[xk] Ignition Coil Failure

XK
1

Wise guys, I’m seeking counsel and advice from the
electrical wizards.

A few months ago my 45012 Lucas coil dated 4-51 decided to
misbehave. The car was running fine and suddenly began to
buck and then stalled about a mile from home. It seemed to
be still getting spark, but after an hour of frustration I
called my brother and we diagnosed coil failure. Burned my
arm rather badly on the water elbow getting the old coil
off. BTW let me say that putting the coil under the intake
manifold on early 120s was the stupidest thing Billy Heynes
ever did! My brother brought my box of spares which happened
to have an above-carb mounting bracket, I changed the coil
for a Lucas Sports Coil also dated '51, now mounted above
the carb, and got home.

On Sunday I went to the Chicago British Car Show and stopped
the car at the registration table. Two minutes later the car
won’t start; the Sports Coil above the carb suddenly quit.
Again it seemed to be sometimes getting spark. But now I
carry 2 spares. Changed it and got home ok.

Yesterday I tried all 7 coils in my stash, including 1 dated
1950 from my Mark V, and all 7 started the car and it ran
fine for a minute or two on each. Including the 2 that had
misbehaved. All 7 have a resistance of about 3 to 4 Ohms
across the CB/SW or +/- terminals.

So my questions are:

  1. What is the failure mode inside these things? Is it heat
    related?
  2. Can they be tested on the bench?
  3. Can they be repaired? I would really like to fix the
    period correct coils that misbehaved.
  4. Should the coil be mounted somewhere away from engine
    heat, like the lower right hand wing valence or the scuttle?
  5. Should there be a ballast resistor in the circuit? Or is
    it internal to the coils?

Let the wisdom flow like a stream of milk and honey…–
XK120 FHC, Mark V saloon, XJ12L Series II, S-Type 3.0
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–

2

Rob
Having three or four 140 period coils that behave about as you describe,
I¹m looking forward to the replies.
Best
Klaus
812394
815666BWOn 9/13/16, 8:20 AM, “Rob Reilly” <owner-xk@jag-lovers.org on behalf of xk120us4@sbcglobal.net> wrote:

Wise guys, I’m seeking counsel and advice from the
electrical wizards.

A few months ago my 45012 Lucas coil dated 4-51 decided to
misbehave. The car was running fine and suddenly began to
buck and then stalled about a mile from home. It seemed to
be still getting spark, but after an hour of frustration I
called my brother and we diagnosed coil failure. Burned my
arm rather badly on the water elbow getting the old coil
off. BTW let me say that putting the coil under the intake
manifold on early 120s was the stupidest thing Billy Heynes
ever did! My brother brought my box of spares which happened
to have an above-carb mounting bracket, I changed the coil
for a Lucas Sports Coil also dated '51, now mounted above
the carb, and got home.

On Sunday I went to the Chicago British Car Show and stopped
the car at the registration table. Two minutes later the car
won’t start; the Sports Coil above the carb suddenly quit.
Again it seemed to be sometimes getting spark. But now I
carry 2 spares. Changed it and got home ok.

Yesterday I tried all 7 coils in my stash, including 1 dated
1950 from my Mark V, and all 7 started the car and it ran
fine for a minute or two on each. Including the 2 that had
misbehaved. All 7 have a resistance of about 3 to 4 Ohms
across the CB/SW or +/- terminals.

So my questions are:

  1. What is the failure mode inside these things? Is it heat
    related?
  2. Can they be tested on the bench?
  3. Can they be repaired? I would really like to fix the
    period correct coils that misbehaved.
  4. Should the coil be mounted somewhere away from engine
    heat, like the lower right hand wing valence or the scuttle?
  5. Should there be a ballast resistor in the circuit? Or is
    it internal to the coils?

Let the wisdom flow like a stream of milk and honey…

XK120 FHC, Mark V saloon, XJ12L Series II, S-Type 3.0
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–

3

In reply to a message from Rob Reilly sent Tue 13 Sep 2016:

With all due respect, electrical devices all have a thermal life. I
would avoid fitting even a never used original coil from '51 for
regular use.

My approach would be to fit a suitable modern coil, selected or
modified such that the original wiring connections would not need
modification, keeping the ancient one if I wanted to show the car
at a concours event.

There are a few places that rebuild original ones if you look on
google.

Rob: Did you check or replace the condensor or flex wire to the
points when this was going on?

Andrew–
The original message included these comments:

buck and then stalled about a mile from home. It seemed to
be still getting spark, but after an hour of frustration I
called my brother and we diagnosed coil failure. Burned my

–
1968 3.8S
Zurich, Switzerland
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4

In reply to a message from JagWaugh sent Tue 13 Sep 2016:

Exactly
At a recent talk to the VSCC, Dick from performance
ignition said that of all the old coils they test, none
are 100%

He recommended Bosch GT 40 as a replacement, and I use
them myself. They come in an unfortunate orangey/red
colour which paint can cure.
Modern push on terminals, but I suppose it’s the owners
choice between absolute originality and reliability and
performance
I suspect a road side breakdown could influence that
decision.
On a similar vein he said that modern plug lead materials
are better than the Ye olde ones we sometime see being
cooked by heads and manifolds.–
The original message included these comments:

With all due respect, electrical devices all have a thermal life. I
would avoid fitting even a never used original coil from '51 for
regular use.

–
Ed Nantes SS
Melbourne, Australia
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5

Coils don’t fail then start working again in my experience.
I’ve not heard of them being repaired as they usually are crimped cans which would be destroyed by opening them up.
They didn’t use ballast resisters till the ‘60s. The idea was to to boost the spark by running a 9v coil at 12v momentarily whilst cranking.

I’d look elsewhere for the fault. Did you check that it was getting 12v to the coil? How’s your earth to the engine, test for high resistance? After that it’s points, condenser and the little earth wire to the points.

Hope this helps.

DJOn 13 Sep 2016, at 13:20, Rob Reilly xk120us4@sbcglobal.net wrote:

Wise guys, I’m seeking counsel and advice from the
electrical wizards.

A few months ago my 45012 Lucas coil dated 4-51 decided to
misbehave. The car was running fine and suddenly began to
buck and then stalled about a mile from home. It seemed to
be still getting spark, but after an hour of frustration I
called my brother and we diagnosed coil failure. Burned my
arm rather badly on the water elbow getting the old coil
off. BTW let me say that putting the coil under the intake
manifold on early 120s was the stupidest thing Billy Heynes
ever did! My brother brought my box of spares which happened
to have an above-carb mounting bracket, I changed the coil
for a Lucas Sports Coil also dated '51, now mounted above
the carb, and got home.

On Sunday I went to the Chicago British Car Show and stopped
the car at the registration table. Two minutes later the car
won’t start; the Sports Coil above the carb suddenly quit.
Again it seemed to be sometimes getting spark. But now I
carry 2 spares. Changed it and got home ok.

Yesterday I tried all 7 coils in my stash, including 1 dated
1950 from my Mark V, and all 7 started the car and it ran
fine for a minute or two on each. Including the 2 that had
misbehaved. All 7 have a resistance of about 3 to 4 Ohms
across the CB/SW or +/- terminals.

So my questions are:

  1. What is the failure mode inside these things? Is it heat
    related?
  2. Can they be tested on the bench?
  3. Can they be repaired? I would really like to fix the
    period correct coils that misbehaved.
  4. Should the coil be mounted somewhere away from engine
    heat, like the lower right hand wing valence or the scuttle?
  5. Should there be a ballast resistor in the circuit? Or is
    it internal to the coils?

Let the wisdom flow like a stream of milk and honey…

XK120 FHC, Mark V saloon, XJ12L Series II, S-Type 3.0
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–

6

In reply to a message from Rob Reilly sent Tue 13 Sep 2016:

So that’s why I didn’t see you except to drive by on the
way out.–
Dan Cusick 1994 xjs 4.0L 1951 xk120 ots 2010 XF
Peru, Illinois, United States
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7

Rob, doesn’t it seem a bit odd that you tried 3 coils & the engine ran a bit & stalled?? You mentioned there was some spark, so before I blame the coil, I’d check the fuel flow to the carbs. or anything else related to fuel flow. Remove the fuel line at carbs. & check the flow, if OK check the floats for sticking. By your description it could be that.

Regards, Otto-----Original Message-----
From: Rob Reilly xk120us4@sbcglobal.net
To: xk xk@jag-lovers.org
Sent: Tue, Sep 13, 2016 9:27 am
Subject: [xk] Ignition Coil Failure

Wise guys, I’m seeking counsel and advice from the
electrical wizards.

A few months ago my 45012 Lucas coil dated 4-51 decided to
misbehave. The car was running fine and suddenly began to
buck and then stalled about a mile from home. It seemed to
be still getting spark, but after an hour of frustration I
called my brother and we diagnosed coil failure. Burned my
arm rather badly on the water elbow getting the old coil
off. BTW let me say that putting the coil under the intake
manifold on early 120s was the stupidest thing Billy Heynes
ever did! My brother brought my box of spares which happened
to have an above-carb mounting bracket, I changed the coil
for a Lucas Sports Coil also dated '51, now mounted above
the carb, and got home.

On Sunday I went to the Chicago British Car Show and stopped
the car at the registration table. Two minutes later the car
won’t start; the Sports Coil above the carb suddenly quit.
Again it seemed to be sometimes getting spark. But now I
carry 2 spares. Changed it and got home ok.

Yesterday I tried all 7 coils in my stash, including 1 dated
1950 from my Mark V, and all 7 started the car and it ran
fine for a minute or two on each. Including the 2 that had
misbehaved. All 7 have a resistance of about 3 to 4 Ohms
across the CB/SW or +/- terminals.

So my questions are:

  1. What is the failure mode inside these things? Is it heat
    related?
  2. Can they be tested on the bench?
  3. Can they be repaired? I would really like to fix the
    period correct coils that misbehaved.
  4. Should the coil be mounted somewhere away from engine
    heat, like the lower right hand wing valence or the scuttle?
  5. Should there be a ballast resistor in the circuit? Or is
    it internal to the coils?

Let the wisdom flow like a stream of milk and honey…

XK120 FHC, Mark V saloon, XJ12L Series II, S-Type 3.0
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–

8

In reply to a message from Rob Reilly sent Tue 13 Sep 2016:

I know two people with coil testers. Back in the analog
age, coil testers and condenser testers were an option that
could be purchased for a Sun Engine Analyzer.

http://tinyurl.com/hf4l279

As I understand the coil testing process, there are three
or four separate tests that are done to determine if the
coil is good. None of the tests, if done in isolation, is
conclusive. Ohms across the terminals is one of the
tests. There are also tests for the continuity of the
internal coils, and there’s a test that measures the size
of the spark the coil will generate.

Bill Tracy used to sell a decent-looking replica of the
Lucas 45012 black coil, which was even date stamped to
12/52. Years ago I purchased one of the Bill Tracy coils,
which only worked for a month or so. After it failed, I
cut it apart to see how it was made. It was essentially a
coil within a coil. It was made by wrapping a small
inexpensive solid state coil in an outer Lucas look-a-like
wrapper.

So this might be a way to restore an original Lucas coil,
which has larger dimensions than modern coils. One could
cut the top off the original coil, removed the insides, and
then find a smaller modern coil to fit inside. Then wire
the three terminals of the modern coil to the three
terminals of the the original Lucas coil. Then bond the
original top back on the original body, refinish and paint.

My understanding is that the Lucas coils do not need a
separate ballast resistor. A ballast resistor is built
into the Lucas coil.

Eventually I did find a used original Lucas 45012 coil,
which seems to operate perfectly, although I have never had
it tested on a Sun coil tester. It’s on my to-do list to
get it and a couple of other coils tested.–
The original message included these comments:

Yesterday I tried all 7 coils in my stash, including 1 dated
1950 from my Mark V, and all 7 started the car and it ran
fine for a minute or two on each. Including the 2 that had
misbehaved. All 7 have a resistance of about 3 to 4 Ohms
across the CB/SW or +/- terminals.
So my questions are:

  1. What is the failure mode inside these things? Is it heat
    related?
  2. Can they be tested on the bench?
  3. Can they be repaired? I would really like to fix the
    period correct coils that misbehaved.

–
Mike Balch
Iowa, United States
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9

A good and cheap replacement coil is the Pertronx Flame Thrower coil.
Cost is around $34 at many places. It is a 40,000 volt coil and has screw
connectors for the wires. The coil wire itself however takes a plug in type
with a rubber boot. It comes with a peel off Flame Thrower label. I had to
wrap a piece of rubber around it to fit my original coil bracket as it is
slightly smaller in diameter. However, once in place, no one would notice
the difference.

While you’re at it, enjoy the benefits of electronic ignition like your
everyday car and replace your old points and condenser and ignition wire with
the Pertronix as well.

John Brady
678462
Bedford MA

10

In reply to a message from Mike Balch sent Tue 13 Sep 2016:

The coil I’m now running is a Bosch.
Bosch silicone plug wires have been in about 4 years.
New Autolite XP64 iridium plugs at the same time.
I drained the tank and cleaned the pump and all the fuel
filters and float bowls last month.
New vacuum advance, clean points cap and rotor and I checked
the little internal wires while I had the distributor out
last month.

Sunday when I had the second failure I checked all that
stuff again.
I was getting 12V at the coil. I don’t see anything in the
circuit diagram about stepping down the voltage after
starting. Does that occur within the voltage regulator?
I disconnected the CB wire and connected a test light there
which flickered when cranking so the points were opening and
closing fine.
Took off a dashpot and a float bowl top and saw normal fuel
level, then switched on the fuel pump and it was pumping fine.
Anyway as soon as I put on a new coil it started right up
and ran fine all the way home.

So it all comes back to the coil.
What is the failure mode inside? Why would those two quit
and then start working again? All I can think of is
excessive heat causing a discontinuity.

These are the old Lucas big body type. There is a screw plug
in the top for filling them with dielectric oil. The case is
steel with a separate crimped ring at the top holding it
together.
I can see what looks like a blob of solder on the bottom.

So I think that it would be a matter of cutting off the
crimped ring and desoldering the other end and the guts
would come out. But I don’t really want to cut into a coil
that still works until I find out more. Has anyone else done
this already?–
XK120 FHC, Mark V saloon, XJ12L Series II, S-Type 3.0
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11

In reply to a message from Rob Reilly sent Tue 13 Sep 2016:

your tale of woe rang a bell…so I did some checking…see
the Bobine article on Lucas coils…but what I gleaned
is…you should before other checks, replace the contact
points and condenser with new of know good quality and at
the correct gap. WHY…because he says the points greatly
affect the coils ability to re-cover and re-charge quickly.
The fact that trying several coils did not correct thew
problem, and that it starts up but then dies…makes the
points suspect…maybe a bad set? or ?? anyway easy enough to
try a new set and see.
Yes, coils are negatively affected by heat so a near the
block location probably one reason so many xk drivers
carried a spare coil routinely.
Nick–
Nick53XK120S
Spokane WA, United States
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12

In reply to a message from Nick53XK120S sent Wed 14 Sep 2016:

There may be a misunderstanding. I did not mean that it
starts and then dies. I meant I tried 7 coils and ran it for
a minute or two and it ran fine with each so I shut it off
with each. Including the two that had died on different
other occasions. My points are recently cleaned and filed
and set correctly.

The first time it died was several months ago with the coil
that I have used for many years. It was after a suburban
drive of maybe 5 or 6 miles and it then began bucking and
then died. Swapping to another coil fixed it immediately.

The second time this past Sunday it died after a suburban
run of 45 minutes. Again swapping the coil fixed it immediately.

Both times the removed coil felt warm but not really what I
would call hot. There is no external evidence of overheating.

I wonder if there is any way to duplicate the tests of the
Sun machine with ordinary lab equipment.–
XK120 FHC, Mark V saloon, XJ12L Series II, S-Type 3.0
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13

In reply to a message from Rob Reilly sent Wed 14 Sep 2016:

Reminds me of some problems I’ve had.

My Alfa would die at highway speeds from fuel starvation. I
‘‘fixed’’ it each time by replacing the fuel pump, wondering
why they wouldn’t last. The problem was a clogged fuel
filter. Evidently, the rust in it would resettle during the
time it took me crawl under the car and replace the pump.

My XJ6 (dual tanks) tended to die at low speed, idling at
intersections. I suspected fuel filters (fool me once)
because the car would start again if I just gave it time.
But the problem turned out to be clogged vents on both
tanks–the tanks would become evacuated, and at a certain
vacuum level the SU pumps couldn’t suck fuel from them. I
discovered this after testing a fuel pump for flow, then
opening the tank lid to return the collected petrol–a huge
‘‘whoosh!’’

So maybe it’s the time taken to replace the coil that allows
something to recover, not necessarily the coil itself?–
The original message included these comments:

The first time it died was several months ago with the coil
that I have used for many years. It was after a suburban
drive of maybe 5 or 6 miles and it then began bucking and
then died. Swapping to another coil fixed it immediately.
The second time this past Sunday it died after a suburban
run of 45 minutes. Again swapping the coil fixed it immediately.

–
Bob Wilkinson, 73 XJ6
Saint Louis, MO, United States
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14

In reply to a message from Robert Wilkinson sent Wed 14 Sep 2016:

Hi Rob, here are few more details to consider. Normally a coil
with 3 ohm internal resistor will run warm from the internal
joule heating over time, not all the heat comes from elsewhere
in the engine compartment. 3 ohms at 10 volts would heat at
about 30 watts.

I’ve seen good coils have a temperature-intermittent connection
at the distributor cap. On some caps, the internal center feed
on the cap can have poor electrical contact at the screw going
into the high voltage center conductor which goes intermittent
when warmed up. I’ve seen corrosion on the screw on old caps,
not visible unless the screw is removed. Changing the coil
would jiggle that wire connection while it also is cooling,
giving either the same intermittent heating problem or a good
result. Not quite enough information to rule the coil is the
problem. There are several temperature-dependent failure modes
inside distributors at rotors and in the caps. Getting the same
heating failure mode while parked in the garage versus driving
is not always easy to diagnose.

Perhaps you can build a small sliding block electrical
connection that has two nails, tips near each other and
coaxial, with variable gap between their tips. When a plug wire
is placed to a lead on one nail head and the other nail head
runs to a ground lead, then the strength of the spark voltage
is indicated by the maximum separation between the nail tips
which will initiate a spark. If the plug wire indicates reduced
spark, then try the nail setup on the output of the coil. That
will tell if failure is at coil or wire after the coil. And
there are the various oscilloscope and resistive ladder
measurement techniques to show both high and low sides of coil
in operation, they take longer than the nails to put together.

In answer to your question, I have not seen how a coil fails
personally. Every time I thought I had a coil failure it turned
out to be failure somewhere in the wiring outside the coil.
Please tell more if the coil truly is the culprit.–
Roger McWilliams
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15

In reply to a message from Roger McWilliams sent Wed 14 Sep 2016:

Some of the distributor testers had just such an adjustable spark
gap. On the one we use I made a bracket out of some Al ‘‘L’’ channel
tapped holes for 6 plugs so that I can connect the Bosch Motor
tester to it and simulate normal operation. For an 8 or a 12 cyl I
can only simulate half at a time.

I’m still sketching ideas for an adjustable gap. Having revived the
distributor tester for the cost of a diode and an afternoons work I
have a certain reputation to uphold with the mechanics that use the
thing.

Andrew–
The original message included these comments:

Perhaps you can build a small sliding block electrical
connection that has two nails, tips near each other and
coaxial, with variable gap between their tips. When a plug wire
is placed to a lead on one nail head and the other nail head
runs to a ground lead, then the strength of the spark voltage
is indicated by the maximum separation between the nail tips

–
1968 3.8S
Zurich, Switzerland
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16

Andrew;
The “Adjustable Gap” on my defunct Snap-On Coil and
Condenser (box) tester was nothing more than a fine threaded
brass rod with a black Bakelite Knob that threaded into
a “Peg” about an inch tall… The other side was a fixed
pointed piece of brass rod… The rods being about 1/4" in
diameter. The Bakelite knob had a large diameter “base” that
was totally flat and it’s radius was within a couple millimeters
of the flat “top” of the box (containing the electronics).
Ultimately, with the tester connected an activated, the
threaded rod would display an intense and “solid” blue arc
until the gap exceeded ten millimeters at a minimum… There
were Green, Yellow and Red markings on the tester box that
indicated such…
Connecting this tester was real complicated… One removed
the HT Wire from the coil and plugged-in a red HT lead from
the tester. A black wire, with an “alligator clip” on the end
was connected to (vehicle) battery power, the white wire, also
with an “alligator clip”, attached to the “distributor side” of
the coil… The Points (Contact Set) HAD to be open… We
just used a “Matchbook Cover” to insulate the points.
During the Condenser Test, the matchbook cover had to
be removed and the CB/Points fully closed…
The entire coil test, heat test and condenser test took less
than five minutes… No “Sun Machine” required… We used the
Snap-On “Anal-O-Scope” (five inch oscilloscope) which was a
separate and stand-alone unit.
Sadly, after my Snap-On Coil/Condenser tester fell on the
floor, it quit working. That little 4"x6" box saved me a LOT
of diagnostic grief over the years!!!
Charles #677556.----- Original Message -----
From: “JagWaugh”

I’m still sketching ideas for an adjustable gap. Having revived the
distributor tester for the cost of a diode and an afternoons work I
have a certain reputation to uphold with the mechanics that use the
thing.

Andrew
1968 3.8S
Zurich, Switzerland

17

In reply to a message from cb@risebroadband.net sent Wed 14 Sep 2016:

Charles,

Thanks for that. Now both brain cells are active.

I’m guessing here, but I think the box may have just had an
oscillator, which simulated driving the coil at some near red line
frequency.

Did the condensor test involve flipping a switch on the test box,
and also the spark gap as a measurement for the performance of the
condensor, or were you using the anal-o-scope for the condensor
test?

As a time served young fart, I have to ask, are you sure you’ve got
the matchbook insulation the right way around? At first glance
having the points closed seems to bypass the condensor.

Andrew–
1968 3.8S
Zurich, Switzerland
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18

Andrew;
In the shop where I worked, one could use the “company”
Anal-O-Scope any time needed and it was not being used.
My Snap-On Coil/Condenser tester was, bought and paid-for
mine… I could use it anytime, without the 'scope… it did not
require the 'scope to perform all of it’s functions (coil test,
coil heat test, and condenser test)
Now, to insure my remaining brain cell was, indeed,
firing, I went out to my shop and retrieved the “Snap-On
MT-335 Coil and Condenser Tester 6 & 12 Volt” tester…

In fact, I pretty much remembered everything wrong!!!
… herewith are the printed instructions (sic)…
Coil Test: Disconnect primary wire from distributor stud on
coil and connect lead with White Boot. Connect lead with
Black Boot to ground. Remove high tension lead and replace
with coil tester lead. Adjust gap for maximum steady spark.

So, I got the connection of the Red (HT) lead correct!!

Coil Heat Test: Used for coil suspected of BREAKDOWN
at operating temperature. Connect the same as COIL TEST.
Heat for five minutes. Repeat COIL TEST.
A GOOD COIL should show the same results as before.

So, except for the “time”, I got that right…

Condenser Test: Reattach PRIMARY WIRE. Hold Points
Open & turn switch to CONDENSER TEST. SPARK GAP
should act the same as on COIL TEST. Connecting WHITE
BOOT lead to battery side of coil will burn out fuse. (in the
tester).

So, my good sir, you caught and named that Faux Pax
100% correctly. Of course, I will use the excuse that I
have not used this tester since the early '70’s (when I
dropped it :frowning: )…

That’s the long and the short of it… This little 3.75" by
10.25" box did all the above all by it’s little-bity lonesome!
The Engine Analyzer was not required, In fact, the only
item one could connect to the engine analyzer was the proper
Snap-On Timing Light… If one had a separate timing light,
it could be attached to the engine in the normal manner,
again, not dependant on the analyzer…
Charles #677556.----- Original Message -----
From: “JagWaugh”

Charles,

Thanks for that. Now both brain cells are active.

I’m guessing here, but I think the box may have just had an
oscillator, which simulated driving the coil at some near red line
frequency.

Did the condensor test involve flipping a switch on the test box,
and also the spark gap as a measurement for the performance of the
condensor, or were you using the anal-o-scope for the condensor
test?

As a time served young fart, I have to ask, are you sure you’ve got
the matchbook insulation the right way around? At first glance
having the points closed seems to bypass the condensor.

Andrew

1968 3.8S
Zurich, Switzerland

19

In reply to a message from cb@risebroadband.net sent Wed 14 Sep 2016:

Thanks for all this great discussion, guys.

I should have mentioned that my method of testing for spark
was an inline ignition tester light tool I got from Harbor
Freight for 3 bucks. On Sunday in bright sunlight with the
suspect Lucas coil it sort of flickered dimly, but when I
tested it today with the new Bosch coil it is very bright
orange. So can I suspect that it fooled me and the
brightness of the light may be an indicator of the voltage
being produced?

Nick, I found bobine.nl/jaguar/ and I learned that my Lucas
Sports Coil is the first HS version 45038 with the brass ID
plate. The stamped lettering on my plate is somewhat
obliterated, I can make out HS LO ER and the number 1 which
I thought was a date is in fact 12V so there is no date that
I can find on this one, no model number either.

Bob W, I understand what you mean and I thought of fuel
starvation, so I tested that the fuel pump was pumping and
there was gas in the float bowls. I had drained the tank and
cleaned all the filter screens only a couple of weeks ago
(there are 4) and the vent was clear then. On a 120 the vent
is a tube spout in the fuel tank filler door well, not a
carbon canister emission recovery system like an XJ6.

Roger M, I found on ebay a cheap Chinese tool with
adjustable pointy screw ends that would test the ability for
a coil to generate a spark to jump a gap as you describe.
You unscrew it until the spark won’t jump. As you probably
know but others may be interested, the pointy ends are
because electricity likes to jump across sharp pointy
things, which is why lightning rods on barns are pointy, and
iridium spark plug tips are so small. Shipping is from China
though, so I may see if somebody local carries them. Maybe
not, it looks a bit dangerous to use.

Charles, Andrew, I found a few of those Snap-On coil
condenser testers on ebay, so I understand what you mean.
This one has a gap calibration printed on the case, so if it
jumps a 1/2’’ gap the coil is good and if it won’t jump a
1/8’’ gap it is no good. I imagine the box has some circuitry
that simulates a set of points opening and closing at a rate
of some hundreds of times per second. The heat test
simulates the points being closed for five minutes while the
coil heats up to operating temperature, then you do the
running test again. I can’t make out the little glass thing
under the screw points, is that a fuse or a resistor or a
light bulb or a mercury switch?

So it seems I should investigate grounding the CB side and
allowing these coils to heat up for five minutes, rather
than just running them cold for a minute and being satisfied
with that. This is beginning to make sense; the engine is
hottest when shut off just after a long run and the air flow
stops and more heat would get to the coil, that’s when I’ve
had the two failures.

This is excellent; knowledge is progress.–
XK120 FHC, Mark V saloon, XJ12L Series II, S-Type 3.0
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20

In reply to a message from Rob Reilly sent Wed 14 Sep 2016:

Rob
Dick from Performance Ign said not to file or sandpaper
points,
. The original finish is ground and filing leaves high
points.–
The original message included these comments:

other occasions. My points are recently cleaned and filed
and set correctly.

–
Ed Nantes SS
Melbourne, Australia
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