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MG MGB Technical - LT side ignition expert required...

Hi
Hoping for some advice on my non-staring UK 1977 (electronic ignition) roadster.

It's been running badly for a number of months (misfiring at 2500 rpm or so, sometimes trying to die when pulling away, seems fine when revving over 3.5k. Also recently noticed tach dropping to zero when heater fan turned on and tach illumination lamp being intermittent.

It has generally been ok at starting, though often needing some persuasion to keep it running for first minute or so.

I was going to take it to garage for proper tune up of ignition and carbs but needed to replace the brake master cylinder first, I did this yesterday, and now the car won't start at all, need to get it running to get it to garage. Symptoms are...

1) No spark on HT lead from coil.
2) 12v on positive side of coil when key in running position and perhaps a volt less when cranking.
3) About 9v on -ve side of coil. Apparently steady although I'm not sure my digital volt meter could keep up with the speed of switching if it was doing so.

Some background facts and possible sources of problem...

1) It strarted fine about 10 mins before I finished with the brakes, I then had to wiggle all the wires near the steering column to put the rubber bulk head blanking thingy back on, so maybe I dislodged something. However, voltage on positive side of coil seems to behave as I'd expect with key position.

2) I probably spilt more brake fluid than is good for the car (but not an excessive amount) in the general vicinity of the dizzy/HT leads, however, as I was styaing it started fine at least once when I'd basically finished with brakes.

3) It seems to have an aftermarket ignition module as it has red and black leads running to the coil. One of the black/white from the coil doesn't connect to anything (it never has in the 18m). The other goes to the tach. My voltmeter says the one to the tach is fine and I've wiggled and tried to tighten up the other wires at the back of the tach.

Any help please!

Darren
D Lewis

Darren. There is an ignition system troubleshooting tech article on my website, MG section, at www.custompistols.com/. However, it sounds like you have the correct figures since you have the "6V coil" in that model year which shows about 8-9V output to the distributor. Thus, the only way you are not going to induce a spark in the coil HT lead (kinglead) is if the coil itself is bad or the electronic triggering device is bad.

There is no way I know of troubleshooting the electronic points replacement devices. You either install another one and see if it works or install a points type distributor and check out any problems.

From my experience, the symptons you describe were the same as when I began to have problems with an electronic points replacement device. It may be that the lack of ignition is not related to your brake work, but, the system has finally failed completely.

If you can borrow a points type dizzy, it should be possible to install it and see what happens. Set the ignition timing, using a dynamic timing light, as the engine is being cranked over as points type systems are often far off of the electronic replacement system's basic timing. If it starts and runs well, you can assume the problem is the points replacement unit.

Your car should have, originally, had the Lucas 45DE4, Opus, system. Most of these have failed over the years and have been replaced by some other form of internals. You would have to remove the system to try to figure out exactly what you have.

Les
Les Bengtson

Darren, you can test the coil by removing all the wires from the neg. post & attache a spare wire. Switch on & touch the spare wire to earth. When you lift the spare wire off earth, you should get an HT spark if the coil is OK. Also ,I don't know which type of ignition system you have ,but if it's Luminition or Baracuda. they are easily damaged by getting the coil leads crossed. HTHs Barrie E
Barrie Egerton

good advice above - if it is a luminition unit- check the earth wire is properly grounded - if you have brake fluid behind it, the contact may have been broken.

++ Usual advice about replacing the ht wires/dizzi cap/rotor regularly and carrying a spare amplifyer.
Roger
RMW

Coil voltages are very difficult to specify without knowing exactly what type of electronic ignition you have, and what voltages they give on a good system. Also most digital meters will not 'settle' on a constantly changing voltage like round the coil, an analogue meter will give an average voltage which can be much more useful.

Failure of the tach when the running problems occur would probably be caused by an ignition LT problem, which could include an intermitently open-circuit LT winding on the coil. However if the tach drops to zero when the heater is turned on, and the engine continues (or did when it was running) running OK, then it is more likely a 12v and/or ground supply problem to the tach and nothing to do with the poor running. If both the tach dropped *and* the engine started to misfire when the heater was turned on then I'd expect a bad connection feeding both. As your electronic ignition is aftermarket the installer could have connected it to any 12v source so it isn't possible to say where that might be. A 77 should have an ignition relay, but the coil was originally fed from the ignition switch (unfused) and not the relay.

Whilst you will get some spark out of a coil just by connecting and disconnecting 12v to it, it will be a weak spark unless you include a condenser like a points system does. Without could lead you to presume your coil is faulty. Intermittent problems can be very tricky and time consuming to resolve, particularly with electronic ignition, but if it now won't start at all it should be much easier. Personally I'd recommend going back to points and condenser, for the elimination or otherwise of problems with the electronic ignition if nothing else. Rubber bumper cars with the 6v coil and loom ballast still in place will show 12v on the coil +ve with the points open and about 6v with them closed. Chrome bumper cars will show battery voltage on the coil +ve (or SW) at all times. All cars will show 0v on the coil -ve (or CB) with the points closed and 12v with them open.

A UK car only ever had points and condenser from the factory, the 45DE4 and 45DM4 systems were only ever for North America (eventually all LHD cars were were to North American spec).
Paul Hunt 2

Thanks for all your ideas. In the end time pressure meant I gave up and got towed to the garage (I hate being towed - it's just 30 mins of staring at a rope, holding a foot on the brakes and preying everytime it goes slack that the lead driver doesn't "nip out of a junction").

It seems the fault was in the ignition module as they put on a second hand dizzy and all is well. But it leads to two further questions...

1) I'm back on points - there are enough other things on the car that keep it a time consuming hobby without needing to replace the points and set the timing every 6,000 miles so I am thinking of getting another electonic module - am I being silly?

2) It now runs very well when warmed up, but in the recent cold UK weather (maybe close to zero centigrade, probably few degrees above), it still takes 15 to twenty seconds of cranking to fire and is then a struglle to keep going for the first couple of mins. It's a newish recon engine, new leads, coil, battery, points. Would you expect it spring to life a bit more easily? Any tips on starting - choke all out?, foot on gas pedal?

cheers
Darren

D Lewis

Darren. All of the Lucas distributors require annual maintenance--cleaning, inspection and lubrication, if they are to deliver their full service life. I find the points last at least 10K miles and replacing them on an annual basis, along with the dizzy maintenance, has never been a problem. One of the aftermarket electronic points replacement systems is not a bad idea, more neutral than anything else. You still need to do the annual maintenace to the distributor, but the unit can hide some of the problems that many distributors have such as slightly worn bushing, slightly bent distributor shaft and slightly inconsistent points cam lobes. Much will depend on the condition of your new/used distributor. With a well maintained, and hand assembled, points type, it will perform as well as the same distributor with an electronic points replacement unit. As the distributor departs from the ideal, the points replacement system will demonstrate its advantages.

Les
Les Bengtson

Darren, I recommend the Crane ignition for your car. It has a feature that no other system (to my knowledge) has. There is an led on the ignition box that lights when the optical trigger is tripped. In other words, as each cylinder fires, the light flashes on. This not only helps you set the timing on the initial install, but also tells you that there is power to the system. The led alone lets you know that the LT side of the ignition system is working properly.

Be cautious of the install. If the sensor is not properly located in the distributor, it can stop the vacuum advance from rotating the points plate. The car will run fine at idle, but you'll be let down with its street performance.
Jeff Schlemmer

Darren,
I'd stick with the points as they are really easy to sort out and cost pence whereas the electronic set-ups are fine until they fail, which they will, when they leave you properly stranded without a hope of fixing them.
If you drive your car regularly I'd expect it to start after a couple of cranks, even in this weather. I use the choke full out, in neutral, feet off the pedals (pushing the clutch in puts a tad extra strain on the starter and my SUs don't like being primed with the accelerator), turn the key and choke in a bit once it catches to stop the revs getting too lairy before the oil pressure is up. Then again, if the car has stood for a few weeks, I usually have to get out the Easystart spray.
Steve Postins

Since I started using dwell to check the points instead of the gap with a feeler gauge I find I don't need to adjust them between replacements, lubrication of the rubbing pad (grease) and weights (oil) is still required of course. Not fiddling with the points also means the timing doesn't need readjusting either. Indeed, they are now well over 10k miles and still within dwell spec. This is on the 4-cylinder which has a wider tolerance for dwell then the V8 (+-3 degrees as opposed to +-1 degree). I did have to adjust the V8 this year, but just over 1 degree drift in about 10k miles is pretty good.

Your 77 has HIFs, I presume. I do find the HIFs on the V8 need full choke to start and for the first few seconds, but the cranking time is barely longer below freezing than in mid summer. The HSs on the 4-cylinder get full choke to start then immediately reduced to half, but again there is little difference between winter and summer cranking. General condition and adjustment of the ignition and carburation components can have a significant effect on starting in cold weather.

Pumping the throttle before cranking with SUs has no effect, unlike fixed jet carbs with accellerator pumps.
Paul Hunt 2

Just to say (again) thanks to all you experts for your time and help. I look forward to the day when I can answer someone's question myself!

Darren
D Lewis

This thread was discussed between 13/11/2005 and 20/11/2005

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