full thottle switch

[treed2 at u.washington.edu (T. Reed)]

>   A voltmeter measures voltage, which is like pressure in a pipe. In order
>   to do this it just "attaches to a fitting" on the circuit
>
>   ***voltage = electrical pressure
>   agreed, but you must attach to both legs of the circuit, i.e. two
> "fittings".   Is this what you meant?***

Uh, sort-of.. yeah. I was just trying to draw an analogy. Like "sticking a
gauge on a tire to check tire pressure" is the equivalent of "connecting a
voltmeter to a battery to check voltage". I simplified things a bit..

The reason for "two fittings" or two wires is obvious when you consider
that pressure is not an absolute quantity. All measurements of pressure
(mmHg's, atm's (atmospheres), etc.) are relative to some base pressure.
Okay, I'll take that back a little -- there is such a thing as "zero
pressure" or a vacuum... but in order to describe a non-zero pressure you
have to compare it to something.

All you're doing with a voltmeter is measuring the voltage difference
between the two sides. If you connect two identical light bulbs in
series to a 3 volt battery.. measuring across one of them you will see
1.5 volts. Measuring across the other you will also see 1.5 volts.

Similarly, say you have a (filled) water tank connected to a drain hose
that is full of water and the end of the hose is uncapped. If you punch a
hole at the middle of the hose and let water start gushing out, there will
be a difference in pressure on the tank side of the hole versus the end
side of the hole. That difference measurement would be the same across any
two sides of any identical hole, so long as there was adequate water flow.

>   And, just like a valve or a "flow meter", it behaves to the rest of the
>   circuit as a normal piece of pipe, but with measuring capabilities.
>   A short circuit (but with measuring capabilities, heh!).
>
>   ***disagree with the 'short circuit' idea.  The ammeter is installed
> in-line with one leg of the circuit, it doesn't short to anything.***

No, it doesn't short -to- anything (i.e. ground) but it DOES short across
the two test leads of the meter. If you connect an ammeter across a switch
that is open (turned off), the circuit connected to that switch will
behave as if the switch was on. So you are shorting out the switch by
connecting the meter. The same thing would happen if you dropped the
switch in to a tank of salt water.

The reason this is a concern is that if you connect an ammeter across an
output signal that is supposed to drive something (say, an idle stabilizer
valve, or a cold start valve solenoid, or a light bulb).. you will short
out the upstream circuit.. and if you're lucky, blow fuses. If you're not
lucky.. you will have to buy a new ECU.

>   ***We may be trying to say the same thing here, just in different
> ways.  If so, I'll concede and we can move on...Larry***

It's happened before ;) Sometimes I'm surprised by regional differences in
English within our own country that I never knew existed! i.e. Who the
hell decided it would be a good idea to start calling a Mountain Dew a
'Coke' ?

-Toby