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MG MGB Technical - distributor diagnostics for one person

Found the appropriate vacuum advance on a junk distributor, so I switched that over to a recently rebuilt body (the distributor I ahd been using with the incorrect vac advance). While I had the whole thing apart on the bench, it seemd like a good opportunity to swap points, condensor, and rotor.

Looks great, unfortunately I can't get it to spark. Given two people, I've done the arc against the block tricks, but on this one I'm on my own. Are there either bench checks I can do with a voltmeter or similar, or is there a simple way to start/crank the engine from the engine bay without frying something I might want later?


Steve Aichele

I would start by putting the old condensor back and trying that - on two occasions I've had brand new non-functional condensors...
Mister T

Classic case of points grounding out. There is a little plastic washer on top the points that is supposed to isolate the points from the post. Sounds like your's isn't
Paul Hanley

A couple of easy checks to make sure you got the insulators correct on the points mounting post. Disconnect the wire from the coil to the distributer. Put a piece of cardboard (piece of matchbook cover) between the points contacts. Check continuity from each point to ground. One should have full continuity and one none. If this is true then you should be able to remove the cardboard, connect the Ohmmeter from the distributer terminal to ground and slowly turn the distributer. Using an analog meter (with a needle on it), you should see meter movement each time the contacts open or close. This is how it should be.
If both point contacts have continuity to ground, remove the condenser lead and retest the insulated (movable) point (or test for continuity through the condenser). There should be none. If you still show continuity through the insulated point then restack the insulating washers on the point spring insuring nothing touches the post. One more thing: make sure the wire from the points to the distributer connection isn't grounded. With points open there should be no continuity from the distributer terminal to ground.
You can do the same checks with a self-powered test light.

A couple of bucks buys you a "remote starter switch" from an automotive store. You want the cheap wire one for mechanics, not the wireless one for lazy people :).

Under the car there you'll spot the starter & solenoid. One alligtor clip goes to the +'ve terminal, the bolt with all of the wires attached to it. The other goes to the spade connector - you'll need to remove the single wire on it that comes from the ignition switch (it's the only one, you can't go wrong).

Turn on the ignition (red ignition light on), hold the plug or wire near ground, grit your teeth and press the switch :).

Careful, you'll get some good sparks if you short out +'ve clips to the body. Otherwise it's pretty safe.

Don't forget to put the spade connector back on when you're done, or nothing will happen when you turn the key.

There might be an easier way to hook up through the starter relay on the top of the car. I've never been bothered to figure it out, maybe someone here has?
Mike Polan

Steve. You might want to check out my website and see if the article on troubleshooting the ignition system is of any help to you. It will be on the MG side under articles.

As to starting or cranking from the engine compartment, I have not tried this. You might, however, take a look at the starter relay which, it would seem, could be jumpered to cause the starter to engage. I suspect that ignition key would have to be in the run position to get electricity to the distributor system.

Are you sure you have hooked the points up correctly? There is a common mistake made and that is to simply remove the nut from the spring arm, then attach the wires from the coil and the condensor. When that happens, there is a direct short to ground and no spark is produced.

The correct way to wire up the points is to remove the nut, remove the plastic "top hat" spacer, install the two wires on the smaller diameter of the spacer, install the spacer back on the post so the wires make contact with the spring arm of the points, install the nut. In other words, exactly the opposite of how the American points systems work and a problem to a lot of people the first time they try it on a Brit car. Les
Les Bengtson

As is frequently the case here, Les had it right on. I had cavalierly connected the condensor and LT leads to the post above the top hat, rather than under the top hat.

Steve Aichele

Steve. I am glad that I was able to help. But, wish you could have run through my tech article and given me some feedback as to how useful it was. I revise them, from time to time, as people let me know what problems they have found with them. I should, very much, like to be able to write an article completely correct the first time. But, I have been doing this sort of thing for a long time and, from time to time, I miss something a "new guy" needs to know or do not explain it as well as I should. So, it is always good to get feedback on how useful someone finds an article to be.

But, at the same time, I am also very glad it was such a simple problem and the car is not running. Hope all goes well. Les
Les Bengtson

Look at the size of the holes in tags.


This thread was discussed between 28/04/2005 and 29/04/2005

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