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RE: [T3] Pressure Switch (was: pressures)


> > I think you have a 68 or 69 (?).  If so, make sure the darn
> pressure switch
> > (not pressure sensor) is hooked up right.  Recently I
> discovered on mine
> > that the hose connecting the pressure switch to the
> manifold was missing.
>
> Oh? I don't think you had mentioned this to us. Good think to notice
> and fix. I see so few of those (68-9 only) that I hadn't thought
> about it.

I noticed this several weeks back when I was looking for bad grounds, bad
wiring splices, bad...  I kept overlooking it because it's so well hidden
under the manifold tubes, and I don't remember ever seeing it mentioned
here.  And anyway it's function is unrelated to my intermittent
missing/no-start problems I was having.  Then one day while thumbing through
the Elfrink FI manual it kind of jumped out at me there.  That manual has a
very good description of this switch and how it works.

So since it's so rarely mentioned, I'll remind everybody with a 68-69 FI
Type 3 to get out there and test your pressure switch!  That switch acts
kind of like an accelerator pump in a carb.  When idling or driving
normally, the vacuum in the manifold keeps the pressure switch open.  But
when you floor it, the throttle opens wide and the pressure in there
approaches atmospheric.  When this happens, the switch closes and the brain
is told to enrich the mixture.  If the hose is disconnected like it was on
mine, the switch is always sensing atmospheric pressure, so the switch stays
closed.

Use an ohmmeter on the contacts while applying vacuum to the hose.  Sucking
on it should open the switch.  Release the vacuum, and the switch closes.
You will run rich if the hose is disconnected or if the switch doesn't open
when it should.  Mine was disconnected (no hose).  Right after I fixed mine,
I had to back off the idle adjustment screw at least a full turn since I no
longer had that big leak in the intake manifold.  (Before then I was
wondering why I had to have the idle screw almost bottomed out to get the
idle down to near spec..)

The function of this switch was first taken over by a modified pressure
sensor for the '70 year.  Then in '72 the throttle position sensor took over
the job.  (That's what I remember reading in Elfrink anyway).

-Mark Fuhriman

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