LED turn signals and hazard lights

I’m currently working on a 1970 mustang and recently replaced all of the bulbs with LEDs. Front turn signals, rear brakes, side markers, etc. I also changed out both flashers with basic low-load flashers you can find on amazon.

The turn signals and hazard lights just stay on now. No click, no flash, just on. To make sure there were no wiring issues, I replaced the front turn signal and brake lights with the original incandescent bulbs. Everything works fine.

Next I replaced just the front turn signals with LEDs, leaving the brake lights as incandescents. The turn signals now work, but the hazards just stay on when activated like before. I’ve heard of the in-line resistors, but that seems like it would only solve a hyper flash issue, not a no flash issue.

I’m afraid I’ll run into this same problem with my 1970 XR7 Cougar, knowing that is more complicated with the sequential turn signals. I know Don sells some kits for the cougar but a mustang should be much simpler, right? What am I missing?

The LED’s do not draw as much current as a conventional bulb. The turn signal flasher in a Mustang is expecting to encounter a certain amount of current. That’s how it works.

For a Cougar Vic Yarberry sells a sequencer that works with LED’s. I don’t know about Mustangs.

The WCCC LEDs for the '70 Cougar work great. Nice and bright and even sequencing.

70 wiring is very screwy with side marker lamps having no ground (turn signal and running lights are the two wires). I’m not sure what happens when LED’s are put there. Try only replacing the incandescents there…

Nor sure what sequencer is used in 1970, so this may not help. On my '67 I got my original mechanical sequencer to work well, and fitted the LEDs - system worked fine The dash warning lights did not flash but an upgrade is available.

You have several problems here.

The use of LEDs means you need to change flasher unit/s out to an electronic unit. This is because the LED’s use so little current draw they can not trigger an original style Flasher unit.

Next, LED means “light emitting diode” and the function of a diode is to limit current flow to one direction only, just like the diodes in an alternator. The only difference here is they emit light.

The problem with the 1970 mustang/cougar is the indicator/hazard lights work by reversing the electrical current direction. That is, current flows in one direction for the indicator function, and then in the opposite direction for hazard light function. (The 1969 mustang/cougar is different)

So, if you fit a diode to the indicator lights, you may find that you have indicators but no harrard lights. If you now turn the LEDs around in the light assemblies, you may now find you have hazard lights but no indicator lights.

The problem can be solved, but complicated.

You may be better off just using incandescent lights and Plasma lights on your two stop lights with a resistor for the rear park light circuit.

(Plasma light globes are so bright on the park light mode, that it is difficult for drivers behind to notice the difference between the stop and park light.)

Peter :slight_smile:

The '70 Cougar represents the hardest of the years to convert to all LED for the reasons explained above but it can be done. If you want to save yourself a LOT of grief, just buy the entire kit from WCCC. It’s pricey but it works.

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As mentioned, the side marker lights are wired uniquely on the 1970-1971 Fords including Mustangs. When the headlights are OFF, the turn signals and side marker lights flash together. When the headlights are ON, the turn signals flash the opposite of the side marker lights. Your setup will work if and only if:

  1. If you install front LEDs, you must use a D0WY-15A434-LSM side marker adapter kit to isolate the side marker light grounds.
    Due to the way the side marker lights are grounded through the front turn signal light element, current goes one direction during turn signal operation and the opposite direction when the headlights are turned on! Most LEDs won’t work backwards. The other problem is that, even if you did use non-polarized LEDs, the current passing through them will turn on the front turn signal LED thus the need for the isolated ground circuit.

  2. Use LEDs marked WCCC1157 or equivalent.
    Some LEDs on the market only use a 50-70 ohm resistor to reduce the voltage for the running lights. The problem is that the current can travel backwards through the other half of the LED and send the wrong voltage signal to other LEDs and the side marker lights (and the flasher).
    A list of tested LEDs are on my website:

A simple test for an LED is to use an ohmmeter to measure the resistance across the two pins on the bottom of the bulb. Measure both directions. It should show as an open circuit (“OL” on many meters).

IMHO, the WCCC1157 bulbs are the best because of the better brake-to-running light intensity ratio. On the other cheaper bulbs, it is hard to tell the difference between the brake light intensity and the running light intensity.

  1. On a Mustang, use a standard LED flasher. The flashers I manufacture for the Cougars/Thunderbird/Shelby Mustangs have a 70% ON ratio and a slower cycle time - designed for sequential lights. The sequential flashers stay ON longer so all the lights have time to turn on. A standard flasher, with a 50% ON time is better for a Mustang because there is only one light per side.