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Ignition Stuff


LOOK! Fitting An MG Metro Dizzy into Mini Engine is:HERE

Ignition, basics

A current flows through the coil. When you open the points, this current is interrupted. The natural tendancy of the coil is to oppose any change in current through it. It does this by producing an emf that seeks to continue the current (Lenz's law). This emf is quite large, large enough to jump the tiny gap between the just opened points. In the same way that you can draw out
a big arc with an arc welder, but not strike it at the same distance, the arc across the points persists as the points gap gets bigger.

So what happens? The current continues through the arc between the points. This is bad for two reasons. It burns the points, and it produces a smaller spark. The voltage induced in the coil is proportional to the change in flux through it (Faraday's Law?). The quicker you stop the current in the primary, the bigger the change in flux (shit, there is a unit for this, and I've forgotten it, less than a week after term ended :-/ ), and so the bigger the voltage in the secondary.

So how does the condensor help? The condensor allows the current in the coil to continue to flow. The condensor (really a capacater) charges up quickly, allowing the full current to flow for an instant. Very quickly, the condensor fills with charge, and the current no longer flows through it. The coil then objects to the reduction in current and produces an emf that tries to jump the points gap.

The trick is to make the delay long enough so that when the peak emf occurs, the points are already too far apart for an arc to form. This way, the current suddenly has to stop VERY quickly, and the change in flux through the secondary becomes HUGE and you get a nice big spark.

You could think of it as a tuned circuit that produces an alternating current in the coil so that it will act as a transformer. I doubt that this effect will occur for more than a cycle or two.

Ignition, coil electrical properties

Typical figures for Lucas Lucas DLB-101 12v coil

Primary (LT):
11.57ohm series resistance

Secondary (HT):
52.41kohm series resistance

Ignition, Coil Numbers ( Lucas)

From a very nice man at Lucas Auto in Southampton....... added by FB 18 April 2003

DLB 198 *** NOT FOR USE WITH POINTS DISTRIBUTORS ** electronic ignition only points will be destroyed ( whether for balast or non balast? don't know)

This answers why a minilisters points were burning out ;)

Balasted systems (pink wire and wire from solenoid to coil) Sports DLB110 ( non-sports DLB102)

Non-balast systems (no wire to coil from solenoid) Sports DLB105 (non-sports DLB101)

Ignition coil, testing

There is a small piece in Haynes '96 ed. , giving a way of checking the ignition coil - std version, presumably.

1) Use a multimeter set to resistance reading to check primaries for continuity.

LT+ to LT-   reading between 5 to 15 kohms
LT+ to HT lead terminal  reading should be similar

2) Using ohmeter or continuity tester, check there is NO continuity between HT lead terminal and coil body

(No, I havent tried it, and yes it has come from Haynes, so I take no responsibility for errors, misprints or their usual cockups.)

A Gunsons Flashtest can also be useful to check performance (in circuit between coil and plug)

Ignition, LT frequency

LT Frequency (Hz) = RPM / 30

Ignition, using MG Metro

I've just fitted a 1990 mg metro engine in my mini,and i would just like your readers to know that instead of spending their hard earned on luminition or other types etc, electronic ignition just go down to their local breakers yard and get a dizzy off a 1989> mg metro, 2 wires to join and away you go, cheap electronic ignition.

 Ignition problems

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Ignition, using MG Metro: wiring up the 65DM4 ( from Peter Kay and much requested - added 29 12 02)

The Haynes "Austin & MG Metro (1980 to May 1990)" reference 0718 is quite a good book.

It covers the engine electrics quite well, with detailed info on on all fourtypes of dizzy fitted.

Electronic module type are either Lucas 59DM4 or 65DM4. The latter having the module attached to its side with two screws, whilst the former has a separate unit which, on the Metro, was mounted on the bonnet slam panel, near to the coil.

I know very little about the former, but the latter 65DM4 type has a 3-pin plug, with connection only made to the two outer pins.

If you have the correct lead, then this will have a white wire and a white/black wire from the plug.

These MUST be corrected correctly with the white to coil positive and other to coil negative. No damage will occur if connected incorrectly, but it won't work!

Timing depends on engine type, and there are lots! I would set at 9 degrees BTDC at 1500rpm, with the vacuum lead disconnected at dizzy, then play from there, but they vary in the book from 5deg to 13deg over the range.

Module is only replaced if it goes duff. They are available as an afternarket part from most sources and even CI (Commercial Ignition) make them.

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