Left blinker does not work when the running lights are on

[robl]

I have a trailer (1997 A-liner) and am trying to get the exterior lights working. It uses a 7 way plug to my vehicle (2008 4Runner).

I have checked the 4Runner’s plug using a multimeter and everything seems okay:


- +12 volt (black) shows 12v
- running lights (green) show 0 when off, 12v when on
- left turn signal oscillates between 12v and 0 when blinking
- right turn does the same ^


I bought a 7-way junction with connector on Amazon to make debugging easier. I have confirmed all the connections in the junction box match the vehicle behavior.


The problem comes when I connect my trailer; specifically, the left turn signal doesn’t work when the running lights are on. Using a multimeter, with the running lights on:


- running lights show 12v
- right signal works, alternated between 0 and 12v when blinking
- left turn signal stays at 12v WHEN running lights are connected and the blinker does not light up; alternates between 0 and 12 when running lights not connected and light blinks as expected


So that’s the weird thing, the blinker seems to not work only when the running lights are connected. I’m also not sure why the left blinker stays at 12v and doesn’t oscillate when the running lights are connected.


Any ideas?

[NamMedevac_70]

Yes resetting the bulb may work or replacing it with a new one. Also look for wire corrosion due to moisture intrusion as I found recently on one of my trucks where the wire connects to the bulb socket. Cheers from the hot spot.

[ppine]

Try a new bulb.

[wa8yxm]

Bad ground on the non-working lamp.

May be as simple as remove and reseating the blub.

WHY.
When running lights OFF the turn signal works because it grounds through the running lights via the common (Shell) connection on the bulb.

With the running lights ON the DIMMER element in the lamp does the same via the turn signal lamps but when both are "Lit" There is no ground.

Start by removal and re-seating of the bulb. Then start chasing fora bad ground on that socket.

[Bobbo]

I agree with eHoefler. You have a bad ground connection at the left fixture. When the running lights are not turned on, the blinker is grounding through the running lights. When the running lights are turned on, they are at the same electrical potential as the turn signal, so they can no longer ground that fixture.

[John_Burke]

I am assuming when you say running lights are not connected the blinker works you mean the lights are turned off and the plug is still plugged in.
If the blinker on the trailer works correctly with running lights off and does not work with the running lights on there is a short between the running lights and the Left turn signal. On an older trailer first place I would look is in/at the light fixture, next would be at the trailer 7 pin connecter.

[eHoefler]

Bad ground at the tail light