LED must have a well working ground and at least 12v

 

Maybe this might help...

 

Some light circuit boards are constructed poorly.
The 88 to 00 gmt400 taillights are notorious for taillight issues due to bad circuit boards as a example.

You will have good power at the plug. Good bulbs and sockets. But the wiring from the plug to the socket has a short/break in the board itself.
Sometimes you can run a jumper, sometimes its better to get a new circuit board.

Unless that plug is connected directly to the socket you (usually) have some form of circuit board. Most of these are built good for several trouble free years. Others are just designed bad and are prone to failure.

Depending on what this is if it is possible to get a new circuit board that would be my move instead of trying to run a jumper of sorts.

 

Couple of things. This probably doesn't apply, but it's worth mentioning. Regular incandescent bulbs really have no polarity. They will work with a positive ground or a negative ground. However, LED's will only work if the polarity is correct. You can check polarity with a cheap multimeter. Not sure what bulbs you have, but some I have seen had a plug that went into the 1157 socket and have wires that go to the actual bulb. You see this often with specialty LED units made for older cars. I have also noticed that electrical stuff made in Asia tend to use white for the ground and black for power. I have ran into this color coding on several offshore LED bulb units.

Also, be aware that even if you use an LED rated flasher, it may still need some load to trigger the flasher circuit. If you still have some incandescent bulbs in the system, you are good. If not, you will need to add some load in parallel to the bulbs on at least one end of the car. A 6 ohm 50 watt resistor will work. I just went through this with my current build. LED's front and rear. They would light up, but not flash. I added the resistors to the front circuit and everything worked as it should. Don't know if all LED rated flashers are this way, but mine was and it was clearly marked LED on the flasher. It wasn't a cheapo unit either. It was a Standard Products brand flasher. This is apparently such a common problem with LED conversions that my local Oreilly's had the 6 ohm resistors on their rack under the Sylvania brand.

EDIT: Make sure the flasher is wired correctly. Mine needed +12 and a ground. I misread the pinouts on the wiring diagram and had them reversed, Nothing worked until I fixed that.

 

I put 1157 running and turn signal LEDs in my "72 Monte Carlo along with the electronic flashers that have a ground wire. Turn signals and hazards flash fine until I turn on the running lights/headlights, then they stop flashing, though the flashers are clicking and the dash indicators glow at 1/2 strength.
I tried switching the leads on the flashers, that didn't work.
Any ideas?

 

Insufficient grounds.

When the head lights are turned on the power they send to ground is higher than the power the LEDs are sending to ground which then shuts off or reduces the current flow from the LEDs. Or independent grind wire from the LEDs to the battery negative post. The whole game is to get the ground side potential down to where current flows through the LEDs.

I’d start with a ground wire from battery negative to the head lights. If that doesn’t work a similar ground wire path to battery negative post with the headlights using the chassis ground they currently have.

You just have to experiment a bit to see what changes the ground potential on the lights.

Bogie

 

Thanks Bogie. This is my 3rd LED conversion. Each time I have used the same CEC electronic 2 prong flashers with a separate ground lead. Previous two I purchased LED bulbs from Superbright, this time I bought from amazon.
Laying under the dash with the hazards flashing and hearing the soft electronic click I pulled the headlights on and heard the flasher 'stuttering', like a starter solenoid with a low battery.
Hmmmm, not enough load? Checked the wattage draw and the superbright bulbs draw slightly more watts than the amazons, imagine that!
Put the incandescant bulbs back in the front TS and everything works fine. Got some load resisters for the front and replaced the LED's up front.
Everything working as it should.
The money saved on bulbs wasn't worth the aggravation!

 

That’s likely the power consumption is not enough for a mechanical flasher. These work by heating a bimetallic switch the current flow heats the bimetallic casing it to make and break contact between cooling and heating cycles. LEDs pulling less current would affect that cycle.

I’m just not impressed with changing service lights to LEDs the power regular bulbs consume is minuscule compared to stereos, air conditioning fans, radiator fans, etc. I suppose as a headlight replacement there’s certainly something to be said for brighter forward illumination, but for marker lights I rather think the old incandescent bulbs are sufficient.

Bogie

 

The issue is what’s called a “floating ground” on your front side marker lamp and the front parking lamp/signal.
LED bulbs don’t have the resistance to provide a ground path for proper function of the front side marker and front turn signal lamps.
Google it to learn more.
I had the exact issue. I have not seen a resolution. What I did was leave a standard 1157 bulb up front, and was able to use LED 94 on the side marker, and all the rear lamps.
But upside is All of my ground connections are clean.