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Re: [Mgs] Starting Problem

To: <DMatt21502@aol.com>, <mgs@autox.team.net>
Subject: Re: [Mgs] Starting Problem
From: "Paul Hunt" <paul.hunt1@blueyonder.co.uk>
Date: Thu, 26 Jan 2012 10:19:48 -0000
With the ignition on but the engine not being cranked a 45DM4 should always 
give 12v on both sides of the coil, unlike points which will usually be 
closed and so there will be an earth on the points side and some voltage on 
the +ve/SW side.  When cranking with a 45DM4 you should have 12v nearly all 
the time on both sides of the oil, not 'nothing' as you seem to have.  That 
implies loss of voltage to the ignition circuits while cranking, which 
implies a faulty ignition switch or ignition relay or associated wiring.

When cranking with points or the earlier 45DE4 distributor I'd expect to 
clearly see voltage pulses on both the coil +ve and -ve of a rubber bumper 
car, assuming it has the original 6v coil and ballasted ignition.  If for 
any reason it has been changed to 12v coil and unballasted (i.e. same as 
chrome bumper) then there would only be voltage pulses on the coil -ve/SW. 
With the later 45DM4 distributor (remote amplifier) the voltage pulses to 
the coil are much shorter so may not be noticeable with a test-lamp 
(although an LED test lamp would stand a better chance by dimming slightly). 
I'd still expect to see something on the coil -ve with a meter, definitely 
with an analogue, with a digital it would depend on the meter being used, 
but with either it is complicated by the 12v static voltage dropping to 
about 10v during cranking anyway.

With none of those showing any indication of pulses, then I'd expect the 
trigger or amplifier.  But under the conditions specified it's more likely 
to be HT breaking down, from damp, and this may be visible as blue flashes 
if you view it in the dark.  It's not the case that an HT problem including 
rotor or cap would only affect running, my V8 would occasionally fail to 
start until a fitted new plugs (even though the old ones had only done a few 
thousand), but once started ran perfectly.  That turned out to be the cap 
breaking down.

PaulH.

----- Original Message ----- 
> Yesterday I tried to start my 1977 MGB for the first time in  about two
> months. It cranked fine but wouldn't start. It was not getting any  spark. 
> I
> took my test lamp and hooked it to the ignition side of coil. With key  on 
> it
> lit up. When I connected the light to both sides of coil and cranked it
> showed nothing. 
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