No turn signals when lights are on

[b5cuda]

Hello everyone, hoping someone has some clues to solve a new mystery on my ’69 cuda. All turn signals work as normal until I turn on the lights (parking or headlamps). Then the turn signals (front and rear) stay lit; no alternating signal. Also, the turn signal indicator lights in the dash stay on constant. When I turn off the lights, everything works again as it should. All the bulbs are incandescent, except the dash illumination bulbs which are good quality LED. Dash turn signal bulbs, as I recall, are incandescent. Could headlamp switch be the cause, or maybe the flasher module? Thanks in advance.

 

[slantsixdan]

It's not the headlamp switch or the flasher. Most likely it's one or more problems with faulty grounds, faulty sockets, faulty wiring, and/or faulty bulbs. The easiest thing to try first is to put in four new exterior bulbs. Use regular 1157 bulbs in back, 1157A or 1157NA in front. Pick a reliable brand (GE, Philips, Sylvania). What you're hoping for here is that one of your old bulbs has dim and bright filaments shorted to each other, or trouble with their base shells or contacts—or that somehow a wrong bulb (a single-filament item such as an 1156 or 1141) somehow got crammed into one of the four sockets, and its single base pin is bridging the two contacts in the socket and creating the path for the feedback you're describing.

 

[b5cuda]

I recently replaced the front turn signal housing bulbs, makes me wonder whether I've got grounding issues there. Thanks Dan, you're the man!

 

[67Dart273]

Likely grounding or bad bulb. Bulbs can be internally shorted. The lamp shell must be grounded. You can easily (if you have a large iron/ gun) solder a grounding pigtail to the socket shell

 

[Red63440]

Pull your signal bulbs and check your sockets and bulbs for corrosion, you have a short that's going to ground and affecting your turn signals.

 

[cosgig]

Try to use the bulbs with the brass caps instead of the nickel silver ones. The brass caps provide a much better ground for the bulb.

 

[slantsixdan]

No, cosgig, you've got that backwards; actually the nickel-plated bases provide a much better and more durable ground for the bulb.

 

[Johnny Mac]

Not sure how similar your harness is to my 74 dart, but I had that exact issue, and it was the ground on the radiator core support. Touched it...and it'd ground. Take pressure off it, and it would kill the ground and the blinkers would stop blinking with the headlights on.
I put a new bolt in and made sure the ground was solid.

 

[b5cuda]

Thanks for all the great input, much obliged! Another question - is it helpful to put dielectric grease in the light socket of the front turn signal housing?

 

[slantsixdan]

Dielectric grease is non-conductive. It's a messy, olde-tyme way of slowing down corrosion between the socket and the bulb. Use bulbs with nickelplate bases (silvery appearance) and the corrosion won't happen in the first place.

Related matters: see here

 

[b5cuda]

You called it! I ran good grounds to both turn signal housings, problem solved. Saved me a lot of time and frustration by pointing me in the right direction - thanks so much!