You do indeed have a three-terminal flasher. The pigtail you grounded is the third terminal.
Your original flasher was a common two-terminal flasher, and you're right that one gets the twelve volts from the signal switch and the other goes to the lights. There is a heating element between the two internal contacts. It takes a minimum amount of current flow through that heater to cause the bimetallic strip to bend. That current is a result of the amount of resistance in the rest of the circuit. To say that a different way, more light bulbs provide more places for that current to flow, and total resistance is real low.
Before the bimetallic strip has bent far enough, current is limited by the resistance of the heater. Only a volt or two is dropped across the bulbs, so they are off. Ten to twelve volts is dropped across the heater, so it gets hot and makes the bimetallic strip bend. When it bends far enough, the two contacts touch. That switches the full twelve to fourteen volts to the bulbs, so they turn on, and it shorts out the heater, so the bimetallic strip cools down and straightens out. That breaks the contacts, and the lights flash off again.
This whole magical story is dependent on current flow through the flasher. When you change over to LED bulbs, they draw very little current, so little current in fact that the heater does not get hot enough to make the bimetallic strip bend and turn the contacts on. This is where adding the resistors gets the current up high enough to make the standard flasher work.
Hazard flashers have to run twice as many lights, meaning both sides at the same time, so their heaters are designed to need twice as much current to make them flash at the proper rate. These are used when towing a trailer too because of the additional lights. Because of their need for higher current, heavy duty flashers are not desirable for use with LED lights.
The LED flasher you installed works differently. It still gets the twelve volts from the signal switch, but it needs a separate ground. Those two are needed to run the internal circuitry. That circuitry has a timer that turns a relay or switching transistor on and off at a predetermined rate. That switching rate has nothing to do with the amount of current going to the lights. Current going to the lights goes out the same terminal as on the original flasher.
I cannot find a diagram to refresh my memory, but I will do the best I can. Do you know what is meant by "series" and "parallel" circuits? If you do not, I will explain that later. All of the tail lamps are in parallel on most vehicles. When one burns out, it has no effect on any of the others. Normally all of the signal lights on one side are also in a parallel circuit, but on yours, the front signal light and the front/side marker light are in series. If either one burns out, both will not turn on. The front bulb, as I recall, is an 1157 with two elements, but only the brighter signal element is used. Current flow comes in on the smaller side marker 194 bulb, goes through it, then goes through the 1157, and then to ground. The 194 has much higher resistance than that of the 1157, so it drops, (uses), more of the twelve volts, lets say eight volts. It looks pretty bright, and the 1157 only gets four volts, so it looks as dim as the smaller filament in a standard running light. Both bulbs look normal when the head lights/tail lights are on.
When the signals are turned on, the flasher switches its full twelve volts to the junction of those two bulbs. Twelve volts is already applied to one side of the 194 bulb through the tail light circuit, and it gets twelve volts now to the other side from the signal circuit. With twelve volts on both sides, the net difference is zero volts, so the 194 turns off. At the same time, that twelve volts from the flasher is at one side of the 1157, and since the other end is grounded, it lights up full brightness. The two bulbs appear to alternate, one is off and one is on.
With the head lights on, I think what is going to happen is the approximately four volts at the junction of the two bulbs will be seen at the flasher's output terminal and cause the circuitry to not function properly. The other problem is depending on how closely-matched the two front LED bulbs are, neither may get enough voltage to turn on. An LED needs very close to 1.6 volts to light up. Multiple LED's are wired in series so their total requires twelve volts. The problem is that except for a small range of voltage variation, LED bulbs can not be dimmed by lowering the voltage like we can do with incandescent bulbs. Basically, an LED is fully-on or fully-off, with little in between. Dimming of LED bulbs is done with "pulse-width dimming", (PWD), aka "pulse-width modulation", (PWM). Circuitry turns an LED bulb fully-on and fully-off roughly four hundred times per second. Our eyes see the average of that. By varying the percentage of off-time and on-time, we can vary what we see as a change in brightness. Pulse-width dimming has been used for dash lights since the early 1990's, so LED bulbs will work fine in those applications.
The problem with putting two bulbs in series in your truck is if they are identical, or draw the same amount of current, they're going to drop an equal percentage of the twelve volts, meaning each one will get six volts. That might not be enough to turn it on, or it could be enough to turn it on full brightness. If one of them turns on with six volts, it will not be much, if any brighter, with twelve volts when the signal circuit turns on.
Without experimenting with this circuit, I suspect the only way to make it work properly is to go back to the original thermal flasher, then replace the original bulbs with resistors. Those will cause enough current to flow to make the flasher work. Now, add the LED bubs in the front, and connect each one across the resistor that replaced that bulb. Adding a regular bulb, as in adding a trailer to the circuit, increases current flow and makes the flasher flash too fast, but adding an LED bulb increases current very little. The flasher is likely to not even notice that.
If you need resistors in the rear, those should not get hot because they will only have twelve volts across them when the signal or brake light is on, and that is very intermittently. On the front, they will each have only six volts across them instead of the normal twelve volts, so current flow through them will be half of normal. That means heat will be lower, but it will be there all the time the head lights are on.
An 1157 brighter filament draws very close to one amp. Ohm's Law lets us calculate that twelve volts divided by one amp equals twelve ohms of resistance, so you would need a twelve ohm resistor in parallel with an LED bulb to cause enough current to flow to make the flasher work. We can also calculate the power rating for that resistor. Twelve volts times one amp equals twelve watts. That is a pretty significant value that will let it absorb and give off the heat it generates. Ten-watt resistors are common in TV repair work. Those should work fine in this application. The most common resistor in TV's years ago was the half watt resistor. Those are slightly larger in diameter than the lead in a pencil, and about 3/8" long. A ten watt resistor is usually about 3/8" square, by about 5/8" long. The larger size is needed to provide more surface area to get rid of the heat. For this application, they will not get anywhere near hot enough to burn your fingers, but they should get warm to the touch. The half watt resistors are made of carbon and have a plastic coating to hold it together. High-power resistors like a ten watt will have a wire wrapped around a ceramic core, and that will be inside a ceramic block that looks like a rock. There are also power resistors that are inside metal housings with little cooling fins. They are made to look pretty in applications where they will be seen, but the advantage is they usually also have two small mounting holes to allow them to be bolted on solidly. The nice thing with adding resistors is they can be, but they do not have to be mounted right by the bulb. They can be put anywhere you have access to the wires. You can even use regular bulbs as the resistors, and they can be put inside a box, or under the hood, anywhere they will not be seen.
Wednesday, November 22nd, 2017 AT 6:04 PM
