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MG MGB Technical - No Spark even with new Coil

Perhaps someone can give me another place to look. My 79 MGB has on occasion just died while going down the road but typically restarts before coasting to a stop. But now it won't start at all and has no spark. So I ended up installing a new coil but to no avail. Hooking up a timing light shows that when the key is first turned on I get an initial blink but nothing afterward when cranking. I've tested the wiring from ignition to coil and from solenoid to coil and also tested the distributor wire. All seem good. The car has electronic ignition and the air gap seems to be correct.....
leeloader

Do you have power to the coil? Get a wiring diagram of your car so that you understand what is happening and what wires are needed to start and run your car.

When the key is in the start mode, power comes directly from the starter solenoid to the input (+) terminal of the coil at a full 12V input. You should be able to read a 12V input to the coil when the starter is cranking the engine over. If not, a problem with the starter solenoid or the line between it and the coil.

With the key in the run position, you should have a reduced voltage to the coil's positive terminal which comes through a resistance wire. If you do not have voltage to the coil with the key in the run position, check out the ignition relay and the power input to that relay. This is the relay near the fuse box which looks like the old 35mm film can.

If you have power to the coil, and power through the coil, the electronic ignition would be the logical suspect.

There is a tech article on ignition system trouble shooting on the MG portion of my website, www.custompistols.com/, which may be of use.

Les
Les Bengtson

Les, I printed off your guide and followed it the best I could. I have voltage to the coil both in the ignition on position and when cranking.
leeloader

Lee - Check the wires in the distributor that none of them are grounded (or if you have electronic ignition, that it isn't fried). Getting a single indication on your timing light, indicates that something on the distributor side of the coil is grounded. Cheers - Dave
David DuBois

Lee: In 1979, your MGB was built with the Lucas Opus ignition - this was the poor design that had the electronics located in a module that was screwed to the distributor housing. Many of these have been repalced over the years with either a dealer installed CEI ignition that had a separate amplifier box mounted under the coil, or an OEM electronic ignition from various manufacturers - usually Crane or pertronix. If you still have the OPUS ignition you can get a Pertronix system to upgrade it. If you have a CEI ignition and it is not producing spark but has electricity at the module and coil, you should replace the amplifier which is a standard GM 4-pin HEI module available at any parts store. If you have a Crane ignition that has failed, replace it with a Pertronix.
John Perkins

Thanks David and John. I do have the Opus and am starting to believe this may be the problem. I've been doing a lot of wire tracing and connection checking but have found no shorts thus far. Would the drive resistor cause a problem? A current light (using AA battery) doesn't light up when hooking to both ends... but the ohms meter shows no resistance going through it. (?)
Lee
lee

This thread was discussed on 31/05/2009

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