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  #1  
Old 02-03-2012, 03:38 PM
Astrowing Astrowing is offline
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Default Turn signal questions

I'm having trouble with the turn signals on my F100. Brake lights work and the turn signal switch appropriately turns off the correct stop light. Yes, it's not a Tbird but it is the same design. Per a post that Simplyconnected made some time ago, the 1157 light bulb is rated at about 24 W, so with one light bulb, it would draw about 2 amps, and therefore you could model the light bulb as a 4.6 ohm resister. When the turn signal is activated, by moving the lever left or right, it is flowing current through 3 light bulbs in parallel. The front parking light, the rear turn /stop light, and the light bulb in the instrument panel which is a 57. If I run three 4.6 ohm resister equivalents in parallel, I get 1.5 ohms. Therefore you would expect it to draw about 7.8A when it is flashing. The circuit protection is with a 14A fuse, so it will only blow with a dead short.

How much voltage drop would you expect to see across the flasher and how much current does it typically require to have it open? I believe my problem is the flasher feed (blue wire) from the flasher.
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Old 02-03-2012, 10:17 PM
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Yep! Ford never made any of the bulbs or flasher units. Like every car manufacturer, they bought them from Tung-Sol or whoever.

The only important bit of info you are missing is...
Your turn signals and brake lights are fed from two different sources. Look at the print; the flasher has its own fuse from the key sw, and the brake lights have their own.

The turn signals interrupt the brake lights. When enough current flows through a proper flasher unit, it acts just like a self-resetting breaker. Add trailer lights, and the flasher interrupts more frequently. My point is, your system (with 'x' number of lights) needs to match the flasher. Some cars only have one bulb in the back, one in the front, and a dash light. Others (like the T-bird or Cougar) may have three lights in the back and one in the front. By federal mandate, they must all flash at a given rate.

Incandescent light bulbs are resistors, but their resistance changes (goes higher) as they heat up. The flasher unit (being a current device) has a bi-metal strip which is yet another resistor that heats up, in series with the bulbs.

"How much voltage drop would you expect to see across the flasher and how much current does it typically require to have it open?" The resistance is low because it is a heater made of Nickel-Chrome and steel. The more current you pass through it, the hotter the bi-metal gets until it flexes and opens its own contacts. Remember, they start with the contacts closed.

Check your Blue wire with a volt meter. That feeds the instrument column with flasher power. Green-Orange is your LH rear signal light, White-Blue is the LH front signal light. If they both do the same job at the same time, why are they separate? Because you don't want the front coming on when you press the brake. So, the turn signal plate disconnects the brake circuit and ties the front & rear signals together just for flasher operation.

Same story on the RH side... Orange-Blue = rear, White-Blue = front.

If your bulbs, sockets and wires are good, you can troubleshoot everything at the base of your steering column, where all the bullet connectors plug in. Hope this helps. - Dave
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Old 02-06-2012, 05:54 PM
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Thanks, Dave. I'll treat the flasher as basically a switch and it should have no voltage drop until it starts heating. I also understand now that if you have too much load, the flasher might not ever open, and the lights would stay on solid. If you don't have enough load, like from a burned out bulb, the flasher will flash very rapidly as it heats up much faster in that case.

My Cougar had the sequential tail lights and you could hear the relays clicking in the trunk so you knew they were working without even getting out to look.
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Old 02-06-2012, 07:13 PM
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Yes Jim, but you have it backwards. Low resistance allows more current to flow. As bulbs heat, their resistance raises (choking off current). So, if one of your bulbs blows out, your flasher may never pass enough current to heat the bi-metal strip inside (and your lights will be on forever).

Adding more bulbs lowers overall resistance, passes more current, and makes the flasher cycle too fast. - Dave
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