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Re: Ignition
Posted by John Hudak - http://saabnet.com/tsn/forms/e.php3?e (more from John Hudak - http://saabnet.com/tsn/forms/e.php3?e) on Mon, 15 Sep 2003 19:23:47
In Reply to: , Kok Chen, Sat, 17 Dec 1988 12:00:00 

> 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? 

Depending on the type and range of you meter, you might not be able to
measure 0.5. ohms. Check the specs and make sure your meter can read this.
Some digital meters are designed to show straight continunity (i.e. 0.000
ohms) for resistances below 1, 2, or maybe even 5 ohms, depending on the
meter).

> 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.  

If you are using a digital meter, and depending on the real meaning of 'a
couple of seconds', you might not see the change in voltage reflected on the
digital display.  Sometimes, DMMs will refresh the numeric display at a
fairly low rate, for example 3-5 times a second to give a person a chance to
read it. Cheap DMMs will refresh the numeric display 1 or 2 times/second.  To
somewhat compensate for indicating fast changing voltages, some DMMs have a
'analog scale' at the bottom of their display that samples and displays
voltages at approximately 10xs the display rate of the numeric display. This
is intended to provide the user with an indication that the voltage is
changing, not necessarly the value.  So, if your meter samples at 10
times/sec, the analog scale would sample at 100x a second.  If your digital
meter can be set up to measure a specific voltage range (i.e. 0-10 VDC), the
analog display full scale range will also be 0-10 vdc.  So, if you see it
jump to about the 1/2 way point, you can estimate that it saw a voltage of
about 5 volts.  Some autoranging DMMs will automatically set the analog
display to range to correspond to the voltage being measure (i.e. if it sees
5 volts, it autoranges to 0-10 vdc (or what ever the appropriate range is),
and the analog scale will also be set to 0-10 vdc.) Newer DMMs have a feature
that will allow it to sample and hold the highest voltage over a predefined
or even user specified peiod of time.    This feature is designed to catch
transients of this type. Best way to see this jump in voltage is with an
oscilloscope.  Also, a good analog VOM is better when trying to see
transients of this type.

> 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?

The buzz is the fuel pump 'topping off' the fuel system pressure.  From what
I recall, it would always go through the brief run period. It has been a
while since I poured over the fuel system schematics, but, I believe the fuel
pump circuit was independent of the control module.  A look at the schematics
would confirm this....
Good luck
John
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