I hate electrics!

Driving home in my Mk1 Rossa the other night the engine cut out, died and would not restart. Problem was eventually traced to a lack of a positive feed to the coil. A temporary bridge from a known live and the engine started straight away and I got home without further problem.

Finding an hour to investigate today I expected a loose wire around the ignition switch area probably disturbed when fitting the new indicator/headlamp switch but no such luck. No evidence of any loose connections or wires. So the problem must be a little more deeper.

I can trace a live feed from the ignition switch via the fuse board to a white (and Yellow?) wire that dissapears into the conduit through the tunnel and emerges to be replaced by a short length (8 inches) to the terminal on the coil. Although this circuit appears OK when tested with a meter it does not cure the problem.

Reading my Haynes manual it talks of a “Resistive wire which is in circuit whilst the engine is running but is bypasased when the starter is operated”. Not knowing the exact donor vehicle it’s hard to know which one of the many wiring diagrams in the manual to refer to. One does appear to show a feed from the starter motor, ie the by-pass for starting.

The Haynes manul goes on to say in the fault dianosis section “The resitive wire may be open curcuit” causing the problem. It does however caution against permanent by-pass of the resistive wire as this will cause the coil to overheat.

So what form does this “Resitive Wire” take? Is it the short wire connecting the coil to the wire in the main loom. I also don’t seem to have the bypass wire. Can you buy these “Resitive wires”? or do others run without them?

Can anyone offer any advice and guidance on this please.

As I understand it - if you have it, whilst operating the starter (which will reduce the voltage available) the 9 volt(?) coil recieves the full voltage available - whilst cranking, likely to be significantly in excess of 9 volt, ensuring a good spark. Circuit reverts to 9 volts (via a resistor), when engine fires and you stop cranking.

Allen

Difficult to say what you've got at the moment without seeing it but a few thoughts on ballasted ignition may help:
On a Mini the 'resistive wire' is a pink cable connected to the coil.
There is another (full voltage) wire from the solenoid to the coil, which is only live when the starter is operating.
These ballast systems are not compatible with electronic ignition, which needs a full 12v to operate correctly.
Many people bin the ballast wire, although you should change the coil for a 12v one if you do this, the ballast coil is designed to run on ~9v and is effectively being overloaded at 12v.
If you have this system you could try running your current coil llike this - it may survive - it may not!

Personally I'd bin the ballast system, fit electronic ignition (Lumentition or Aldon Ignitor) and fit a new 12v coil.

Is this "Pink Cable" the short length about 8" long?

The next step is to remove this short wire and see if it runs without it. Its just that there not much room down the back there to unplug and re-plug connectors

I've only got one one wire going to the +ve on the coil. Is there a relay involved because would'nt the running feed back feed the solenoid?

Maybe !
In your original post you said the circuit tests out OK. Could you be a bit more specific?
I'd expect to see: Battery voltage, less a bit (maybe 1/2 v) at the white wire running from the ign switch
then ~9v at the coil end if it had a ballast system, or the same 12v 'ish at the coil if it doesn't.


That's wrong for a ballast system - but it wouldn't affect the running, only the starting would be more difficult.


Well there is a relay, but not a seperate one - it's part of the solenoid.
On the solenoid there are, or at least should be:
2 large connectors - battery cable in (always live) and starter feed out.(live when you operate the starter)
A large sized spade - main feed to the wiring loom (always live)
a normal spade - the input from the starter position on the ign switch
another normal sized spade - the 12v feed to the coil (the ballast bypass) Only live when the solenoid is engaged. When the starter is not operating this connector is floating - it's neither live nor earthed, it's this feature that stops any 'backfeeding'

The above assumes a non pre-engaged starter, if it's pre engaged, ie the type with a solenoid piggybacked onto the starter the connectors are a bit different but the principle's the same.

Big But ... we haven't yet established if the system is a 'ballast type'

It would really help to know:
Age of the engine?
does it have electronic ignition?
type of coil fitted - any markings such as part numbers or voltage?
pre-engaged starter?
Any pics?

Thanks for all the posts, still lots of investigation to do but I now have a lot more info to show me what to look for. It great having all these experts to help.

Progress a bit slow because of finding the time and I'm away on holiday next week.

Coil off next to check for markings and readings. I have a separate solenoid if that makes a difference. Oh and yest there are two wires from the +ve terminal on the coil. The second disappears down under the Dizzy somewhere.

Car was built 1986/7 using a MG Metro engine, which I assume is still the same one, if that helps to identify if a Ballast system is likely.

Watch this space.

No sign of a Ballast Resistor and no feed from the Solenoid to the coil so I think I have a non-ballast set up.

No model marking on the Coil which Has a Unipart sticker on the side but has "Ducellier" moulded into the top. It also has 12V moulded in as well. Coil is not in good shape and appears to be leaking oil but car still works when 12v applied to it from a good feed. I've traced the feed wire to where it appears in fron of the gear lever and car runs if I apply 12v at this point. I need now to trace this wire back through the mass (or is that mess) of wires behind the dash. Although I can get the car working OK, I would still like to know why it suddenly stopped. It obviously not at the engine end.

Out of interest what is the advantage of using the ballast resiter system and does anyone know part number and where to buy one?

Ir you have a Mg Metro lump then the wire from the coil that disappears under your dizzy will go to the little black box (igniter ) screwed to the aforementioned dizzy, this is part of the electronic ignition. If this is what you have then everything needs to be 12v, coil, supply etc

The old ballast system was to enable a fat spark with the old points set-up when cranking the engine.

Totally agree with Tim, don't even think about a ballast system they're just not relevent in the age of electronic ignition.
It looks like your fault is more than likely just a broken wire / corroded terminal somewhere.