Subject: Re: [Mgs] Won't start

From: "PaulHunt73" <[email-masked]>
Date: Thu, 25 Apr 2013 09:38:02 +0100

I'm wondering whether you mean the ignition relay doesn't click when you 
turn on the ignition.  The starter relay doesn't click until you turn the 
key to crank, and is drowned out by the solenoid clicking if not the engine 
cranking.  I presume the engine is cranking?  Originally the coil was 
powered from the ignition relay, and if that isn't operating then you would 
have no spark, but you would have no power on the second fuse up neither 
would you have battery voltage on the coil +ve.

You say your car has a dizzie, but they all do.  I'm assuming you mean it 
has a replacement points dizzie of some type.  If so, then it all depends on 
whether the coil and coil wiring was changed when that was fitted. 
Originally your car would have had a length of resistance wire (the ballast 
resistance) in the harness in series with the coil.  With the points closed, 
and ignition on, if that circuit is good you should have 6v or less at the 
coil +ve, not battery voltage.  Battery voltage there implies there is no 
continuity through the coil, the points, and the distributor earth wire, or 
maybe no ballast resistance.

Late MGBs with the 45DM4 distributor may well have had a 32C5 coil, which 
may have measured as low as 0.8 ohms on the primary.  Using one of these 
with points ignition, or some other non-standard ignition system, could well 
result in significant overheating and damage to the coil.  A 0.1 ohm coil, 
if that is what you are saying you have, has either gone short-circuit or is 
completely the wrong coil and is one intended for modern electronic systems 
that use a very short pulse of high voltage.  However you need to remove the 
wiring from the coil before you can measure its primary resistance as there 
are other components on the wiring that goes to it that will affect the 
reading.

You are going to have to go back to first principles and determine whether 
you have the ballast resistance in circuit or not, which will determine 
which type of coil you should have.  If your coil doesn't measure up to that 
it has probably failed and will need replacing with the correct one.  Then 
you will need to check the circuit through the points and the distributor 
earth wire, as excessive current that is enough to damage the coil will 
likely damage these as well.

PaulH.

----- Original Message ----- 
> Any suggestions of observations from the above?
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From: Monte Jane Morris <[email-masked]>
Date: Thu, 25 Apr 2013 09:24:55 -0500

Paul,
You've probably "hit it on the head".
Yes, I do have a points dizzy (from a 74B) that I've used for about ten
years now when I converted to HIFs. I never changed the wiring at all when
I did the conversion and kept the original coil. There is a "drive
resistor" (Moss #131-560, pp. 86) still wired in.
When I replaced the original 79 coil back in 2004,* I used a 79B
replacement at the local parts store*.
Therefore, I would assume that the coil is shot and that's the cause of my
problems.  I should now replace it with one from a 74B, correct?  When I
install the new coil (Moss#143200) do I just disconnect the drive resistor?
Will anything in the ignition have to be adjusted with the new coil?
I guess having the wrong coil does not affect (effect?) the performance of
the car since it runs real well (when it runs:)?.

On Thu, Apr 25, 2013 at 3:38 AM, PaulHunt73 <[email-masked]>wrote:

> has a replacement points dizzie of some type.  If so, then it all depends
> on whether the coil and coil wiring was changed when that was fitted.
> resistance) in the harness in series with the coil.  With the points
> closed, and ignition on, if that circuit is good you should have 6v or less
> at the coil +ve, not battery voltage.  Battery voltage there implies there
> is no continuity through the coil, the points, and the distributor earth
> wire, or maybe no ballast resistance.
>
> with points ignition, or some other non-standard ignition system, could
> well result in significant overheating and damage to the coil.  A 0.1 ohm
> coil, if that is what you are saying you have, has either gone
> short-circuit or is completely the wrong coil and is one intended for
> modern electronic systems that use a very short pulse of high voltage.
>  However you need to remove the wiring from the coil before you can measure
> its primary resistance as there are other components on the wiring that
> goes to it that will affect the reading.
>
> which type of coil you should have.  If your coil doesn't measure up to
> that it has probably failed and will need replacing with the correct one.
>  Then you will need to check the circuit through the points and the
> distributor earth wire, as excessive current that is enough to damage the
> coil will likely damage these as well.
>
> PaulH.
>
>

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From: Monte Jane Morris <[email-masked]>
Date: Thu, 25 Apr 2013 10:31:53 -0500

BTW, This 79B has a 45D distributor with points and condenser. I'm now
confused as to which coil to order. I think my last post was wrong. Since I
have not changed the wiring and the "drive resistor" is still connected, I
should order Moss coil #143-230 (for a ballasted system) and leave the
resistor and other wiring intact, correct?

Whew, back 12 years ago when I bought a 74 parts car, trashed the ZS carb
on the 79 and converted to HIF's, trashed the Opus electronic ignition and
switched to a points dizzy, used parts from the 74, bought other parts new
and used.... I can't remember what parts came from where, so the ignition
system as well as the fuel and exhaust system are HYBRIDS!!  Several times
I've mistakenly ordered parts for a 79 when I should have ordered parts for
a 74:). I suppose many MGs on the road are now hybrids.

On Thu, Apr 25, 2013 at 9:24 AM, Monte Jane Morris <[email-masked]>wrote:

>> You are going to have to go back to first principles and determine
>> whether you have the ballast resistance in circuit or not, which will
>> determine which type of coil you should have.  If your coil doesn't measure
>> up to that it has probably failed and will need replacing with the correct
>> one.  Then you will need to check the circuit through the points and the

[email-masked]

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From: "PaulHunt73" <[email-masked]>
Date: Thu, 25 Apr 2013 16:46:35 +0100

OK, the Opus was the earlier 45DE4 with the additional drive resistor, so that
explains that particular component.

PaulH.
  ----- Original Message -----
  ... trashed the Opus electronic ignition and switched to a points dizzy...
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From: "PaulHunt73" <[email-masked]>
Date: Thu, 25 Apr 2013 16:50:13 +0100

Sounds like the harness ballast is still in circuit then, do the resistance
checks to confirm, and if so get a rubber bumper ignition coil e.g. Moss
143-230.

PaulH.
  ----- Original Message -----

  I do have the two white/green wires on the coil positive.
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