[DML] Re: 3 post idle microswitch

["endotex23" <endotex23@xxxxxxxxx>]

Good Lord, such a fuss over something so simple. Instead of giving out
fish let's try and teach fishing. Apologies to those who already know
this very basic stuff.

As Martin points it's a single pole, double throw switch. All switches
have two parts 1) The electrical portion that does the actual
switching of current and 2) The actuating portion. The two portions
are mechanically interconnected but electrically isolated from each
other. (After all, you don't what electricity on your switch
actuator).

The term single pole refers to the electrical portion. It simply means
the switch is designed to switch only one electrical circuit, or
"pole". The normally open (NO), normally closed (NC) and common (C)
refer to the switch's contact state when the actuating portion is
*not* active. The "throw" part also refers to the electrical portion
of the switch. Pressing the lever results in the internal contact
"throwing" one way and releasing it results in it "throwing" the other
way. It's the same as "throwing" any manual switch (like the light
switches in your house) and in fact is where the term originates from.
The common terminal is just that: a common connection between the two
throws.

The actuating portion can depend on any number of things. It can be
manually actuated or actuated by high or low (vacuum) pressure, flow,
temperature, or any number of other physical properties. In this case
it's a manually acutated lever switch. Depressing the lever cause the
electrical portion of the switch to change state from what it was when
"normal". This the key to understanding it. So what's considered
normal?

Take any switch out of the box and put it on the table in front of
you. In this state there is no actuating force. Thus there will be no
continuity (no contact closure) between the Common and Normally
Open terminals. Why? Because that part of the switch is in it's
"normal" state. After all, the switch can't get much more "normal"
than sitting on the table staring back at you. Using the same line of
thought there will obviously be continuity bewteen the Common and
Normally Closed terminals because that part of the switch is
"normally" closed.

It's only when you actuate the switch using whatever force variable it
needs (in this case by depressing the lever) does the switch change
state. Now there will be continuity bewteen C and NO because the
switch is no longer in the "normal" state. Before you pressed the
lever the path between C and NO was open, now it's closed because all
normalcy is gone.

Think of the pushbutton on your doorbell, a perfect example of a
NO switch. It doesn't have a second NC contact in it so it's only
"single pole, single throw". Pushing the doorbell button gets you a
closure between the C and NO terminals. It has a single throw, it's
all you get. Yes, when releasing it you get a second throw but since
the switch has no "guts" for NC operation it's considered single
throw. The term "throw" only applies to an action that causes a
contact closure.

There is no need to label such a switch C, NO, or anything else
because it's designed to perform only one function, the NO function.
You only have two terminals so no labeling is needed. No mystery
there, one must be common and the other must be NO. (Since such
switches also come in normally closed it's up to you to be sure when
buying the switch it's a normally open one for doorbell use).

In this case the idle switch we're talking about needs to be wired
just like your doorbell. You want the switch to close and complete the
circuit when the throttle screw pushes on the lever at idle. The
throttle screw is your finger on the doorbell. Thus you want a
Normally Open function because when the throttle screw *isn't* pushing
on the switch lever it's the same as when the switch was on the table.
Your Otterstat is the same way, it's open until a certain temperature
(it's actuating "force") closes it. Same with your WOT and Lambda
Thermal Switch, further examples of NO switches. In the box, on the
table, or when cold or *not* at WOT they're all open. Think about what
is "normal" with no actuating "force" applied. Simple huh? Now you
know.

A look at the car's schematic confirms this, it shows you want the
switch to be closed at idle so it can supply power to the idle ECU. 
By opening when you step on the gas it kills the ECU. (The switch also
serves another function but it's not important to this discussion).

Remember, switches and relays are always drawn in their "normal" state
on schematics. This is true for switching devices on any schematic.
It's up to you to remember what is "normal" when they're installed or
if other forces are affecting them at any given time. For example, you
could wire a switch normally closed and then hold it open. Such a
switch in a doorbell would require you to release it to ring the bell.
The point is switches are always drawn in their normal state. (So are
the contacts of relays. They're always drawn in the state they'll be
in when no power is on the coil, it's what's "normal" for them).

Confused? Two minutes playing with a ohmmeter and the switch will tell
you everything you want to know about how such switches work. Adding
poles doesn't change this. Add more poles and a coil to supply the
actuating force and you have a relay. In fact, if you examine the
internal contact arrangement of a relay and forget about the coil
you'll see pretty much what's inside your idle switch. Often called a
microswitch (in reality a brand name), it's correctly called a
snap-action switch. The click you hear when actuating one is the snap
action mechanism in operation. I suggest you carefully crush your old
switch in a vise and inspect it's internals. (Keep in mind that
whenever you toss something in the trash without an autopsy you may be
losing a valuable opportunity to learn something).

There is much more to know about the application, testing, repairing,
and cleaning of snap-action switches (even when they're sealed) but
I'll leave that for another day. Just remember Common is the
terminal that affects all other terminals in that "pole" and
"normally" means just that: the state of the switch when at rest in
the box, on your workbench, or anytime other time actuating force is
not applied.

And you thought something so simple couldn't possibly consume so many
words to describe. Maybe so, I've been told if you ask me what time
it is I'll tell you how to build a clock. If you think this is long be
thankful I'm not explaining how to set up or troubleshoot idle
problems. But don't ask, it would take too long and if you didn't know
how to wire the switch I suspect anything more technical would be too
much, too soon. You have to learn how to bait the hook before you can
fish.

Greg








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