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Discussion Starter #1
I have converted all the exterior lights and the instrument cluster lights on the wife’s 66 to LED’s. I had some weirdness with the taillights that I traced to bad grounding, which was fixed, so they work fine now. But I have been troubleshooting the front turn signals for two days now and can’t figure it out!

Here is what is going on... when I click the turn signal left or right both front turn signals blink, the commanded light blinks bright and the other side blinks dimly. Now when I turn the headlights on (past the parking light detent) and click the turn signal left or right, the commanded turn signal blinks from high brightness to low brightness and the other side is on low brightness with a dim flash.

When I take either front LED turn signal bulb out and replace it with an incandescent, it fixes nearly all the problems (still have a slight dim illumination on LED bulb with headlights on and blinker on). And with the front LED’s installed, headlights or parking lights on, when I click the turn signal the dash signal indicator will stay lit with a very minor change in brightness as the flasher clicks along.

I have installed a three wire flasher. I found an issue with a previous set of LED bulbs where an internal resistor burnt up, so I replaced the bulbs and still no go. I even went as far as unsoldering the bulbs to make sure they had a diode installed to prevent backfeed, and they do. I am trying to avoid installing ballast resistors as that seems like a bandaid to me, a hot inefficient bandaid. I also took apart the turn signal switch and activated each contact independently and I get the same results.

Anybody running a full set of LED bulbs successfully without using ballast resistors?
 

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I'm no electrical expert but electricity seeks the path of least resistance. It appears that the path of least resistance to ground in your situation is through the other LED. An incandescent bulb has a lot more resistance than an LED and that's why it doesn't illuminate when swapped out for an LED, the electricity finds a route to ground through something with lest resistance like a ground wire.
 

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Had a similar problem - turned out to be the idiot light on the dash for directionals. Needed to grounded by it self - found that out from the guy from Vintage LED's - $10 bucks for the LED and all fixed.
 

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I suspect a slight ground problem. A LED bulb had very high resistance and a incandescent bulb has very low resistance. When you have a series circuit everything has the same amperage going through it. You add up the resistance of everything then divide that into the supply voltage and you get the amperage. Then you take that amperage and you multiply it by the resistance of each device, that gives you how much voltage that device has on it. The higher the resistance, the higher the voltage. The voltage of each device added up will equal the source voltage. To go one step farther, if I take a battery with nothing connected, the resistance is infinite. 12 volts divived by infinite resistance will produce amperage. Impossible to read amperage but theoretically yes according to Ohms law. So if you put a LED bulb in series with a incandescent bulb the LED bulb is going to be the choke point in how much amperage. It will be a small amount, enough to power the LED but not enough to operate or produce the needed calculated voltage to make the bulb work.
 

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I meant to add to what I was saying before about the parking and directional lights. The bulbs have 2 filaments that share a common ground. If you have either a poor or no ground on that bulb the power is going to look for one. That means since the 2 filaments share a common point on the shell of the bulb as the ground, the power will go through the other filament to another bulb’s ground. It becomes a spider web for the power to flow.

LED’s have significantly higher resistance then a incandescent bulb which means significantly less amperage flow and heat. Adding a resistor is only going to reduce amperage draw and heat even farther. Bottom line LED’s are going to me more susceptible to small, stray currents and working.
 

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Discussion Starter #7
Had a similar problem - turned out to be the idiot light on the dash for directionals. Needed to grounded by it self - found that out from the guy from Vintage LED's - $10 bucks for the LED and all fixed.
That’s one thing I haven’t explored yet and is next on my list. Did you just have to run a separate ground to the gauge cluster and/or bulb socket or did you install a special LED bulb?
 

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Special LED with a built in ground. I know this will sound like something coming from leftfield but can you check for a bad coil wire - OK guys stop laughing but my cousin had an issue with a steady on idiot light and very slow/stopping directional LED's - found the coil wire had a small hairline crack and was jumping over to the fuel line - was running a little rough - we changed the coil wire and his LED problem cleared up - was FM coded fix - Fu*king magic.

Flasher note - I used one with a built in ground wire that needed to be hooked up....Just FYI.
 

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Had a similar problem - turned out to be the idiot light on the dash for directionals. Needed to grounded by it self - found that out from the guy from Vintage LED's - $10 bucks for the LED and all fixed.
Yep!
 

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I run all LEDs outside and inside. Have had some weird issues in the past but they were all caused by either the turn signal switch (replaced it), the flasher (installed one for LEDs w/ ground wire, $9 on amazon), or bad fuse connections (cleaned contacts).

You've already addressed most of these but I would recommend swapping the bulbs in the dash for LED ones as well (or at least the direction ones). You can't dim them but it removes the risk you are causing problems having incandescent bulbs and LEDs on the same circuit.

I also would take a look at your turn signal switch; given you get at least some light in both signals when you click it left or right it sounds like there's a switch issue too since those should be separate circuits.
 

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One way to solve LED issues is to run them off parallel circuits with incandescent bulbs. Just some small pigtails with 194 sockets/bulbs hiding somewhere. Wire the 194s in parallel with the turn signal filaments. I did this on my TBird in the rear because there were issues with electric bleed through and also the cruise control. Now there are 2 194 bulbs hidden in the trunk that flash or light with the brakes. Sounds dumb, but it worked perfectly.
 

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Discussion Starter #13
Had a similar problem - turned out to be the idiot light on the dash for directionals. Needed to grounded by it self - found that out from the guy from Vintage LED's - $10 bucks for the LED and all fixed.
Yep!
I have a sneaky suspicion this is the cause. It gets really whacky with the headlights on, so if the ground from the cluster to chassis is amiss I can see it finding a path to ground through the gauge cluster backlight bulb wiring which would in turn light up the running lights portion of the front LED’s.

I’ll dig into it again this evening and will let you know what I find out.
 

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What year mustang is this? The 1970 mustang parking / directional signal lighting is unique for only one year. If the headlights or parking lights are on, then the marker lights on that side will flash alternately with the turn signals. If the headlights or parking lights are off, then the marker lights on that side will flash in unison with the turn signals. 69 mustang side markers don't blink.
 

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Actually the grounded turn signal indicator form Vintage Led's is for 64.5 and 65 cars using the single turn signal indicator instrument panel. So unfortunately that shouldn't be the problem on a 66 unless it has a 65 dash.
 

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I tried like the dickens to get LED front directional and side markers to work when installed on my 1970 Mach 1. I finally gave up. Back to incandescent and no problems. Seems 70’s are a problem.
 

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I recently converted all of the exterior lights to LED and add sequential brake lights to my Mom's 1965 Fastback. I pulled the instrument bezel and had it resting on the steering column with all of the instruments still wired. I installed a rally pack and a hot lead for a relay switch onto the rear post of the ignition switch. I started experiencing numerous electrical issues where the headlights would flicker and the signals would do weird things. After numerous attempts to troubleshoot, I grounded the instrument bezel via an alligator cable and all my problems were solved. So if you are trying to test the electrical system and your instrument cluster is resting on your column or unscrewed (not grounded), give it a try. Just thought I would throw it out there as knowing this could have saved me about a day of troubelshooting.
 

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Discussion Starter #19
So I yanked out the instrument cluster this morning and this is what I found out...

- The cluster to frame ground was fine but I cleaned all connections anyway.
- I ran a separate ground to the turn signal indicator bulbs and it didn’t change anything.
- I cleaned all light sockets and the bezel frame with a dremel wire brush to ensure good socket to cluster ground, no change.

I am at a loss now. I can remove an instrument bulb from the cluster, turn the headlights on, and just with me holding the bulb socket in my hand the LED bulb will dimly glow. It doesn’t take much continuity to make a complete circuit to light these LED bulbs!

And just to clear some things up I have LED’s everywhere in the car except the glove box light and the shifter light. The turn signal switch is new and so is the headlight switch. I even went as far as taking the turn signal switch apart and jumpered each connection, one at a time, to rule out any cross feeding. The grounds at all four light corners were cleaned and either new ground wires run or jumpers run to prove out existing wiring.

I appreciate the ideas to run an incandescent bulb in the circuit somewhere (which is similar to adding a resistor), but I’d like to get it functional without doing that. Something in these old cars is causing this issue I just need to figure out what.
 

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Wow! I don't have a picture but the LED signal indicator light I used but had a ground built-in from the bulb itself - just like LED flasher I used.
 
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