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MG MGB Technical - Ignition Wiring Problems, elect-back-to-points

First the questions / comments then the rest of the story:

- 1977 MGB, ballasted ignition was removed by PO, installed 25D carb w/appropiate coil.
- car started last summer in driveway ( refurbishing, never driven), ignition system worked at that time but due to wear the engine was just replaced
- no current to "+" on coil when key is on "run" ( this is why I wasn't able to "time" as mentioned in previous post), I assume with key on run "+" on coil s/b hot ?
- drive resistor still in-line... I assume this should be removed?
- when engine is cranked plugs spark, probably from starter solenoid circuit.
- not sure fuse box input/outputs are correct, any source for correct connections?

That's the situation. The PO's intention was to take off the electronic ignition and go back to points. With the right coil what if anything else should have been done? That it ran last summer and not now makes me think something failed, but I'm not sure. My next step is to replace the ignition relay. If that doesn't work I'll start un-wrapping the harness at the "+" coil lead and work backward.

Any suggestions and/or the identification of the "usual suspects" to investigate are appreciated.
Brian Denis

Regardless of the fact you now have a 25D distributor whether the loom ballast should be bypassed or not depends on whether you have the original 6v coil or the earlier 12v coil. Check which you have by measuring the primary resistance, a 6v coil is about 1.5 ohms and a 12v coil is about 3 ohms. Note that some sport coils can have a primary resistance of 2.4 ohms and these are 12v coils as well. If you have a 12v coil then you should bypass the loom ballast by connecting a suitably coloured wire from the white/brown at the fusebox to the coil ballast. If you remove the white/light-greens from the coil +ve be aware that they are live (or should be!) with the ignition on so must be carefully insulated and taped back to prevent them grounding. But it does no harm to leave them connected.

The loom ballast resistance has a white/light-green wire at the coil end (+ve) and a white or white/brown at the fusebox end. I think by 77 the cars had an ignition relay, if so the colour is white/brown. If none of the other ignition circuits are working with the ignition on like instruments, turn signals, washers, wipers, heater fan, brake lights, reverse lights then I would suspect the ignition relay. If the fuel pump (assuming SU) isn't clicking either when first turning on the ignition then I would suspect the white from the ignition switch, the brown to it, or the switch itself. Does the ignition warning light come on? If so the brown, switch, and part of the white at least are OK. The relay needs a white and a black (ground) on its winding terminals, and a brown and white/brown on its contact terminals. If some at least of the ignition circuits I mention above are coming on then the relay must be OK and its connection to the fusebox, which only leaves a break in the loom ballast itself as a possibility. You don't really want to go digging in the loom for this, it goes all the way to the end of the loom at the front on the right-hand side and back again. If you originally had the 45DE electronic ignition there would also have been a separate ballast resistor nearby for the electronics with white and white/blue wires, and a white 12v wire. If this last wire is still present, and carries 12v when the ignition is on, then this can be used to power a 12v coil. I wouldn't like to say whether the separate ballast resistor could be used in place of the loom ballast or not without knowing its resistance and wattage, but an aftermarket coil ballast resistor could be, with a 6v coil, if it is about 1.5 ohms.

Fusebox connections should be:
Bottom fuse - brown at the front and purple at the rear.
2nd fuse up - white or white/brown at the front and green at the rear.
3rd and 4th fuses up - a single red/green at the front on either fuse and four reds at the rear, two per fuse.
Paul Hunt

This thread was discussed on 20/04/2004

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