Re: Spider distributor

["Evans, Tom" <tevans@domain.elided>]

Dave & Charlie,

I'm with Charlie on this one.  I don't believe it!  You are talking about
disconnecting the two color-coded connectors sitting under the wiper fluid
bottle, right?

Here is my understanding of the Bosch Ignition system on these cars.  The
Bosch Ignition ECU's primary function is to control the ignition coil.  It
functions to set the ignition timing, dwell angle and duration, and spark
intensity under various operating conditions.  The ECU replaces the old
points-type or mechanical/electrical advance mechanisms inside traditional
distributors, and it controls the ignition coil directly.  The output signal
from the ECU goes to the coil terminal #1, which also goes to the Bosch
L-Jetronic ECU, the fuel pump relay, and the tach.  The output signal is a
square wave, the voltage going to +12 volts to energize the coil, and
dropped to zero to discharge the coil (spark).  Positive battery voltage is
sent via a fused wire to terminal #15 on the coil.  The ECU alternately
opens and closes the circuit between coil terminal #1 and ground to charge
and discharge the primary coil at the proper times.

The ignition ECU has to decide when to begin to energize the primary coil
and when to cut off the current to the primary, thereby inducing high
voltage into the secondary coil and causing the spark.  The longer the
current goes through the primary coil (the dwell), the higher the discharge
voltage to the distributor and spark plug.  The exact point in time at which
the current is cut off to the primary coil is the ignition point.  The ECU
combines several input factors to determine the ignition point, but the most
important are the reference point and rev sensors.  Furthermore, the ECU
will drop current to the coil under a certain RPM to avoid spark when the
engine stops or stalls.

I don't see how the ignition coil can discharge without the ECU grounding
the primary coil.  And how can the ECU do that after taking away it's
information about engine speed and crankshaft position?  Dave, is there any
chance your ignition system has been modified?  What year is your Spider?

If anything I said is wrong, please correct me.  I'm very interested in
understanding the Bosch system in these Spiders.  I wish my Spider was
running right now, because I would run right out and try disconnecting the
sensors.  Seeing is believing!

p.s. for Charlie:  If I get the Bosch system figured out, I think a lobotomy
would be more appropriate than a Nobel Prize. :)

-Tom
'86 Spider

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