Coil Test procedure ?

[Tinster]

Is there a test procedure to determine
when an ignition coil is about to fail?

I'm replacing most of the ignition system in
my TR6. The Lucas Sport coil "seems" to be
doing OK and I'd hate to rip it out if it's
still got some life in it.

Presently a little over 1000 miles on it, I think.
I've kinda lost track of the number of coils I've
installed. 5 or 6 or 7 sounds in the neighborhood.
The Lucas has lasted the longest.

Edit: I've also got 1500+ miles on my rebuilt distributor.
Is it time to send it back to Advanced Distributors for an
overhaul?

Thanks,

dale

 

[startech47]

Dale,

The distributor and coil should easily last for a minimum of 50,000 miles without replacement or overhaul. Tune ups for the ignition system should include cap, rotor, point, condenser, and spark plug replacement. Of course you should clean and lubricate the distributor and visually check the internal and external wiring.

When you have multiple problems at once, the fuel pump and the condenser, it is much harder to trouble shoot fix the car. Great job.

Phil

 

[angelfj1]

Dale: Do you have a volt-ohm meter? Here is a simple test you can make. See really bad diagram below. The ignition coil (conventional Lucas type not electronic) is a simple auto-transformer. It has a primary winding of perhaps hundreds of turns and a secondaty with many thousands of turns. First, isolate the coil, that is, disconnect the wires that go to the small terminals on top of the coil(one from the wiring harness , the other to the ditributor). Leave the high voltage lead (the one that comes out of the top of the coil) connected to the coil, but disconnect the other end where it attaches to the dist. Take your ohm-meter and set it at the LOWEST setting (maybe 1 - 10 ohms) Now connect the meter probes to the 2 low-voltage coil terminals. What do you read. You should measure between 3 - 5 ohms. Now set the meter for something like 20,000 ohms. Connect to terminals 2 and 3. You should measure maybe between 5000 and 15000 ohms. I have a Lucas Sports coil on the TR250 and I just got a reading of 3.1 ohms for the primary winding and 8.46 k ohms for the high voltage winding.


o(1)---MM(lv winding)MMM---o(2)---MMMMM(HV winding) MMMMMMMMMMMM--->(3)high voltage to top of dist.

In other words:

For my Lucas Sports Coil

from terminal 1 to 2: - <span style="text-decoration: underline">3.1 ohms</span>
from terminal 2 to 3: - <span style="text-decoration: underline">8460 ohms</span>

These reading are from a known good coil that has never acted up. But, these readings only tell you that the windings are not open or shorted, when the coil is not energized and dead cold.

What can happen is that heat will build up in the coil from the current and also because the coil is mounted on the engine block. The heat can cause the high voltage winding (very small wire, many turns)to expand and contract. Sometimes there will be an intermittant open circuit when it gets real hot. When it cools, the circuit will close and everything looks normal. This can be frustrating. To simulate this problem you would have to take the readings while you applied some heat source, like a heat lamp. Your not going to hurt the coil because it gets pretty hot under the hood of a TR6. if you do this experiment and you see no appreciable change, the coil is probably OK. If the readings get very high or if your meter indicates infinite or open circuit , the coil is probably bad. it is also possible for the coil resistance to decrease with temp, but this means that part of the windings are shorting. This also indicates a bad coil. Your resistance reading will increase a little due to the normal change in conductivity with increase temp, but this will be nominal.

You can also test the coil to see if it is producing High Voltage either hot or cold. If you have a 12 volt power supply or even your car battery (please connect a fuse in series if you use your battery), connect 12 volts across terminals 1 and 2. Now, if you disconnect one of these two wires, the magnetic field in the coil will collapse and a high voltage pulse should be produced at terminal 3. This only lasts for a moment. You can test for high voltage at terminal 3 by taking a screwdriver with a plastic handle and bringing the high voltage wie near terminal 1 or 2. Be careful or you 'll knock yourself on your keester! A nice fat blue spark (maybe .25 in. or more) should jump to the terminal. If the spark is yellow and very faint, you either have a bad coil or low battery. If you have a battery charger, connect it for a while and try this test again. Fat blue spark = good. skinny yellow spark = bad. This may only happen when the coil is hot or maybe no spark at all.

Good luck!

 

[BobbyD]

Back before I converted to TBI, I had something going on with my coil. Basically the hotter the coil got, the worse the car ran. It would backfire, miss, stumble etc and the coil would be so hot that you couldn't touch it. And I mean it was HOT. I had some test procedures but nothing as detailed as what FJA just gave. The only thing I'd add is that an internal versus external ballasted coil have different readings as I recall. I just can't recall what those readings are.

 

[angelfj1]

The only thing I'd add is that an internal versus external ballasted coil have different readings as I recall. I just can't recall what those readings are.

hey Bobby - great advice. I had forgotten about the balast resistance. My 250 and my earlier 4A and the 3A did not have external resistors.

Dale: what kind of coil do you have at this time?

 

[swift6]

TR6's didn't get a ballast resistor until into 1973/74. Dale should have the un-ballasted ignition. Unless his new wiring harness changed that.

 

[TR3driver]

TR6's didn't get a ballast resistor until into 1973/74. Dale should have the un-ballasted ignition. Unless his new wiring harness changed that.

Or he's added an external ballast.

A coil that requires an external ballast should have a primary resistance around 1.5 ohms, instead of around 3 ohms.

 

[Tinster]

Frank, Thanks--

My coil is a 12v "no external resistor required".

I got 3.7 ohms testing as you instructed.

I have only a + and a - terminal
No terminal No3 to test.

Both my meters simply read ohms- I cannot preset
anything.......... that I am aware.
So what do you think? good or bad coil?

I spoke with Dan Masters, while I was at the store
purchasing a new coil, so I am fairly confident the no
external resistor model is correct for the Power Block
wire harness.

Standard coils run about $50 here on-island, I better play
it safe and install a new one.

thanks,

Hoping to soon see maybe another 5 miles or 5 minutes
driven w/out breakdown.


d

 

[swift6]

Dale,

Terminal number three was the lead that goes to the distributor. Which is why he advised care when testing it as its the "business" end of the coil. Your coil is probably fine.

 

[Tinster]

Thanks Shawn-

I got 10.7k from the middle hole of the coil.

Frank- that other testing sounds grand for an expert.
I would probably mess up and end up dead on the floor
from getting myself "sparked".

Thanks !!

It appears I will be "loading up" the trunk again with
spare parts and tools to install same. YeeeHaaaa!!

d

 

[Tinster]

<span style="color: #990000">grumble, grumble, grumble </span>