Howdy all,

Nothing much to learn here; just a curiosity.

So I was changing out the turn signal switch due to it no longer cancelling. During the installation, the flasher switch was on for quite a while and the flasher unit (a mechanical unit) got quite hot. The emergency flashers worked, but the turn signals would stall in the unlit position. It seems there is a point when it gets hot enough the solenoid won't fire. Once everything cooled down, it worked fine.

 

Interesting. Don't our trucks have a separate relay for the turn signals and the emergency flashers? If the relay got hot enough that it seized up you might consider replacing it (especially if it's the OE part).

BTW, did you do the test with the truck idling or OFF? With the alternator running the relay might never seize up.

 

That's how the flasher units work, they work on heat. Unless they are the electronic type.

There is a bi-metallic piece inside. Bi-metallic is two pieces of metal bonded together that have different expansion rates. So when the bi-metallic arm heats up, it curls.

The circuit for the lights runs through the flasher. The circuit runs inside, and they wrap a piece of wire around the bi-metallic that has the lighting current running through it. So the power for the lights starts at one flasher terminal, runs through a wire running around the bi-metallic arm, and then leaves the flasher on the other terminal.

As the lights are burning, the current running through the flasher and the coil around the bi-metallic starts getting hot. It is heating up the bi-metallic arm. When the arm gets hot enough, it curls and on the end of the bi-metallic arm is a contact that will break the lighting circuit. When the lighting circuit is broken, no more current is flowing to the lights, so the bi-metallic will cool off and uncurl. The contact on the end of it will complete the circuit, the lights start burning again, and the cycle starts over, heating the bi-metallic arm again.

That is why when you have a light burned out somewhere in the turn circuit, the flasher will not flash. There is not enough current flowing through to heat the bi-metallic arm. That is also why when you hook a trailer to this same circuit, the lights will flash rapidly, there is a lot more current flowing so it's heating the bi-metallic much faster than normal.

A heavy duty flasher is wired differently inside. It works on the same principle, but instead of the current for the lights running through the bi-metallic "heater", it has a branch circuit usually running to ground. Or they will have some electronics inside the flasher if it's still two pin that do not require heat to work.

Same reason when converting to LED's, they sometimes do not blink. Not enough current flowing to heat the bi-metallic inside the conventional flasher.

 

Excellent point. If your charging system fails and system voltage drops slowly while driving, it often goes unnoticed at first. If your truck has an ammeter, it's easy to miss. If equipped with an idiot light, the bulb could have failed. Either way, one of the first symptoms you might notice is the turn signals don't flash when switched on, and merely light up solid.

 

I had the battery on a charger while I worked. Working on electrics, I often do to avoid draining the battery.


What was the real surprise for me was that the flasher unit is STILL mechanical! I figured it would be some chip magic made to look like a read solenoid.

 

The technology has been around for generations. Simple to manufacture and the tooling has been amortized decades ago. Easy money to keep manufacturing a product that works.

 

True, but the voltage regulator in my Failrlane looked like the mechanical ones, but it was only a chip in an old style box. Somebody may still make a mechanical one, but I opted for new old replacement stock.

 

Wrong thread, sorry.

 

wrong thread maybe?

Back to the OP
You know when changing out anything electrical the first thing the book tells you to do is disconnect the battery.
Changing the switch the key should not have been on to have power to the switch and if it was on the battery was not hooked up so no big deal.
Now after the new switch was installed but before the wheel put back on you can hook up the battery for testing then disconnect to finish the job.
Dave ----