The Triumph Dolomite Club - Discussion Forum

I had the same with this system on my 1300 Dolomite and replaced coils ballasts etc. without solving it. It also suffered from a very annoying miss or stutter when cruising at 50/55. I asked for help on here for advice on coils and sorting but could not get any satisfactory answer.

After changing every thing from several coils, plugs, leads and fitting additional earth leads the problems persisted, even a new coil was provided under warrantee. During investigations by myself and a Classic Restorer we found that the unit itself in the distributor was unbelievably hot, it was impossible to touch. It was removed and the base plate was found to be lose although the screw was fully tightened up. So we took the base plate off and found that it was black underneath where it had been shorting out on the distributor plate. We replaced it with points and condenser and the problem was resolved never to come back. This problem had been going on for the best part of three years and I had changed jets, well everything because everybody kept telling me that this piece of kit was brilliant, it works perfectly all the time or doesn't work at all, it wont be it.

Since solving mine by removal of the Lumenition kit I came across a post advising the use of small spring washers on the fixing screw of the base plate to stop this very problem. It was apparently a "known issue" that the screw provided is too long in some applications and will not secure the base plate sufficiently leaving it loose creating a bad earth and causes the issues I had with missing, hot coils and also starting problems. Found this out to late for me but it may be what's causing your problems.

This should be made a sticky
Thanks for making that clear Rob

Tony

I had the same with this system on my 1300 Dolomite and replaced coils ballasts etc. without solving it. It also suffered from a very annoying miss or stutter when cruising at 50/55. I asked for help on here for advice on coils and sorting but could not get any satisfactory answer.

After changing every thing from several coils, plugs, leads and fitting additional earth leads the problems persisted, even a new coil was provided under warrantee. During investigations by myself and a Classic Restorer we found that the unit itself in the distributor was unbelievably hot, it was impossible to touch. It was removed and the base plate was found to be lose although the screw was fully tightened up. So we took the base plate off and found that it was black underneath where it had been shorting out on the distributor plate. We replaced it with points and condenser and the problem was resolved never to come back. This problem had been going on for the best part of three years and I had changed jets, well everything because everybody kept telling me that this piece of kit was brilliant, it works perfectly all the time or doesn't work at all, it wont be it.

Since solving mine by removal of the Lumenition kit I came across a post advising the use of small spring washers on the fixing screw of the base plate to stop this very problem. It was apparently a "known issue" that the screw provided is too long in some applications and will not secure the base plate sufficiently leaving it loose creating a bad earth and causes the issues I had with missing, hot coils and also starting problems. Found this out to late for me but it may be what's causing your problems.

The 1-line Management summary is … Yes ! Ignition coils can run very hot. But it depends a lot on how you are running the engine.
Now I do these experiments so you don't have to. So don't try this at home as ignition coils by design generate very high voltages.
First some numbers to work with.
The temp; co of copper wire is +0.393% Deg C
Sprint dwell angle is 51 deg ON and 39 deg off.
Ignition coil primary inductance is approx; 10mH
Primary resistance of the Bosch 12V coil is 3.26 ohms at 24 Deg C
Secondary resistance of the Bosch 12V coil is 7.36 K at 24 deg C.
Note that when you quote a d.c. resistance of a copper wire wound coil the temperature should also be quoted due to its enormous temp; co; of resistance.
Now the worst case conditions for coil dissipation ( it gets hottest ) are at idle where the the inductive time constant of rate of rise of coil current forms the smallest portion of the coil ON time. The coil dissipation is the product of coil current^2 and the d.c. resistance
Example: with L= 10mH and R = 3.0 ohms the time constant to 63% of peak current is L/R seconds = 3.33mS or 16.5mS to 99% to full current.Which is quite a surprising result as at 6,000 rpm you have 1 spark every 5mS so the peak current gets nowhere near its peak value of 3.93A ( This determines spark energy )
Some more maths;
Assuming idle speed is 600rpm ( = 1 spark every 50mS ) and lets approximate the charge time of the coil to 3.3mS using the dwell time of the Lucas distributor as 51 deg, the ON time of the coil is (51/90 x 50 )mS . Or about 28.5mS less the charge time of 3.3mS. Lets call this 25mS in round numbers which is the time the coil current is at or near maximum or about 50% ON/OFF duty cycle in round numbers.
So if the coil was ON continuously it would dissipate 12.6V/ 3.2 ohms = 3.93A or 49.6W !
So if we put 50% of that power into an ignition coil that will give us an idea of its worst case dissipation.
So from Tamb = 24.4 degC, Rpri = 3.26 ohms, Rsec = 7.36K with 8.97 V across the coil it takes 2.75A = 24.6 W. A good start.
Now due to the enormous temp; co; of resistance of the copper wire its resistance increases and the dissipation decreases at constant voltage. The coil primary and secondary resistances increase.
After 20 mins we have 8.94V, 2.38A and the coil resistance is 3.80 ohms
After 30 mins we have 8.94V 2.25A and the coil resistance is 3.97 ohms
At 40 mins we have 8.92V 2.20A the coil resistance is 4.05 ohms which is where it stabilised. The case temp; was 74 deg C
So yes a good coil at idle will run hot.
Interestingly you can deduce the internal temperature of the coil windings by measuring the secondary resistance at room temp and again at 74 deg case temp.
At Room temp we have 7.36K and 10.12K at the 74 deg case temp, which from the temp;co; of copper wire gives 95.4 deg C + 24.4 dg C =119.8 deg C

Don't forget; today is a hot day at 24 deg C here in the U.K. At +4 Deg C in winter the coil case will only be at 54 deg C to use round numbers.

Looking at the other end of the rev; range at 6,000 rpm you get 1 spark every 5mS , and with a dwell angle of 51 deg the ON time is (51/90 x 5mS ) = 2.8mS but with an L/R time constant of 3.3mS the current will be around 50% of maximum so the dissipation will only be 1/4 of maximum so the temperature the coil gets to will be a lot lower. But the spark energy will also be a lot lower.

One other thing I found is that both the Bosch 12V coil and the Intermotor 6V ballasted coils I have are not wound according the the "classic" coil diagram of the H.T. return being connected the the negative terminal of the coil. Both are wired to the positive connector. There are no connections to the coil metal can.
So what is the return path for coil HT current ?
Through the battery / IGN switch / coil positive ?
Or through the capacitor across the points and the coil primary ?
Anyone still awake after reading this ?
Tony.

My 1850 with Lumenition optronic ignition seemed to have an issue with overheating coils. Having run ballasted and unballasted I found the only way to keep my coil cool was to increase the ballast in the system either by running an unballasted coil with a ballast or by a larger than 1.5 ohm resistor on a ballasted coil.

In the end if concerned about hot coil i see no issue with limiting the ignition 12 volt supply with a ballast resistor, my alternator is feeding 14.5 volts via ignition live to the coil, a 1.5 ohm resistor dropped the feed to 11.5, a bit low for an unballasted coil, so a 1 ohm resistor should have got me a very healthy 12-13 volt feed and a luke warm coil. Dont reconnect the cranking 12 volt feed from the starter though as it wont be needed. As its still essentially an unballasted coil just having the 12 volt supply adjusted by an external ballast.

I may still do this myself in the future.

It may be incorrect info from Lumenition that the coil gets hot normally, as there are many knowledgable posters on here who say the coil shouldnt get hot.

Hi Rob, Sorry I didn't answer your question directly. Yes, when I did that posting I did a quick hand-hot temperature measurement of my 12V unballasted ign coil after a trip back along the main road, also with the Lumenition Optronic ignition system in the middle of Winter when it was quite cold outside . Hence my estimation of "just hand hot coil temperature" .
Tomorrow I'll see if I can do the same measurement on the car after some time at idle at , what 22 deg C its been around here today.
And yes, I do like your idea of adding a small amount of resistance externally to the coil. Anything that helps to keep electronic temperatures down while ensuring adequate spark is good.
Tony.