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Ignition
Posted by jfrancis - http://saabnet.com/tsn/forms/e.php3?em= (more from jfrancis - http://saabnet.com/tsn/forms/e.php3?em=) on Mon, 1 Sep 2003 12:34:46
In Reply to: , Kok Chen, Sat, 17 Dec 1988 12:00:00 

Dear Fellow Saab Enthusiasts,

Please pardon the long post here, but I have a few very perplexing
questions.  If you are not interested in the classic 900 ignition
systems, feel free to skip to the next posting!

First the car:

1984 900T 8 valve, manual transmission, with a little over 300,000 miles
on it.  Bought from the original owner in amazingly good condition.  The
engine, turbo and transmission have never been opened; it still even has
the original CLUTCH!   It had quit last Spring - would start and die,
start and die, etc.  I diagnosed it over the phone as a bad ignition
control module (it was the original, too).  Got a used one from a friend
out of state, put it in the car, and drove it home.  Started it the next
day, it ran fine.  Worked on the interior for day three, then got in it
the morning of day four and it wouldn't run.  It would start and die,
start and die, then eventually not start at all.  One of the things I
did was to take out a couple of fuel injectors, put them in a coffee
can, and try to start it again.  The injectors put out a nice spray for
about a second, then promptly shut off.  I also noticed that after that,
when I couldn't even get it to fire, that I was getting no spark either.
I started going through the Bentley ignition diagnostics.  Rotor, coil
wire, spark plug wires all checked out with the proper resistance.
Power to the Hall sensor - check.  Power to the ignition control unit -
check.  Hall sensor output signal - check.  Power to the coil - check.
Coil primary and secondary circuit resistance - didn't get anything
close to what Bentley says I should.  More on that later.  Substituted
the coil into a running 16V car, (part number is the same)  the car
still ran, so the coil appears to be good.  Ignition Control Unit
Switching Function To The Coil - again, did not do what Bentley says it
should.  Checked my running 16V car with identical test, it didn't do it
either!

So here are my questions:  

1)  Bentley says I should have .5 to .9 ohms resistance between the plus
and minus terminals on my coil.  I have no resistance - straight
continuity.  However, I have four coils here at the house, two of them
in running Saabs.  All of them show straight continuity between the +
and - terminals.  What's going on?  

2) According to Bentley, I should be able to clip my voltmeter to the +
and - terminals on the coil, disconnect the Hall sensor wire, turn on
the ignition, and see a rise in voltage to about 5, then drop to 0 in a
couple of seconds.  This is supposed to test the ignition control
module.  My new car sits stubbornly at 0 volts.  However, a running '87
900S also sits stubbornly at 0 volts.  What's going on here?  Why
doesn't this test work on a running car?  Is there a way to test the
ignition control module other than substitution?  Unfortunately, that's
not an easy option for me, as my two running Saabs are both 16V and the
new car is an 8 - different control modules.  

3. When the car ran, you would get in, turn the key to 'on,' and hear a
hum or buzzing noise for a few seconds - possibly the fuel pump?  Now
that doesn't happen.  Would a bad control module cause that to happen as
well?

Thank you for your patience in this long posting!   Any advice you can
offer me would be very much appreciated.  

John Francis
Rolla, MO
1990 900    124,000
1987 900S  160,000
1984 900T   300,000
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