ISV control module schematic

[Ero Rademer]

Hi!

John Sherrow sherrow at westol.com wrote:

> It seemed to me that the car did not have the classic fast idle (an
> early 1200 rpm dropping to 750 after 30 seconds or so), except when
> cold (below 50F). I put a 1.5 kohm resistor between the sensor and
> the ISV control. The car fires right up fast idles briefly then
> settles down to a normal idle hot or cold.

If you *really* put the resistor *between* the sensor and the module
and not *instead* of the sensor the behaviour is clearly correct.
You "say" to the module, that is colder than in reality, so it
"thinks" it's cold and does it's "cold-behavoiur".

I did a (partial) sketch of the schematics inside the ISV module
years ago to find a bug in the decel-valve-circuit but unfortunately
lost that piece of paper since the car was totaled 6 years ago...(:

But: Since the module is fine art analogue-only-technique all
"functions" are controlled by a bunch of OpAmps, resistors,
capacitors, and transistors. I never got the schematics in total,
I stopped when identifying the broken semiconductor...

Most probably the cold-start-reving is controlled by a branch
in the circuitry, loading a capacitor depending on the temp sensor
value and a comparator -- but thats only a wild guess.

But also: It is possible that the cold start reving is simply
an artefact of the (analog!) circuitry settling up and pulsing a
short but heavy current through the ISV only because some capacitors
are loading. The circuitry to power the ISV was quiet complicated
with a current limiting power stage to power the ISV which also
only works when the ISV resistance was in a defined range (somewhere
between 20 and 200 Ohms) So it might be a bug, not a feature...:)
Since the decel-valve is non-operative when the engine is cold it
could "feel" like a 30 second 1200rpm boost dropping slowly...


For a long time I drove the car with a potentiometer in series
with the temp sensor to play around and to rise the idle or to
disable the decel valve by simulating "cold" engine temp. The
point (or area) where the module "switches" between cold and warm
operation was quiet small.


[All information refer to a '87 KP engine with the "simple" ISV,
the one that closes completely when NO power is applied, that is.]

So long!
Ero.
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