wrong and I didn't do all of the wiring but I did question my friend several times and I looked over most of it last night. He doesn't live close to me and I was in his area late last night.

Forget about the Jeep, we are testing with external magnetic trailer lights that are new.

The wires are spliced into the wires that go to the turn signals and tail lights. Originally straight out to the external trailer lights but now going to relay board. The ground was the same chassis ground as the lights but now coming directly off the battery. I even had him try the coaches accessory battery, no change.
 

Thread Starter

PaulCalif

Joined Oct 14, 2018
8
...specific wiring diagram required for a better idea of what the problem is.
Yes! But I dont think that's available and even if it was, a lot of the wiring looks like it has been modified.

I just dont get it, coach likes work, we are wired directly to them, not pulling any power to speak off since we are going through the relay.

I'm thinking about throwing diodes on each of the blinker and the tail light wires going to the relay.
Trying to isolate the problem.
????
 

Hymie

Joined Mar 30, 2018
710
These relay module boards are mostly available in 12V and 5V versions, are you sure that you have not installed a 5V unit in error, which could cause all sorts of problems.
 

drc_567

Joined Dec 29, 2008
729
If nothing else works, try placing an electrolytic capacitor to ground in parallel with each jeep blinker light. Maybe the blinking is creating an inductive voltage at the blinker lights and can be shunted to ground through a capacitor.
 

Thread Starter

PaulCalif

Joined Oct 14, 2018
8
These relay module boards are mostly available in 12V and 5V versions, are you sure that you have not installed a 5V unit in error, which could cause all sorts of problems.
Yes, 12V, I've used several of these boards for other projects. As I noted, we had the same results without the relay board. Bizarre that this is happening.
 

Thread Starter

PaulCalif

Joined Oct 14, 2018
8
If nothing else works, try placing an electrolytic capacitor to ground in parallel with each jeep blinker light. Maybe the blinking is creating an inductive voltage at the blinker lights and can be shunted to ground through a capacitor.
Wow, now that is thinking! What size cap would you suggest?
 

drc_567

Joined Dec 29, 2008
729
Try 100 uF maybe ... check that polarity is correct ... + to 12 volts, - to ground.
100 volt or 50 volt rating ... whatever is available.
 
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