GTV6 L-Jet throttle / fuel pump cut offs et al.

["Scott Murray" <scott105@domain.elided>]

Hi folks,

I've recently done a lot of trouble shooting on my '85 GTV6 to try and find
the cause of a rough idle, and strange off idle performance.  Here's what I
found about throttle switches, AFM's, Fuel Pump Cutoff's and so on.

In the L-Jetronic (I have -zero- knowledge of how the motronic system works -
that's my little caveat :)  )   the fuel pump is cut off by (1) the famous
Inertia switch  or (2) There is a little contact built into the AFM on the
engine side of the air filter box.

When the engine is running and happily sucking in some air, the AFM flap opens
just slightly, and is enough to make the small internal contact close and
switch on the fuel pump.   So, if the engine stalls for any reason, airflow
through the AFM stops, the flap closes and the pump stops.

This small contact is wired to the mysterious combination relay that energises
the fuel pump.  Often the small contact ( or those wonderful bosch connectors)
fails, and rather than purchase a complete ($$$) AFM, some folks will bypass
it with either a switch under the dash somewhere, or wire it to the RUN
position on the ign switch.  While this makes the car run ok, it removes the
safety aspect that is the whole point of the system.  My car has one of these
little switches from sometime in it's illustrious past.


The switch on the throttle body itself does two things;  The first is to tell
the L-Jet box you have the throttle Wide Open, making it pretty much dump as
much fuel down the injectors as it can without flooding.

The other switch is made when the throttle is closed.  It's purpose is to tell
the L-Jet box to enter Idling mode.  Also when you have the revs at -say-
5500rpm,  and then lift off the accelerator completely, the throttle closes,
the idle switch closes and the L-Jet box will actually cut power to the
injectors until the revs decrease to ~ 2000 rpm.  It then turns them back on
and attempts to maintain a nice idle.   It does all this to both bring the
revs down quickly, and try and minimise emissions. I guess it saves a bit of
fuel too :)

Of course, everyone's entitled to correct me where necessary !


Best Regards

Scott Murray
'85 GTV 6 2.5

scott105@domain.elided

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