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MG MGB Technical - Ignition Circuit Failure

Hello all,

I was hoping someone could anyone offer some pointers on where to go next.

The problem:

Just before Christmas I was driving down the motor way and sudenly lost power, the OD disengaged and the tacho dies, no ignition light. I managed to cost into the hard sholder and as I was costing along the ignition came back and the car fired stright back up and I went on my way, just a little slower.

Around 100 miles later the same happend, no ignition for a few seconds and then back.

While on the drive looking into this it is either working completly or not at all, I can not seem to find what bit is doggey.

As a matter of precausion I swapped out the alternator and cleaned up all the fuses and connections and the ignition switch.

I have tried tracing the failure with the multimeter but am not sure where to look.

When I turn the ignition on now, the fuel pump starts and I can here the ignition relay click. The ignition curcit at the fuze box is live and the engine cranks over but will not run. The radio also does not work but the handbrake warning light does.

I have checked all the leads at the fuse box, cleaned them up, the wires at the starter look fine too as do all the relay's I can find!

What should I check, has anyone seen this sort of problem before. I am guessing it's a loose or coroded connection but where!

Any help or guidance would be appreciated.

Thank you.

Paul
Paul Fryer

Paul. The first step is to get a copy of the wiring diagram for your car. The wiring changed over the years and, since you do not note the year of your car and whether it was a "home market" vehicle or one which has been re-imported, it is difficult for us to consult a wiring diagram on your behalf.

Generally speaking, your ignition works off the white wire circuit, as does the fuel pump. If you have a low tension problem, the tachometer will drop, immediately, to zero when the engine cuts out. If the tachometer continues to read and winds down as the engine slows, it indicates a high tension problem or a fuel problem.

You seem to be indicating that the car will not start, but will crank over. You, also, indicate that there is power to the white wire circuit at the fuse box.

Is there power to the input side of the coil when you are cranking? If not, trace the current backwards from the coil to see where there is a problem.

If there is power into the coil, is there power through the coil? At the distributor? Do you have the easy to test points type distributor or the much harder to diagnose electronic distributor? What model?

Have you hooked a stroboscopic timing light to the coil lead to see if it is flashing or removed the coil lead and held it near the block while turning the engine over? Is there spark at the spark plug leads? All of them?

I have an article on trouble shooting the ignition system on the MG section of my website, www.custompistols.com/ which might be of assistance to you. But, you need a wiring diagram for your car to trace the wires backwards if the ignition system is not getting power.

Les
Les Bengtson

Okay, raining and dark now but I will start looking again on Tuesday if the weather holds off.

I should have said the car is a 79 B, the ignition has been updated with Magnatronic and the appropriate coil which has a permanent live I think.

Is the lumanition coil supposed to be 12V un-balasted or 6V?

I have gotten the wiring diagram. I checked all the fuses including the extra one included in the latter wiring (In line).

Are there any key point's people would check to quickly narrow this down a little more?

Again thanks for any input.

Paul
Paul Fryer

Paul. Again, the first thing to check, when the engine will not start, is the coil lead. If there is no spark at the coil lead, you need to investigate the low tension circuit, starting with power to the coil. If there is power to the coil, then check for power through the coil. If there is power to the coil, there is either a problem in the secondary windings of the coil (rare) or in the make-break circuit (points or electronic points replacement system).

Les
Les Bengtson

If the OD disengaged at the same time the ignition cut out then you have lost power from the white circuit that feeds both. If the ignition light did not come on, then this potentially narrows it down still further, to somewhere between where the warning lights comes off the white circuit, and the white circuit splits for coil and OD.

Very conveniently, all four connect together at the fusebox as white/browns. The 12v feed comes in from the ignition relay to one of the spades, another wire should come out of that wire connector to one of the other circuits, then there should be another two white/browns in another wiring connector on the other spade for that fuse beside it. Either one or other of the wiring connectors isn't making good connection with its spade, or just maybe the conductors have fractured inside the insulation on one of the wires.

Radio should come off the accessories position of the ignition switch, which is a completely different circuit. But if an owner has fitted that it could be connected to anywhere.

Handbrake warning light comes off the green circuit, which is on the fuse that comes off the white/brown circuit previously mentioned, so power is getting from the relay to that at least.

Rubber bumper cars had ballasted ignition and a 12v coil originally, but what you have now and how it is wired could be anything if it has been changed to Magnetronic (a module under the distributor cap with two wires to the coil). Whatever, it isn't going to be associated with this problem, unless the radio and OD are purely coincidental and unconnected.

The first thing to check with a multi-meter is that both spades and wiring connectors with white/brown wires show 12v with the ignition on. The problem here is that as soon as you poke a meter probe at them you will disturb them which will quite likely bring the connection back again, for a while at least. You can test two of those connections elsewhere close by - at the fuse for the electric fan (white/brown to green, or does this come on if you bridge the connections on the radiator sensor together?), and by looking at the ignition warning light. If the pump clicks I suspect the light will be working normally, but then that circuit also feeds the OD, which raises a question mark over that disengaging when the problem occured. The third connection was originally to the ballast resistance for the coil, but as I say with a different ignition system that and the coil could be wired to anything, so you need to see what connections are on the coil +ve and the Magnetronic red.
P Hunt

Thanks guy's I will have a look tomorrow and let you know how I get on.

Paul Hunt, I have read your site a few times, where in the midlands are you? I am from Tamworth orig, now I live in Hinckley so not a million miles away.

Thanks

Paul
Paul Fryer

Solihull, Paul, Tamworth would have been easier to pop over :o), although I'm touring Oz for a month from Monday. I got my V8 from a chap in Hinckley.
P Hunt

This thread was discussed between 24/01/2010 and 28/01/2010

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