Very difficult to diagnose a system with electronic ignition except by
substitution unless you know exactly what the output to the coil 'looks'
like and can measure it. The remote amplifier system you have (45DM)
replaced the Opus self-contained system (45DE) which was very unreliable.
The later system is usually very reliable, so carefully check all
connections and the continuity of the wires, you can find a schematic at
http://www.mgb-stuff.org.uk/wn_electricsframe.htm and click on 'Schematics'
and 'Ignition'.
Failing that, substitute a 45D distributor with points and condensor - any
will do at this stage - which will allow you to diagnose either the
coil/wiring or the box of electrickery, then you can go from there.
Incidentally an article in the MG owners mag this month on the long-term
ownership of an MGB reports that the only time it ever conked out on the
road was when the owner-added Lumenition electronic ignition failed - twice.
In general terms the original points and both electronic systems of the MGB
all connected 12v (via a ballast resistor on rubber bumper cars) to the +ve
of the coil, and unless there is something grounding the -ve side of the
coil i.e. the points being closed, or the coil is bad, you will always see
12v on the -ve side too. AFAIK the 45DE is much like the Pertronix in that
they are not much more than an electronic 'switch' which energises the coil
much the same way as points did, the benefit being no mechanical contact
breaker to deteriorate over time. I also though the 45DM was much the same,
just more reliable, but others claim it is a CEI system in which case it
probably uses a high-voltage short-duration input pulse and you would need
an oscilloscope to measure it. But unless you intend to open the box with a
view to repairing it if there is no output you junk the box. After first
having made sure the problem isn't in the trigger, which will also have a
unique voltage and duration input pulse.
----- Original Message -----
From: "Tom Bott" <tbott1@bellsouth.net>
To: "MG Net" <mgs@autox.team.net>
Sent: Tuesday, March 04, 2003 12:44 AM
Subject: No Spark
> Patient: 1980 MGB LE all original with electronic ignition including
external
> amplifier (mounted on inner fender below coil).
>
> Symptoms: no spark at plugs.
>
> Diagnostics: I am getting 12+ volts out of the coil. But I also noticed
that
> when I ground the voltmeter to the engine and put the positive probe on
both
> positive and negative sides of coil, I am still getting 12+ volts. Is it
> normal to get voltage from both sides of coil? Or is something plugged
into
> the wrong side?
>
> Also, what type of volt reading should I be getting out of the amplifier
to
> the distributor? And does anyone know of a simple test for the
distributor?
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