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MG TD TF 1500 - ignition system issue

This post is similar to the coil problem post but I am pulling my hair out trying to solve this issue. I had the same problem a year ago and I messed with so many things I never knew for sure what fixed it but I got 1000miles of good running since. I decided to change the points and condenser to new and now it is doing it again. First let me say. The point gap is perfect, the timing is perfect, the distributor is properly clamped, the engine ground strap is in place. The problem happens on either of 2 original type coils I have, I have tried multiple condensers and more than one of the red rotor buttons, The bolt stud connector that passes thru the distributor is properly isolated, the wires and distributor cap are like new and the carburetors are in good tune.
Here is the problem: The car starts cold and runs really good and strong for ten miles and then starts missing slightly that can quickly degrade to not running in a few more miles. I have about 5 condensers and every time I install a new one the problem immediately goes away or at least changes for the better until it gets warm again. So if it was the coil what are the chances they would both be bad and why did it work well for 1000 miles after I experienced the problem before? Since changing the condenser resolves the issue temporarily, it seems to me something is making the condensers fail. I even moved the condenser outside the distributor but it made no difference with the problem but does keep it cooler. The wire from the coil to the distributor is good. When the engine acts up it is very symptomatic of condenser failure. Any ideas? I have checked and rechecked everything.
BEW Brett Wright

Buy a new condenser from Advanced Distributors in the USA or the Distributor Doctor UK. Or get one that will fit from an older American vehicle. Paul can help on this?
Dave H
Dave Hill

Brett, if your car is an early TD with the external clamp type of distributer hold down, it needs a ground wire. (see photo)

The clamp is a floating part that does not always provide a solid ground to the block.

Richard Cameron

My distributor is the later type with the locking bolt that has a rounded section removed to clamp the distributor shaft housing. I don't believe it ever had this type of ground wire.
BEW Brett Wright

Condenser at your local auto parts store
Len Fanelli

Are you sure it is ignition? How did you verify that? (Did you pull a wire and verify you do not have a good blue spark to the proper plug at the proper time?) arcing between plug wires, across distributor cap. Corrosion at plug wire connections..wire ends or in the cap. Poor connectors on the plug end of the wires, etc.

Verify what element of the basic "good spark at proper time and to the proper plug, compression, proper mixture delivered to the combustion chambers" is not there...

Have you thought of an intake leak or mixture issue? Regards, tom
tm peterson

I have had a similar problem in the past and found it was due to the overheating of the coil. In other words the coil was breaking down - but not all the time.
I changed the coil and it fixed my problem.
John Walton

You say "2 original type coils." Are they actual old ones or newer ones made to look old? Are they "3 Ohm" of 1.5 Ohm or less? Model number may be on the bottom. What part number condensers have you been using, from where? Is the condenser a big one as the original, and the one (G120P) Len F suggests, or a small one? Are your spark plugs a normal TD gap?

D mckellar

Brett, your right the later distributer with the cotter bolt and cast mount doesn't use the ground wire.
Richard Cameron

I will try to answer numerous questions. The condensers are almost all from Abingdon Spares for the TF. Some say west germany on the back Others have a solder bead where the wire come thru the top. They are not as large as the long original but the engine has accepted them before. The spark plug wires and distributor cap are like new and all ends and connections have been inspected in good order. since the problem is heat related and the car must be in motion I have not been able to pull wires to see which cylinder or cylinders are affected as things are breaking down. I do know as soon as I change a capacitor, I am up and running again for a while. The plugs are gapped correctly and I have changed them out with no effect. I have no idea if the coils are original or more recent replacements but they are both identical and have the proper mounting bracket and the proper terminal identification letters. They both say 012 with LO below and on the 3rd line one reads ER 12v-948 and the other reads 12v 553 the 4th line reads 5820 on the first one and 0450on the other. I could get a third one and try it. I don't know what ohm they are but I could find out how to verify and do that. I don't see how it could be intake leak since it resolved itself for 1K miles and it is not all the time. It is does not act like a compression issue because the engine is still as strong as ever on my test hill until the breakdown occurs and it does not smoke or use oil. I am incline to track down some condensers that Len
bew wright

Brett Ė Does the 1st line say O 12 or Q 12? If it's actually a Q, then one of your coils has the same markings as my failed Moss reproduction coil, except for the numbers on the 4th line whose meaning I donít know. I donít have a picture, but original coils have LUCAS in raised letters on the top and date of manufacture on the end. I would suspect your coils have both failed.

Joe Olson

Try swapping the distributer cap. The next time it acts up, pop the gas cap, leave it open and try to drive it again.
D. Sander

Suggest locally buying a cheap internal ballast ignition coil, likely around $20 and try that. Also condenser from From The Frame Up or a different supplier. Re-reading your initial post: was it running fine for 1000 miles, then you changed the points/condenser and the trouble began? That would seem condenser. Someone had an article out that described the horrors of new condensers, possible bad new ones? George
George Butz

Brett - I think you have answered your own question. I would buy a new coil (not one of the reproduction original types) and a few quality condensers.

Check your secondary coil wires as well. Those wires break down over the years. It mat be time to re wire your distributor.


79 MGB
gary hansen

Bret - Have you checked the fuel side of the equation? The next time the problem manifests itself, check the fuel level in the float bowls on each carburetor. Before you start troubleshooting next time, turn the ignition on and wait for the fuel pump to stop clicking - shut the ignition off and check the fuel level in the float bowls and record the level (I would measure down from the top of the float bowl, so you know where it was when you start so you can tell if it is lower when the problem arises).

Next thing to do is to take all those capacitors (condensers) and put them in a box and stash them somewhere that is inconvenient for you to get them (my first thought is the trash can), then get a known good capacitor from Advanced Distributors and install it in your distributor. Finally start keeping a detailed log of what you have done and the results of what you did. This will keep you from running around in circles, redoing things that you have already tried. Cheers - Dave
DW DuBois

It will likely be Saturday before my next attempt. The wire to my distributor is new. I have procured a new coil from AS and purchased some new condensers that Len recommended. I have a new distributor cap too. I doubt the problem is fuel related but I will check the float levels. I know the gas tank cap does not make an air tight seal on the tank. the O may be a Q on my coils. They are painted and very hard to read for sure. I have been trashing condensers as they act up. If you go back to my original post, I mention that I changed the condenser that was working. I am going to reinstall that particular one for a test with the coil it was operating well with, as a control check. Duh... should have tried that by now. I really want to change one thing at a time to arrive at what the problem is. It is time consuming because it can take 30 minutes of driving to reproduce the issue. I will systematically insert one new part a time to see what helps. At least it is a fun car to drive!!! I will report back either way when I have actually done something.
bew wright

Brett, you have a series of really good people advising and you will find the problem. I did have similar problems and ended up being spark plugs of all things. I was using NGK plugs and had issues like you describe. I changed to Champion, and so far the issue has not returned after 5000 mi.

Just a thought.... CR
C.R. Tyrell

You may have missed it but I did change all 4 plugs to a spare set I had without any change to the issue. I now have 4 new plugs I can try. I am not putting much stock in that being the problem because it deteriorates to the point it won't even run at all. I would expect it to be running on 3 cylinders if it was a plug. I do enjoy all of the thoughts and recommendations and it arms me with some ideas I haven't thought of. No doubt I will eventually find the issue.
BEW Brett Wright

I had EXACTLY the same issue, it was DEFINITELY the coil
M Lees

I agree it is likely something about the reproduction coils you have, but one thing not identified yet is the type of spark plug wires. Are they the kind with the stranded copper wires running through the center? Look for writing on the outer insulation if you are not sure. Modern plug wires likely have too much resistance for that coil, if the reproduction coils are accurate (internally) reproductions. Too much resistance can damage a coil.
D mckellar

First I wanted to say hooray for this forum. For anyone with the fortitude and interest to get to 22nd entry of this discussion, I believe I have the problem conquered! There was no problem when I started as I was just making routine tune up changes. So I dug out the condenser that I originally replaced and hooked it to the coil it was operating on and it fixed the problem! My theory is that the condenser I installed and maybe the next one was bad and then I thought it must be something else. At that point I switched coils and all of the rest of my time consuming effort was conducted on a bad coil that was destroying one condenser after another. That bad coil was one of the reproduction kind of look alike coils that others have had problems with. The coil that the car likes is the original 63 year old lucas coil. Now that I got it running I installed one of the long (more in line with original size) condensers Lew recommended. Bigger is better with some x rated body parts so why not with condensers?
The spark plug wires are not resistance type, by the way.
I hope this is not a premature victory lap but it has traveled 30 miles of hard running, at least, without any sign of breakdown. Headed to the British Invasion in Stowe this weekend and feeling confident!!!
Thanks for all the help and similar experiences!!!
BEW Brett Wright

Brett - You have found out first hand something I was told by a shop superintendent of the electronics shop at the Puget Sound Naval Shipyard - we were just chatting about any and everything and started talking about cars. On the subject of ignition, he said "when replacing the points, DON'T change the capacitor (condenser)". He went on to ask, how many times have you seen a bad capacitor (other than an electrolytic capacitor). On reflection I had to admit that I couldn't remember a time when a capacitor in a circuit had been the cause of problem.

Later, when I was working on our TD (that we got 10 years earlier in 1974) I got to thinking about his advice and the fact that in all the time we had been driving the TD, I had not replaced the capacitor in the distributor because that particular capacitor was no longer available. I had always cringed a little when I replaced the points and not the capacitor, but I also realized that the capacitor had never failed. That same capacitor was still going strong when I installed an electronic ignition about 10 years ago (that capacitor is still on the original plate with a set of new points that is in the tool box of the TD against the day that the electronic ignition may fail (and I am very confident that if that plate is called on to get us home, the capacitor will still be good.

Bottom line - when changing the points, DON'T change the capacitor!. Not only do the original capacitors rarely fail, it is a far better chance that the replacement capacitor purchased today will fail out of the box (or shortly after). Cheers - Dave
DW DuBois

Dave, Could a dodgy coil kill a new spark plug and maybe an old original condenser ?.



Tony - Not unless the coil was putting out way to much voltage - which doesn't normally happen. Cheers - Dave
DW DuBois

Hi Brett, first of all congratulations on finding the fault on your car. second I must agree with Dave on his comment regarding not changing the capacitor if it works then don't change as there is nothing to go wrong with it, it has caused so many problems for so many people by changing a working component. I had exactly the same fault as you did, the car would run well for many miles then start to miss fire, this continued for several years would you believe but would clear if you drove carefully for a couple of miles. I eventually found that that the spark plug caps were causing the miss fire due to the resistors inside them were breaking down when they became hot, I didn't know there were resistors in them and nobody I spoke to have heard of this problem, however I replaced the plug caps and it was like driving a new car. If your problem starts again I suggest you change your spark plug caps with none resistor type such as NGK they are very cheap and maybe solve your problem.
Kind regards Adrian Wells. Lincoln UK
Thomas WELLS

This thread was discussed between 06/09/2016 and 13/09/2016

MG TD TF 1500 index

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