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MG MGB Technical - No spark - Please help.

Hello everyone,
I've just got hold of a 79 Roadster (my first vintage car) - Drove it home 30 miles with no problem.

The next day (After a wash), I turned the engine on for run and could hear and electrical click in the engine bay. I opened the bonnet to see the coil arcing from the bottom to the nearest wires. Moments later the engine lost power and I haven't be able to start it since.

I have replaced the coil like for like (ballast 12V), plus a new distributor, and new spark leads - all with no success.

Though.. I have discovered that there is no spark coming from the coil itself - yet there IS a healthy voltage running to and from it so by all account it should be working?

Does anyone have any helpful ideas on why this won't spark - or could I be missing something else?

Thanks in advance. Ed
E S Bird

Sorry, forgot to mention - It's a rubber bumper model, and has been converted to an electronic starter system.
E S Bird

Ed. What distributor did you replace your old one with? If you have power to the coil, and power through the coil, there are only two things that will cause the lack of spark at the coil lead--bad distributor internals and bad coil internals. If a points type distributor, most commonly the Lucas 25D4, the points can be hooked up incorrectly, leading to a direct short to ground from the coil.

Take the wire going from the coil to the distributor loose from the dizzy. Turn the ignition switch to the run position. Hold the coil lead near the block and briefly touch the distributor wire to the block, grounding it. When the wire is removed from the block you should see a spark at the coil lead if the coil is good. If you do see a spark, look to the distributor as being the problem. If you do not see a spark, bad coil. But, make sure that you have power to the coil with the switch in the run position (through the resistance wire circuit) because you have a secondary coil input through the starting circuit. You need, first, to insure the coil is getting power in the run position. Otherwise, this test will not work.

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

Les
Les Bengtson

Hi Ed,


Assuming you mean by "electronic starter system" you mean some sort of electronic ignition?


Can you tell which system it is, usually a makers name on the electronic wizardry?



Regards Steve
SR Smith 1

Hello,

Les - thanks for your thoughts, I read through the ignition trouble shooting on your site... It appears most definitely to be an issue related to the coil - as there are no sparks coming from it when cranking, though there seems to be healthy power going to it when ignition is on.

Could it be something when turning the engine on, that a break can develop in the coil system causing no spark? (remembering that power is good to the coil when ignition is on, and the coil is brand new).
[Something that could be of note, is that moments before the engine lost power, the coil was arcing from the base (start of the distributor lead) to one of the connection wires.]

Steve, yes - I think I meant electronic ignition. I'm not sure what system at the moment but will get back to you asap. I'm also fairly confident that problem is before it reaches ignition - as mention above.

I really appreciate your help, as I'm still fairly novice. This is my first ever hands on car to enjoy.
Thanks. Ed
E S Bird

Hi Ed, we were all novices once....
Hang in there and we'll help get you up and running again. Tracking from the coil HT to one of the LT terminals presents no problem with points but may have killed an electronic ignition unit.
You say there is power to the coil so I'm assuming you have a meter to check this. In which case the next step is to confirm there is 12v on the coil +ve when the engine is cranking eg with the key held in the start position. Then you need to check the voltage at the -ve side (again whilst cranking). This should be something less than 12v but exactly what depends on the type of electronic ignition/points in use. If that checks out okay you should get a spark from the coil... check this by holding the main HT wire about 12mm from the block and look for a spark.
If you still have nothing you need to look at the dizzy and the electronic ignition system. Report back what's fitted in the dizzy and we'll advise further.

Best of...
MGmike

M McAndrew

Hi Ed,

Assuming you have an internal conversion to electronic ignition and the distributor is really just a standard 45D, I would suggest you revert back to a points system.

Fresh points/condenser, fresh lead from the coil to your distributor and I bet it starts with no problem. There is absolutely nothing wrong with a well sorted points ignition system on this period of car.
There is a little earth lead in the distributor that must be in good condition and often it is not, worth checking/changing.
M F L Sherrit

If the HT lead at the coil is arcing to the LT lead of the coil, doesn't that mean that there is very high resistance downstream of the coil lead? That is, it seems the fire is not making it into the cap, and the fire is looking for ground anywhere it can get it. I would be looking at the carbon contact in the dist cap and the rotor.

Charley
C R Huff

Yes Charley, has a very good point, even if it looks like a new cap, the quality of the carbon contact is very poor on many of the Chinese produced caps. Simply removing the cap and placing it back on is enough to dislodge these poorer contacts.
Ed, I notice you wrote that the distributor is new. The cap that came with my new distributor was just terrible.!!
M F L Sherrit

I second what and MGmike have said about the normal route for the spark not being available, and quite likely having destroyed the electronic ignition module. Given that I rather suspect the coil isn't at fault.

Also depending on what electronic ignition you had, and how it was wired, will determine what coil you need. Originally your car would have had a 6v coil, and fitting a 12v coil will give weak sparking. But you are only going to diagnose this properly by going back to points and condenser, in order to get the engine running, as usually electronic modules can only be diagnosed by substitution.

You can determine which coil you need, if indeed you do need a new coil, as follows: Connect an earth to the coil -ve, usually has two black white wires, and measure the voltage on the coil +ve, which usually has two white/light-green wires. Turn on the ignition. If you measure 6 to 8 volts on the coil +ve (it will depend on whether you use a 6v coil or a 12v coil) then you need a 6v coil. If you see 12v on the coil +ve, tap the earth on and off the coil -ve, and see if it sparks. If it does you need a 12v coil with your wiring.

You can't go by what manufacturers or suppliers say about their coils, you have to measure the primary resistance to be sure. 6v coils measure about 1.4 ohms between the spade terminals (wiring removed), 12v coils measure about 3 ohms.
PaulH Solihull

Hello everyone,

Thank you so much for your helpful comments, ideas and suggestions. I studied them all and eventually diagnosed the problem as a faulty electronic ignition.

I've made the change back to the original points system (which came spare with the car), and it runs fine, with a little tune up needed.

The car is now struggling with overheating problems and conks out when idling if it's hot (doesn't start until it cools down). But I think this is a combination of water leaking from heater outlet, and a small fan being mounted backwards in front of the radiator. (a 30 mile trip would see the expansion tank empty)

To combat this I'm planning to install a larger fan on the engine side of the radiator (correct way round) and replace the heater manifold to stop the leak.

If anyone has any helpful tips for cooling / fan placement etc, then I'd be grateful. Otherwise, thanks for all your help on this.

Ed

E S Bird

Ed,
I would check for a faulty radiator cap first, cheap and easy to do. Unless you are in a lot of traffic, your fan should rarely come on, so I don't your emptying expansion tank in 30 miles is connected with your fan.
M F L Sherrit

You really shouldn't have these problems, especially at this time of year. Installing an extra fan is only getting round the real problem - maybe.

What do you mean by a small fan being mounted backwards in front of the radiator? The factory fan is only small and is mounted in front of the radiator, with the blades closer to the radiator than the motor of course. That should rotate in an anti-clockwise direction when viewed from the front of the car, to blow air through the radiator. If the fan turns the wrong way the electrical connections have been reversed.

What does the temp gauge go up to? If it's not steaming or pushing out coolant then it's not overheating. Of course if you have known leaks they must be fixed. A pals car of a similar vintage used to chuck coolant out when switched off, but that was because his 15lb cap was only holding 3lb! A PO had fitted a mechanical fan to the water pump, but that was turning the wrong way, blowing air at the radiator, i.e. reducing the amount of air the electric fan could blow through to do the actual cooling.

When an engine does overheat it is either because it is generating too much heat, or the radiator can't get rid of it fast enough. If the radiator is equal heat all over, with the outlet not much cooler than the inlet, then the engine is generating too much heat. If there are cool spots in the middle then it is partially blocked. If the radiator gradually cools from the inlet to the outlet, but the inlet is too hot, then the coolant probably isn't circulating properly, which may be a partially closed thermostat.

Does it have a thermostat? Taking it out can result in too fast a circulation, and localised turbulence and hot-spots.

Physically stopping and not being able to restart because there are no sparks is probably the coil either overheating or failing anyway. The former can be caused by excessive dwell, or the wrong coil, or the coil ballast having been bypassed. If you had an aftermarket electronic ignition system before they may have done anything with the coil and its wiring, it needs checking for resistance values.
P Hunt

If the expansion tanks empty, the fan sensor may not operate. If it is the rad cap, you may find a new cap although it cures your problem, the extra pressure being retained within the system may force the sensor to blow out.
c cummins

Just have to question
"I have replaced the coil like for like (ballast 12V),".
A ballast coil will NOT be 12volts. If you have a multimeter check the resistance across the low voltage terminals. Should be in the region of 1.5ohms. 3 ohms is a 12volt coil. Running a 12 viol coil with a resistor wire is not good, as Paul said.
Overheating can have a several causes. Incorrect timing (too advanced), too much Dwell. In view of your problems above quite likely. Siezed brakes, weak mixture, slipping fan belt, worn water pump, faulty pressure cap, stuck thermostat (closed), under inflated tyres.
Allan Reeling

There is a huge amount of confusion over coils, not least with the people selling them and even the manufacturers. Helping someone sort out an ignition problem at the moment he purchased two coils, one labelled "6V No External Resistance Required" and the other labelled "12V External Resistance Required"! In the event they both measure 1.4 ohms i.e. are both 6v coils which need an external ballast on a 12v vehicle like the MGB. The only true way to know what you have got, is to measure it.
Paul Hunt

This thread was discussed between 24/04/2013 and 05/05/2013

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