[Mgs] Coil resistance for Lucas 25D

[Paul Hunt]

As they came out of the factory the primary for a CB should be 3.1 to 
3.5 ohms.  For RB cars and points ignition they were 1.5 ohms, with the 
45DE4 electronic distributor they were 1.4 ohms, both with a 1.5 ohms 
(approx) ballast resistance contained with the harness, which meant the 
overall resistance and hence current are about the same for both CB and 
RB.  With the 45DM4 distributor they were 0.8 ohms with no ballast, 
which gave a very high coil current but the 45DM4 is a variable dwell 
system which gives very short pulses of current to the coil, much 
shorter than points.

An easier way to check the overall system is to remove the wiring from 
the coil -ve or CB, turn on the ignition, and connect an ammeter between 
the coil -ve or CB and earth.  With points and 45DE4 you should see 
about 4 amps.  With 45DM4 you should see about 15 amps which may well be 
above the capacity of your ammeter, disconnect immediately, only run 
that coil with a variable dwell system.

Lower than 4 amps you may well have a 12v 3 ohm coil in series with a 
ballast resistance which will give a reduced spark.  Higher - up to 
about 8 amps - you may have a 6v 1.5 ohm coil with no ballast 
resistance, this will overheat on anything except a variable dwell 
electronic system, and with points will burn them.

PaulH.

On 25/02/2024 21:20, William Killeffer wrote:
> Hello everyone,
>
> My car's a 1974 MGB with an 18GK engine and twin HS4 carbs. I've been 
> running a Pertronix distributor with its matching coil for awhile, but 
> just got the car's 25D back from Jeff at Advance, and it looks better 
> than what it probably did when it was new. I'm going to go back to 
> using points and condenser with the rebuilt distributor.
>
> I still have the factory Lucas coil that came with the car when I 
> first obtained it. I checked the resistance to see if it was still 
> useful, and here's what I found:
>
> With a small handheld analog meter, it registered around 6700ohm 
> across the secondary, and the little meter's not capable of giving a 
> reading for the primary.
>
> On the new-to-me Sears Engine Analyzer, the primary shows 5.6ohm and 
> the secondary shows 5100ohm.
>
> Either way, that looks too high given some of the other numbers I've 
> seen. I haven't checked the Pertronix coil, but will before trying to 
> use it.
>
> Am I right in believing this original coil shouldn't be used?
>
> Thank you,
> -Bill
> Sent from my iPhone
>
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