See oops below...

At 04:43 PM 12/7/2013, Neil N wrote:
>Thanks for the correction David! I assumed the white wire from ECU 12
>to fuel injectors only supplied a switced ground. I think I understand
>what you're saying; this wire switches from + to -  (engine running)
>to open/close the fuel injectors.

Not at all.  It does indeed switch the ground, because this is
electronically easy and efficient to do.  When the transistor is
conducting (injector firing) the great bulk of the voltage drop will
be across the injector, since the transistor has a low
resistance.  But when the transistor is *not* conducting, then all of
the voltage drop will appear across the transistor.  Thus if B+ is
not present on the negative side of the injector connector, it means
there's an open circuit somewhere upstream of it.  If B+ *is* present
on the positive side, that locates the open circuit somewhere between
the two sides of that connector.

Same trick you use to locate a break in a window defogger -- on a
good one you can run your probe along the paint stripe and see the
voltage gradually decline (or rise, if you're going in the other
direction).  But on a broken stripe you'll get either nothing or B+
right up to the break, where it will switch to the other one.

AAAAANNNDD...I forgot to mention that you have to unplug the other
three injectors, because all four of them are feeding that negative
side through their common connection to the ECU.  So only plug in one
injector at a time for this test.  Big Oops.  Sorry, Steve.

Yrs,
d