Hi everyone,

Finally got my Mk2 to run on all cylinders after fixing float valve problem not allowing fuel into front carby. Amazing how these engines will even run on 3 (rear) cylinders!

However, after engine has run for a while, I noticed the ignition coil (Bosch GT40) is very hot (hardly touchable). Is this normal?

After fiddling around with a few spare coils collected over the years and doing resistance checks, I notice some variation in the secondary windings ( dizzy output terminal) to positive (ignition switch) terminal.

So my question is, would a coil with a high resistance (say 13K+ ohms) heat up (significantly) more than one with a lower resistance?

Can't seem to find anything on Youtube that would indicate what is an acceptable ohms reading - maybe it is not critical?

Any ideas?

Thanks.

 

It's getting hot because you have a coil in there that's designed for an external resistor.
I couldn't find any specs on that Bosch coil, but measure the resistance of the primary windings (the negative and positive terminals). I'm betting that it's 1.5 ohms or less.
There is no external resistor on the MK 2 so you need a coil with at least 3 ohms across the "-" and "+" terminals.
And the secondary winding resistance is not that critical.
Crash course on coils...

The current from your battery flows across the negative and positive terminals of your coil through your ignition points.
When the points are closed, electricity flows through the primary windings, building a magnetic field.
When the points open the field collapses on the primary windings inducing electricity on the secondary windings _ and you get spark.

At idle the points stay closed longer and more current flows through the primary windings.
The bad part about that is that there simply is too much current flow at idle and this causes the coil to heat up.
If you continue to use the coil in this fashion it may get so hot that the internal resistance of the primary windings will become so high, you will get a misfire.
As the coil heats up the resistance also increases in the primary windings to the point where the voltage in the battery can no longer over come resistance and you get no spark.

The solution is to add an external resistor (usually around 1.5 ohms) between the battery and the coil OR add more windings to the primary circuit of the coil to get al least a 3 ohm resistance. (Some coils can have 5 ohms resistance).

The resistor not only increases the resistance, it heats up as well, like a heating element on a stove. This increased resistance cuts down on the excessive currant flow at idle. This also has the desired effect of keeping the coil cool at idle.
When you're traveling down the road at speed, the points stay closed "less". With the points staying closed less, there is not as much current flow going to the primary windings.
Because there is a drop in current flow, the external resistor "cools" off and the car gets proper voltage to the coil.

On my 6 cylinder Bentley, it has 2 sets of points and 3 lobes inside the distributor.
Because there are only 3 lobes this causes the dual points to stay closed even longer _ the idea behind this is to get a very hot spark to the secondary windings when the field collapses.

The problem with this set up is that even at speed the points simply stay closed for too long.
What was done was they put a coil in there with a 5 ohm primary winding resistance.
The 5 ohms cut down on the current flow and kept the coil from getting too hot.

A coil with 3 and 5 ohm resistances achieve this by manufacturers simply adding more windings to the primary circuit to add resistance or by using finer wire.
There is no such thing as a coil with an internal resistor.

And one more thing _ to answer your question, even if you have the proper coil in there, they can get pretty hot.
What manufacturers to cut down on cost and weight to build a 3 ohm coil is to use finer windings in the primary circuit.
Although this increases the resistance to the desired 3 ohms it also has the adverse effect of the coil heating up.
The 3 ohms are achieved by forcing the current to flow through finer windings _ the fine wire acts like resistance.

On a better coil it will simply have more windings of thicker wire to achieve it's 3 Ohms.
This creates very large coil and is expensive to make.

 

Thanks Jeff for that comprehensive lesson. Everything I need to know about coils but was too embarrassed to ask!

I was thinking of re-locating the coil to a cooler spot. (My aspirations of originality have long since vanished).

But maybe its situated in the best spot possible- just above the fan. In any case, there is no room on the driver's side of the engine bay (having fitted power steering & pancake air filters) and the exhaust side would be even worse.

Maybe a case of letting sleeping dogs lie...........

 

IMO, the spot where Jaguar put the coil is in the worst possible spot, right at the top of the rad where all the heat is.
As my engine bay comes back together I do plan on moving mine, the better place would be on the fender-well next to the distributor.
There it can get some of the air movement from the fan with out it being cooked from all the heat collecting at the top of the rad and engine bay.
The more I think about it, the spot where it is now, is the worst possible spot _ heat rises and gets trapped right where the coil is _ didn't some engineer think of that.
Or maybe they were thinking that the coil performs better when it gets really hot _ I don't think so...

One other thing. An oil filled coil is meant to be sitting up-right, not on its side. Oil filled coils have a tendency to leak when they're on their side and if the coil is not filled properly then not all the windings will be submersed in the oil.

 

I use a Bosch "blue coil" on my 3.8 Mk2. It is epoxy filled and has an internal ballast (resistance is 3.4 ohms). I use it on other cars as well (Alfa Romeo 1.3 and 1.6l) and I had a couple of spares, which is why I tried it on the Mk2. It always provides a strong spark, even in that awkward position.
JP

 

Great discussion on how coils work- thanks! It is interesting that the (1960) 3.8l has the coil upright in the middle by the top of the radiator as Jeff describes, and that the (1967) 3.4 has the coil relocated over on the fender well near the distributor. I just thought it was the difference between a 3.4 and 3.8, but perhaps they moved it there for cooling reasons too.