Hi Pete :
Well, it's always dangerous to apply stock troubleshooting procedures to
a modified car ! See below for comments.
Pete & Aprille Chadwell wrote:
>
> I have a Lucas Sport coil with an Allison XR-700 electronic ignition
> system. I don't think the ignition system has anything to do with
> the problem
> it's the purple circuit fuse that is more directly
> related, I'm sure.
While they shouldn't be related (the purple fuse feeds things that are
always active, like the horns, interior lights, 4-way flashers, etc.),
it does seem awfully suggestive that both things happened at the same
time. Might be a short between the two circuits, somehow.
> The next test is to check continuity between the coil
> negative wire and ground while cranking the engine. Here's where I
> get confused. Dan's book says that resistance should alternate
> between zero and infinity as the engine turns.
The problem is that this test is only valid for engines with points.
_Any_ kind of electronic ignition will most likely make this one fail.
The Crane XR700 (which I assume is the same as the Allison XR700)
instructions say to connect a test lamp between coil negative and
ground. The lamp should flash while cranking.
If not, they suggest first substituting a known good coil, and then
checking the electrical connections to the module (especially the ground
connection). Check that the connections to the optical pickup are clean
and dry.
As a last test, turn the key off, disconnect the plug to the optics,
turn the key on, and try shorting between the white and gray wires to
the module at the plug. If that causes the lamp to flash, then the
optical pickup is misaligned or bad. If not, most likely the module has
failed.
> Well, it appears to
> hover at just below 1 ohm while the engine is cranking.
That suggests to me that your module is bad, or possibly the wire from
the coil negative to the module is shorted to ground. I don't have an
XR700 handy to try it on, but it's very rare for a good transistor to
look like less than 1 ohm (unless you have a very old ohmmeter that uses
several volts to test resistance).
> Dan discusses two conditions
> other than the zero-infinity oscillation
> a constant infinity reading
> and a constant zero reading.
Under 1 ohm is 'zero' for most purposes. But again, Dan was talking
about points. This test looks directly at the point resistance.
> I have a rudimentary understanding of how
> the points type ignition works, even with respect to the electrics
> involved. But I have no idea what's going on inside that Allison
> XR-700 box!
The output is just a big transistor that performs the same function as
the points.
> Thanks in advance for any suggestions!
You're not going to like this, but my suggestion is to convert back to
points, and proceed from there. I suspect that will get the engine
running, but even if not, you can use Dan's book to troubleshoot
whatever problem remains. And, you can drive the car while you send the
module back to Crane (who bought out Allison some years ago) to exchange
for a new one.
I use a Crane on my daily driver, but keep a set of points already
mounted on a spare point plate in the trunk. Swapping the points back
in is quicker than waiting for a tow truck !
One way to look for a short (like you apparently have in your 'purple'
circuit) is to connect a test lamp across the fuse terminals, turn
everything else off, and then use a kid's magnetic compass to identify
where the current is flowing. The compass needle will deflect when you
get near a wire that is carrying current. Or, JCW will sell you a kit
that is basically the same thing for $25. Look for "Short Circuit
Finder".
Randall
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