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MG MGB Technical - No spark at all

Hello all,

apologies for opening with a question, but needs must. My partner and I have just bought a 1971 MGBGT that has been off the road for 4 years. Its not in bad condition but does not run.

It has/d an MSD 5 ignition system, but ironically does not ignite...no spark. We have spent the weekend following all sorts of online guides, have replaced the coil, distributor cap, rotor arm, condensor, points, coil to distributor lead. We have also disconnected the MSD system for all tests, so that we have effectively run all voltage drop/resistance checks with it in place and with it disconnected.

Still no spark.

So we've checked voltage from coil to distributor and both sides of the points, have removed the lead from the distributor and no spark against the block. Although it did spark twice when we stopped churning the starter and returned the key to "off".

We have removed the brown and white wire from the starter relay and bridged it against the fuse box (with points 5 and 7 (white and brown) bridged, still no spark. If I remove the brown and white wire, touch points 5 and 7 with the fly lead, then the wire from the coil will spark against the block at the same time as the fuse box connectors spark.

I'm lost.

There are only two things left in my mind. The new coil is faulty as well as the old one, or the ignition switch is faulty.

Can any one help?

I should note, that the twin 6v set up has seemingly been replaced with a single 12v setup. Beyond that I have no idea what has changed from standard.

This is our first MG and first car of this era. All of our other projects have computers of one sort or 'tother. This is great to learn, but after this weekend, I'm out of ideas.

Many thanks in advance.

Daniel
DP Lawrence-Sansbury

Just a couple of thoughts. Check that the ignition relay is working. Has a later ignition switch been fitted? - later cars had a lower voltage coil with a resistor in line , but when the ignition switch is turned to the start position, the coil is then fed from the starter circuit/relay.Check what happens at the back of the ignition switch.
As a temporary measure, why not take a wire with a switch in line direct from the battery to the coil just to see what happens?
Yours, Dave
DM Gibson

The fact that it sparks as the key is allowed to return to run is significant, suggesting you have 12V at the coil on "run" but not on crank. Try a bypass wire from the fuse box bottom terminal to the coil pos (this is permanently live so remember to disconnect after the experiment) if it fires then try shifting the fly lead to the 2nd fuse up to the coil pos. Then try again. If the first works and the second doesn't it suggests a switch problem either faulty or incorrectly wired.
Allan Reeling

No ignition relay on a 71.

Chrome bumper cars had a 12v coil connected directly to the ignition supply, rubber bumper had a 6v coil connected via a ballast resistance inside the harness. But either coil will produce sparks in the 'wrong' configuration, albeit overheating in one case and weak sparks in the other.

Does the ignition warning light come on with the ignition? Do the fuel gauge, wipers, heater fan work? If so the ignition switch is OK.

You talk about taking wires off the starter relay, but that will only affect or be relevant if it wasn't cranking. I'm not exactly sure what you are doing and seeing here. I'm assuming it cranks, but doesn't fire up.

Exactly what voltages did you get around the coil? You should have 12v on one side (white wire) all the time (will drop to about 10v while cranking with a good battery, lower with a weak battery). This voltage comes via the tach until the 1973 year, not from the white at the fusebox as with 1973 and later.

What you get on the other side (white/black wire) depends on whether the points are open or closed. With them open you should also have battery voltage, with them closed you should have an earth i.e. zero volts. Remove the distributor cap, manually open and close the points, and check you get those voltages on the coil terminals.

With the ignition on as you flick the points open and closed you should get a spark from them, if you get that you should be getting a spark from the coil. Remove the coil lead and connect it to a plug laying on the block to see.

If the voltages are OK and you get a spark at the points but still not at a plug it could be the condenser. Try connecting another one - ideally one that is known to work as new condensers are problematic - between the white/black and earth.
Paul Hunt

thank you all for your thoughts. I have been up to the garage again this evening, and run the various suggested checks.


Voltage at the positive terminal of the coil is 10.9v.
Voltage at the distributor is 10.82v
Voltage at the condensor wire is 10.9v
Voltage at the points when closed 10.9v and 10.9v
Voltage at the points when open 10.9v and 9.6v

Bridging the bottom fuse to the coil did not result in a spark.

The voltage at the negative terminal of the coil is 1.5v and 0.2v.

Something isn't right.

Thanks again

Daniel
DP Lawrence-Sansbury

The voltage at the points when closed ought to be zero, or very close to it, as closed points have a direct connection to the distributor baseplate and thence to earth. You have new points, so check the earth strap from the baseplate to the distributor body.

Other thoughts:
Are those voltage readings with the engine stationary or cranking?

If stationary, have you measured the voltage at the fuse box?

If the fuse box voltage is ~12.5 V then it appears that a ballast resistor may have been retrofitted, and the way it's wired may be part of the problem.

If the system voltage is at the same ~10.9V, then the battery would not appear to be healthy.
Paul Walbran

A ballast is unlikely from the readings - when the points are open you see 12v on both sides of the 6v coil with a ballasted system. It's only when the points are closed that you see 6v on the supply side and 0v on the points side.

But the readings are definitely not right, even ignoring the low voltage.

You can't get 10.9v at the points when you have between 1.5v and 0.2v at the coil negative, unless some very strange wiring mods have been performed. That latter reading (0.2v at any rate, 1.5v is a bit high for decent points and connections) is what I'd expect when the points are closed, and I would expect to see very nearly the same reading on the moving contact at the points when they are closed, 12v when they are open. The fixed contact of the points should always be zero volts.

The readings need to be taken again, plus on the brown at the fusebox with the ignition off, and the brown and the white with the ignition on. I'd expect to see 12v on the brown, and only a couple of tenths less than that on the white, and on the coil +ve. If you still see below 11v then measure the voltage between the battery posts (not the connectors) with everything switched off. If you still have the twin 6v batteries then measure both and add them together. If you still see less than 11v then either your meter is incorrect (check another vehicle), or the battery is flat, although in that case I wouldn't expect it to be able to crank the engine.

Paul Hunt

The 10.9v is most likely down to the continued efforts to diagnose the issue and cranking the engine. Perhaps that therefore is a red herring.

Thanks again for your thoughts. Regrettably I am out of town for the next week or so, so will have to reconvene next week.

At this point I may remove everything and just start with a clear mind.
DP Lawrence-Sansbury

Daniel - where do you live?
Paul Walbran

Hello there all,

again thank you for your efforts. I have just got back home after an afternoon tracing.

We have removed and cleaned the distributor which was pretty filthy, have checked both with both old and new condensors, checked the points again, and then started on all of the wiring.

Success!

The wire between the coil negative and the distributor had a break. Have connected a fly lead across the two points, and we have life. We have stripped the wire back and there are signs of a previous break being soldered. I will focus my attention here next weekend.

With her now running though, the oil is filling my parents' garage floor quite quickly...oh, well, that can be the next project. Buy a car that hasn't run for a few years, you can't really complain :-)

Thanks again.

Daniel

PS. I am in Auckland Paul. I'm sure we will come say hello to you soon enough. This will be an ongoing project I think.
DP Lawrence-Sansbury

This thread was discussed between 18/10/2015 and 01/11/2015

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