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MG MGB Technical - Problems when warm, hot coil, hot dynamo...

Any ideas on this anyone? I've been having a problem whereby the car runs fine when cold, then after 10-15 mins begins to miss a little and get rough. Sometimes is conks out, sometimes not, but once it does it won't re-start. If I substute the coil, it restarts fine until the same thing happens. I have worked through 5 coils so far, but the same thing happens with each one. The coils are very hot (almost too hot to touch). All show 3.5 ohms.

I've replaced the control box twice incase that was a problem. One of them was letting the voltage get up to 16-17v. Now sticks around 14.5 - 15v.

I've also noticed the body of the dynamo gets very hot. Can't remember how hot it should get normally. Testing the dynamo, if I connect the D & F terminals together, I get 25-27v at about 1800rpm. Is the generator overdoing it?

Is it possible that the batteries have been damaged with the higher voltages previously and are making the dynamo work too hard, with some effect on the coil? They show 13.5v with engine off.

I can't keep stopping every few miles to swap coils, but could really do with bottoming this out.

Car is positive earth

Thanks in advance.
Mike Winter

In fact you shouldn't rev the engine to get more than 20v from the dynamo when the D and F terminals are connected together as it can damage the dynamo! You should get 20v by 1000rpm.

3.5 ohms is correct for your engine. The coil does get warm in use, but shouldn't get too hot to touch unless the ignition is left on with the engine stopped. One cause of a hotter than normal coil can be if the dwell is too high i.e. the points gap too small, but I wouldn't have expected it to make that much difference. Another cause can be the voltages you were experiencing, the maximum should be 15.5v at 10C/50F (control box temps), lower at higher temps. But again I wouldn't have thought it would make that much difference.

Do you have the electronic tach or the mechanical rev counter? If the tach what does that do when it starts missing? If it starts flicking down to zero then that shows the ignition LT circuit is breaking down, i.e. ignition supply via the tach, coil primary, points, distributor ground wire. When it gets to the point of not starting you need to do some voltage tests through this circuit to determine whether it really is the coil that is at fault, or something else. If you have the rev counter you will have to do this anyway.

If the tach doesn't drop but continues to show the correct engine revolutions when it has cut out but the momentum of the car is still spinning the engine, then the problem isn't ignition LT (with the possible exception of the condenser) but ignition HT or fuel.

Again when it won't start clip a timing light onto the coil lead and watch the flashes while cranking. If the timing light flashes regularly and consistently then it isn't coil HT either. Do the same on each plug lead, and if one or more of these aren't flashing regularly then the rotor and/or cap is breaking down. If they all flash as they should, and you get around 8 to 10 degrees of advance when pointing it at the crank pulley when on 1 and 4, then it is probably fuel (or condenser). After cranking for a bit with no start remove a plug, and if it doesn't smell of fuel then the problem is probably fuel. Other wise you need to check the condenser.

With the distributor cap off and the coil lead removed from it and laying near the block, flick the points open and closed. You should get a spark jumping at least 1/4" with a load crack. If it will barely jump a plug gap and only fizzes, and there is lots of arcing at the points, the condenser is probably gone. You can compare condenser and no condenser by doing the same check but this time tapping the white/black wire on and off the coil spade (should be CB or +ve, with white on the SW or -ve) while the points are closed, and this simulates a failed condenser.

I realise changing the coil seems to cure it, but I'm a great believer in diagnosis from first principles rather than relying on assumptions.
Paul Hunt

Thanks very much Paul. I've a few things to try there and have wondered if the condenser is faulty, but didn't know about testing it in the way you describe.

I have the mechanical rev counter.

I think it must be electrical rather than fuel in that the car runs very well as soon as the coil is substituted, and only plays up again once that coil has got hot. Will work through the tests you suggest and report. Out of interest do you know how warm the body of the dynamo normally gets?

Thanks
Mike Winter

Mike - the best way to test the condenser is to replace it. If your troubleshooting points to a fuel delivery problem, see the article, Fuel Delivery Troubleshooting Guide in the SU fuel pump Articles section of my web site at: http://homepages.donobi.net/sufuelpumps/ Cheers - Dave
David DuBois

Hi Mike,

Iwould go for the points first. As they age (wear) the gap gets smaller, and unless you have regularly regapped them they could get to the point where they are closed most of the time, give too long a current flow in the coil causing you heating. It could also reflect into the dynamo which now would need to generate extra power for the coil. Maybe when the engine is hot and stops, the points don't actually open any more.

Also check the capacitor across the points, as suggeted by others.

Herb
H J Adler

David is correct in that only replacement will show up a *partially* failed i.e. 'weak' condenser, but the test I have described will reveal a completely failed condenser.

Forgot about the dynamo. Heat in that will be partly from the block and partly from current it is delivering. Under most circumstances it will be the dynamo that is running all the cars electrics and not the battery. With the higher voltage you had you will get more current and hence more heat, but again it shouldn't be that much. If something in the cars electrics is demanding more current than it should then it will be hotter than it should. You should have the third 'current regulator' relay in the control box, so could do some tests to see if that is operating which would indicate high current. If you lightly touch the armature and feel it buzzing then it is operating. The voltage regulator relay does this, so if only one of them is doing it and your voltage reading is normal i.e. around 14v it will be the voltage relay you are feeling, which according to the Workshop Manual is on the end of the group of three, the current regulator is in the middle.

Some time ago for some reason I did some 'how hot does it feel' tests on my alternator-equipped roadster and found the alternator was warmer than the coil but the oil filter was hotter than the alternator. Haven't thought to test it since with my laser-pointer thermometer (yet).
Paul Hunt

Mike

Are you getting all the coils from the same place eg Moss in bradford as the coils seem to be only puttig out 1.7 and the sports coils about 2.9 which causes them to over heat to the point the last one i changed was still untouchable after two hours.
Go to a local factors like andrew pages and get an intermotor or bosch one instead but make sure you dont get a ballisted coil by accident.

Ste
Ste Brown

Thanks to all of you for your comments. I tried some of Paul's tests this morning in between the rain showers. When testing the condenser I got a fat spark, then very little then another fat spark, so seemed to be an intermittant spark followed by weak ones. I subbed a new condensor and got nothing at all, so the new one was entirely stuffed.

I ordered new points and condensor at the weekend and will get them in a couple of days. In the interim I decided to refit my Newtronics elec ignition system which I took of a while ago when this problem first started, at the recommendation of the RAC guy who rescued me.

However, this was when the system was running at 17-18v and I think this caused the coil to overheat and fail. Having replaced the control box and that coil and having 14.9v this morning at 1800 rpm, I recon it all might work. So if the weather perks up I'll take 'B' for a spin and see how far I can get...
Mike Winter

Well, I've been out for about 45 mins and the car ran really well. No missing, ran beautifully. Once I got home, I stopped it and checked the temps. Coil, really hot, as hot as the oil filter. Dynamo much cooler than before, just a bit warm really.

Interestingly, I tried restarting and got similar results. Lots of cranking and not much action. It did eventually fire and ran again. I checked fuel levels in the float chambers and these were both fine with delivery when floats were lowered.

The coil I ran with today is a Lucas Sports on (DLB 105) with a resistance of 3.5ohms. Is it possible that all the coils I've been using could have bought it if they'd been running at the much higher voltages I'd previously been experiencing? I have 2 other standard coils, one shows about 1.7 ohms (from Moss as it happens Ste..) the other from MGOC 3.5. These are DLB 101. I do also have the original Lucas coil which was on the car when I bought it, much larger body and SW and CB markings on the connectors, so probably an original +ve earth coil.

I'm obviously a bit reluctant to buy another coil, but did wonder if all the above could have been cooked at 18v+?

Thanks
Mike Winter

Check your heat shield to see if your carb is vapor locking. Had this experience and the heat shield had been partially removed by the PO
Ellis Carlton

Stamped on the bottom of the coil is a number, which is the design voltage. 12 for non ballasted systems, about 3-3.5 ohms. 6 or 7 for ballasted systems, about 1.5-1.7 ohms. 18V is 150% of nominal 12V design (300% for a "6" coil), or near 30% over actual high limit spec, so current flow would have been at least that much over spec, maybe a lot more (and constant rather than the intermittent draw of the points) if the condenser was shorting internally.
Coils short internally between layers of windings, so lower than spec resistance is a sign of slow death. It can result in heat, high current draw, but low HT output.
Your OE coil is probably the best of the lot - they were a "semi-sports" on MG.

FRM
FR Millmore

Thanks again. I checked the heatshield yesterday Ellis when I was looking in the float chambers and the carbs were certainly cool enough to handle comfortably, so I think that is ok. If I had a vapour lock, how would I diagnose it? The fuel filter was full and when I opened the float chambers, fuel flowed in through the needle valve.


FRM, I put the original coil back on yesterday, a Lucas one marked 12V and 2 64, so I think it is the original Feb 1964 coil that went out of the factory with the car..
Mike Winter

I'd suspect your ohmmeter readings if you got 3.5 ohms for a DLB105 Lucas 12v Sport, mine measures about 2.4 ohms. There is also definitely something wrong somewhere if two coils are labelled DLB101 but one measures 3.5 ohms i.e. is 12v and the other measures 1.7 ohms (but both a little on the high side) which is 6v i.e. for a ballasted system. This is why I always say measure coils before you fit them, and before even paying for them if buying over the counter. DLB101, GCL101 and GCL110 should all be 12v, DLB102 and GCL111 should be 6v, AFAIK.

I've done some temperature tests today and a standard coil after a few minutes running was a couple of degrees hotter than the alternator. After another half-hour or so of running the alternator temp had dropped a few degrees, presumably as there was less demand on it after recharging the batteries. The oil filter was quite a bit warmer to the touch than the coil, about 70 degrees as opposed to about 55 degrees. If your coils are as hot or hotter than the oil filter than that seems too hot to me. However the temps in the oil filter can vary quite a bit depending on whether you have just been running in cold air i.e. cool oil, or whether it has been running at a fast idle for a while i.e. no cold air blast over the block, sump and through the oil cooler if you have one i.e. hot oil. So really you need temperature measurements, which can then be assessed absolutely as well as comparatively. Maplins are (or were) doing a reasonably priced infra-red non-contact thermometer, but you have to get pretty close to small objects like the coil as even with a 10:1 aspect ratio you don't have to be very far away before it starts averaging the object being measured with its surroundings.

I have found that when I left the ignition on for sometime and forgot I couldn't get the engine to start until after I had switched off and let them cool down. Whether there is a safety cut-off i.e. a thermistor in the coils I don't know, I can't imagine what else would cause that, unless it is a broken turn in which case I'd expect it to happen at other times as well. There doesn't seem to have been any lasting damage. I cooled mine down with a wet cloth, and you can do the same on the jet pipes between the float chambers and the jets of HS carbs, which is pretty-well the only place vapour-lock can occur. But it's rarer than people suppose, especially in the UK, and especially in November.

SW and CB *are* the markings on +ve earth car coils, although I didn't think they were larger than the later + and - marked coils for negative earth cars.
Paul Hunt

The 1.7 ohms coil could of course be a partially shorted one that FRM mentions.

The reference to '12v' electrics is only nominal, the designers know that the typical running voltage is going to be about 15v and take that into account. If a supply of 18v is going to burn out a coil when running, then having the ignition on without the engine running, the points usually being closed, is going to have much the same heating effect even with the lower voltage. And we know coils don't burn out that easily.

The 'other' end of the HT winding doesn't go to earth via the can, but is connected internally to the CB or - terminal. I've not heard it causing this problem before, but do you have the coil connected round the wrong way? For positive earth the SW or - terminal should have the white ignition supply wire and the CB or + terminal go to the points. For negative earth they should be reversed.
Paul Hunt

PPS. The Workshop Manual states that 'ignition on' current is 3.9 amps, and running at 2000 rpm is 1.4 amps. However this second figure is what is displayed on an *analogue* instrument, which is mechanically averaging the current flow whilst the points are opening and closing, a digital instrument may indicate something completely different or simply not 'settle' and display a consistent reading. I've just checked one of mine at very close to the above figures, it would be interesting to know what Mike's shows. Also what a timing light is doing when it won't restart.
Paul Hunt

Thanks for all that Paul. The coil which showed the low resistance (1.7 ohms) packed up after just a few minutes running and was very hot, so I suspect that one is genuinely faulty.

I'll have a go at current draw tomorrow, and I have a digital multimeter so we'll see what it says.

However since I put the elec ignition back on and reverted back to the original coil it seems a little better, but I need to take it for a spin to see for sure. I have a timing light in the back so will utilise that if I get stuck...

I was wiring -ve terminal to ign and +ve to earth. Can test the spark direction with a pencil between HT lead and plug.
Mike Winter

An interesting thought, while HT current will flow in the 'other' direction with a reversed primary I can't say I've ever noticed the spark direction! Definitely one for the test bench in an idle moment.
Paul Hunt

That's why the polarity is marked and matters. A spark can be initiated more easily at a pointy hot place than a cold flat one. So, you want the spark to jump from the center electrode of the plug to earth - it reduces the max voltage required to cause the spark, by 10 or 15% typically. Putting a pencil point in the spark path is the old time test to see which way the spark goes - the spark actually knocks some graphite off the point and causes it to burn, making the spark yellow on the "leaving" side.

FRM
FR Millmore

Right then... Enough of a break in the rain (really selling 'Up North' well aren't I?) to test the current flow. Remember I am now using an electronic ignition box rather than points condensor.

I get 3.2 Amps ignition on, not running, then 2 to 2.1 Amps when running at about 1500-1800 rpm. I'm using a digital meter and the current briefly drops to 0.5A, then hovers at 1.9 for a fraction before going back up to 2.1 again. It cycles like that about once every 5 seconds or so. So i assume my multimeter is trying to take an average. This was all measured on the original 1964 Lucas +ve earth coil.
Mike Winter

Not really comparative, by using a digital meter and electronic ignition (was it in series with just the coil or the coil and the electronics?) but in any case that doesn't seem excessive. In theory the coil current running should be around 2/3rds the static value (i.e. 2.6 amps) with a 25D distributor, or 1/2 with a 45D (i.e. 2 amps), because of the different dwell or percentage of time they are closed compared to open. But then you have to take into account the higher system voltage when running of typically 14.5 to 15v compared to the static of 12v which would increase that by another 20% or so. The 1.2 amps running current in the manual is significantly less than these, but this shows the effect of a 200v or so back emf for a very short time, which is tending to throw an analogue meter needle back further than simple Ohms Law would indicate. If you run a distributor with a simple resistor instead of an inductive coil you get the 'correct' i.e. Ohms Law running current. You would really need to see what your digital meter on a standard points system shows, or an analogue meter on your system, to see if yours is in the right ball-park or not.
Paul Hunt

Right, I have finally solved this problem, after a break for the really cold weather.. :-) I did adjust the output from the control box to within limits, so I only peak at 15v rather than 18v, but the actual problem was one of fuel..

Yesterday, I stripped the carbs down and found that the jet on the front carb was flush with the carb body, whereas the rear was about two full turns down, as it should be. Reality is then, my MG has been running mostly on the rear carb, with the front helping when cold and choke out, but offering virtually nothing when cranking when hot. However using the choke when hot made no difference, which is probably because the rear carb was overchoking the rear two cylinders.

I'm posting this in case it helps others with a similar hot start problem. I did get someone to 'tune' the car for me a while ago and I think they forgot to adjust the front jet once they had 'zeroed' it with the carb body, but worked on the rear.

Anyway, it's running better today than it has for a long time. Thanks to all for your contributions a few months ago, it is amazing how one can be misled into thinking the ignition was an problem 'cos the coil was so hot, when it was really down to imbalanced carbs.
Mike Winter

Makes a change from "90% of SU carb problems are caused by Lucas ignition". In this case put a couple of HT leads down your mechanic's trousers.
Paul Hunt 2010

This thread was discussed between 14/11/2009 and 24/04/2010

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