Ignition control unit and coil

[Huw Powell]

> Can someone explain the relationship between the ignition control unit (aka. 
> hall control unit) and the ignition coil?  If one fails does it all have to be 
> replaced or just the failed part?

Taking this backwards, no to the second question.

First question... here's an abbreviated simplified explanation of how it 
works... hope I get it right.  The coil a very simple device, it just 
has two windings inside, one with a lot of turns, one with a a lot less. 
  They share a common connection, which I think is the +12 one.  The 
control system, whether it's a transistorized module or points, 
alternately charges the short winding by grounding it, then discharges 
it by "ungrounding" it.  When it is opened like that, the field 
collapses, and generates a much higher voltage in the long coil - which 
is connected to the distributor.

My apologies if I may have reversed any circuits or functions in this 
description, it is generally correct but I may have the grounding cycle 
backwards or something.

Whatever is controlling the coil grounding is set up to be in sync with 
when the dist. rotor is pointing at a spark plug wire terminal.  Like I 
said, it can be a set of points in the dist, doing the job mechanically, 
or an external electronic control unit, which usually gets its info 
about where the rotor is pointing via a Hall sender or the like.

Some ECU's just fire the coil when told to be the Hall sender and some 
are more sophisticated, with the capability to adjust the timing 
depending on various factors (rpm, load, knock sensing).

Either way, one can be bad and the other still ok - but there are no 
guarantees that they aren't both busted.

I believe the only thing you have to do to test the coil is to measure 
the resistances of the two windings, and also make sure they aren't 
shorting to each other.  Shop manuals usually have this resistance spec 
in them.

Testing the ECU is a bit less hands on, in that you probably can't 
really do it.  Swapping in a known good unit, after making sure 
everything else is good so you don't let the smoke out of it, is usually 
the method.

On our electrically controlled cars at least (and perhaps all?) there 
will be a tach feed coming off the switched ground terminal (on CIS/CISE 
cars this is the red/black/yellow wire) - even if there is no tach - it 
is used also by the fuel pump relay and the oil warning control unit as 
well.

I hope that somewhere in what I said you'll find part of what you need 
to know.  I might further suggest googling somethig like "gasoline 
engine spark control" to see if you can find some nicer, better 
explanations out there.

-- 
Huw Powell

http://www.humanspeakers.com/audi

http://www.humanthoughts.org/