jim beam wrote:

<snip>

> > Voltage signals are the wrong concept since the devices work
> > by current like 74 series TTL does. If you uncouple the signal]
> > wire to measure it on voltage alone, the lack of current may lead
> > you astray!

74 series TTL came with totem-pole output and open collector output.
7400 is an example of the former, 7401 of the latter. I gather you mean
the ECU output is open collector.

<snip>

> hence now i want to
> dry test the igniter. and i want to try this without stripping the
> working car back down again because i have a broken finger which makes
> for a little bit of a dexterity problem!
>
> any thoughts?

I would hook it up as follows (terminal numbers pr.
http://www.gcw.org.uk/rover/igncloseup1.jpg):

0V: = 0V (ground)
T1: = leave open
T2: = 1 k ohm resistor to +12V
T3: = +12V
T4: = Leave open for now.

Then measure the voltage at T4. If the ECU is open collector (or likely
open drain) the ignitor would need to have a pull-up device. You should
see either about 12V or about 5V with T4 left open. If you have a
function generator, set it to a square wave, 25 Hz, negative peak to 0V,
positive peak to whatever you measured at T4 when it was left floating.
Hook the function generator to T4. This simulates the engine running at
750 RPM.

Now you should see square waves at both T1 and T2. You can also hook up
the coil (short the secondary to avoid painful surprises!) and measure
if you still get a square wave at T2 with the actual coil in the system
instead of the 1 k resistor. The resistor is a much lighter load than
the coil.