TR4/4A TR4A Won't Start

[KVH]

I have an ignition related problem, and I'm correcting an earlier post. Here we go:

When I couldn't start, I could tell the ignition had no spark.

And, yes, I'd just left the ignition key on for four straight hours before I noticed it!

So, I opened the distributor and found everything fried. I needed a new condenser, internal wires, and low tension lead.

I thought that I'd just replace everything on the hunch something just "short circuited". That was not so, because everything immediately started to burn up again.

I'm assuming that my three year old Lucas coil is the problem?

I there a way to troubleshoot that? Did I fry it by leaving the key on?

Here's something else that may be relevant--I just replaced my battery with a large Bosch model. Could that burn out my coil?

Thanks for the help

 

[Banjo]

Quite possible. If you replaced the points and condenser, and it did it again, most likley that coil is feeding back into the control side. Yes, it should have lasted much longer, but it's not unheard of.
Also make sure(I'm sure you already know this) not to leave the ignition on with the car not running. If the engine stops where the points are closed and the ignition is on for a while, it can fry the points, condenser, and take out the coil.

 

[TR3driver]

I'm assuming that my three year old Lucas coil is the problem?

Are you sure it's the right coil? Contrary to popular belief, Lucas coils come in two flavors, one of which requires an external ballast resistor to limit current through the coil and points. Later Triumphs have the ballast resistor incorporated in the wiring harness, but a 4A would not.

Also contrary to popular belief, coils that require an external ballast are not always marked "6V".

Lucas DLB101 and DLB105(B) do not require a ballast (hence are suitable for a TR4A).
DLB102 and DLB110 do require a ballast (which can be added if necessary).

 

[KVH]

Thanks for the comments. BUT let me correct my post by saying that when I couldn't start, I could tell the ignition had no spark.

And, yes, I'd just left the ignition key on for four straight hours before I noticed it!

So, I opened the distributor and found everything fried. I needed a new condenser, internal wires, and low tension lead.

I thought that I'd just replace everything on the hunch something just "short circuited". That was not so, because everything immediately started to burn up again.

I'm assuming that my three year old Lucas coil is the problem?

I there a way to troubleshoot that? Did I fry it by leaving the key on?

Here's something else that may be relevant--I just replaced my battery with a large Bosch model. Could that burn out my coil?

Thanks for the help

 

[Banjo]

Quite possibly having left the key on has fried the coil. It overheats, then shorts out. the short then causes too much current to flow through the points, and wiring. That's most likley your problem.

 

[Geo Hahn]

...I'm assuming that my three year old Lucas coil is the problem?...

I would certainly try a known-good coil. I always carry a spare as that is such a common failure (last time I replaced it under a Mesquite at the corner of Pontotac & Sunrise while the rest of the club watched.

If that 3-year old coil is a Lucas Sport Coil from Moss via the fellow whose name begins with 'E' perhaps it was from the same batch as mine (which lasted about a year).

 

[KVH]

I changed the coil and it started right up. The old coil dripped oil everywhere when I pulled it out.

I fried it alright. Thanks for all the help.

My old was from Moss. My new one is the Lucas Sport Coil, also from Moss, via local.

I've gotta say: what great cars.

Two bolts, a coil, a .015 feeler gauge, and back on the road again!

 

[Andrew Mace]

I've gotta say: what great cars.

Two bolts, a coil, a .015 feeler gauge, and back on the road again!

AMEN to that! Especially compared to the (needless?) complication of today's totally electronically controlled cars (yeah, my son and I are still frustrated by a stupid Toyota speedometer, but that's not for this forum)!

 

[Geo Hahn]

...Two bolts, a coil, a .015 feeler gauge, and back on the road again!

And on a rainy afternoon on the shoulder of US180 in New Mexico, the spare coil can be 'installed' with duct tape and the points set with a matchbook cover.

Great cars indeed!

 

[KVH]

I'm buying a spare coil next week, Geo. I thought a coil would be a "20 year" thing, but mine sure fried fast.

By the way, I know you're a scholar in these things: What is all that oil doing in the coil, and what is the function of the coil--with its negative lead going to the distributor?

I've been over-thinking that issue a bit.

 

[TR3driver]

The oil is for cooling, it conducts heat from the windings and core to the outer metal can.

The coil is used to convert the 12 volts or so from the battery/generator to the 10,000-plus volts needed to generate the spark that ignites the fuel/air mixture inside the motor. When the points are closed, current flows between the two smaller terminals on the coil, building up a magnetic field inside the core. When they open, the magnetic field collapses, generating a much higher voltage in both the primary and secondary windings.

 

[KVH]

OK, so the center plug wire on the coil sends high voltage current to the distributor cap, where the rotor sends the current to each plug wire in sequence, causing each spark plug to fire.

And, the (-) negative wire to the distributor means that it's the negative feed that is "cut" each time the points open? The whole bottom apparatus of the distributor is just to open and close that negative circuit from the coil.

So, I guess, that means that the distributor really just operates to cut the negative feed precisely as each plug gets ready to fire.

But how is it that cutting the negative feed (if that's what's happening)at precisely the right time for all four cam lobes on a TR4 (or 6 on a TR6) causes the plugs to fire. That seems backwards somehow. I think I must misundertand something about the negative feed or the coil itself.

I'd like to understand it all better.

thx

PS I guess I could "Wiki" the whole thing.

 

[TR3driver]

Well, running current through a wire generates a magnetic field around the wire. For just a straight wire, the magnetism is too small to notice, but it can be increased by winding the wire into coils so that each turn of the coil reinforces the field from the next turn of the coil. This is how relays and motors work.

Also, moving a wire through a magnetic field induces a current/voltage in the wire. Again a pretty small effect if the wire only goes through the field once, but it can be greatly increased if the wire is coiled so it goes through the field many times. This is how generators, etc. work.

Turns out, it doesn't matter if the wire is moving through the field or the field is moving through the wire, it still induces current (or voltage if no circuit exists) in the wire.

So, the current flowing from the positive (battery) terminal of the coil, through the primary winding inside, and from the negative terminal of the coil through the points to ground builds up a magnetic field inside the coil while the points are closed.
When the points open, the current ceases, and the magnetic field drops to zero very quickly. This produces an apparent motion of the magnetic field through the windings, which induces a voltage in them. The primary winding gets a few hundred volts; the secondary winding (to the center tower) has roughly 100 times as many turns and so it gets 100 times as much voltage, a few tens of thousands of volts. When the voltage rises high enough, it jumps the gap at the rotor, and the gap at the plug.

This whole obscure (but very effective) system was dreamed up mostly by Charles Kettering, one of the founders of Delco.

The Wiki article doesn't have much, but there is more at:
https://www.triumphexperience.com/article/understand-ignition.html