[xk] Ignition Coil Failure

XK
21

In reply to a message from Rob Reilly sent Thu 15 Sep 2016:

Rob,

Yes, I bumped into a couple of pics last night.

The tube is a neon tube, presumably to indicate if the coil is
putting out anything at all, but not enough to jump the gap (Same
thing as the old fashioned screwdrivers with a built in voltage
indicator). The neon may also be there as part of the electrical
circuit, rather than just a light bulb indicating ‘‘Igor, don’t
touch!’’.

I would say that the tester has to run at something near red line
frequencies. When a coil starts arcing internally the performance
initially degrades as revs (i.e. duty cycle) go up, until enough
carbon has been deposited, or the area is hot enough that it just
keeps sparking there, then the degradation also happens at idle.

The closed points heat test is also worth doing, although this
won’t test the condensor heat response, and ignores the ambient
heat change as a result of running an engine.

Earlier I was more inclined to just replace coil and condensor.

We have a known good set of points and condensor in the garage. It
only takes a few minutes to temporarily connect these two, if the
car runs better then we usually just replace both. Having a motor
tester to actually look at what is going on is a bit more
informative, but the symptoms and troubleshooting ususally give
ample clarity for a diagnosis. We use the same test coil/condensor
for everything from a 4 cyl to a 12 cyl.

Still haven’t decided if I want to make a test unit. I hear a lot
of ‘‘Lucas is crap, Bosch is better.’’ I suspect the truth is more
like ‘‘Old is Old, New is New.’’ but it would be nice to show the
difference visually.

Andrew–
The original message included these comments:

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

–
1968 3.8S
Zurich, Switzerland
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22

In reply to a message from Ed Nantes sent Thu 15 Sep 2016:

Ed,

Sort of. If you use a points file, and do it properly by finishing
with polishing with 800 grit paper, and the contact surface
material is thicker than a gnats spinal cord, then it isn’t
necessarily bad, especially if the points have already developed
their own ‘‘geography’’.

If possible, new points are to be preferred - The last 4-5 years or
so I hear many stories about new points that don’t last very long,
last summer I had a new set (Alfa) which didn’t last long enough
for me to set the dwell on the distributor tester, so don’t throw
your old points away, and get a file and some 800 paper. (you do
have to get the filed/polished material out of the dizzy with
proper cleaning).

Andrew–
The original message included these comments:

Dick from Performance Ign said not to file or sandpaper
points,
. The original finish is ground and filing leaves high
points.

–
1968 3.8S
Zurich, Switzerland
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23

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

Rob no big point but I doubt Heynes made that decision. Mounting
on the block was practice for many makes in that period and I
doubt the coil gets much hotter there as it does above the
radiator but I have never measured so no real idea. It is a pain
to get too which is perhaps what you were getting to but as far as
function the only concern I have had was a spark wire with an
issue coupled with a weepy carb. that said the XK engine sure
looks slick with the wires entering from the rear and not breaking
up the lines of the cam covers.–
The original message included these comments:

manifold on early 120s was the stupidest thing Billy Heynes
ever did! My brother brought my box of spares which happened

–
George Camp
Columbia SC, United States
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1 Like
24

Rob R;
That “little glass thing under the screw points” is (was in
my case) actually a “Neon Light” of some sort… When the
coil it producing it’s spark (while being tested), that little
tube lit-up a pinkish-red… I recall it flickering, at a high
rate, in concert with the blue arc buzzing… My “neon light”
broke, but it never affected the tester’s ability to work… only
dropping it on the floor killed it!
If those Snap-On Coil/Condenser testers are still around,
AND they work, I’ll be looking to buy another one!!
Inside the box are, indeed, some form of “electric”
componnents, akin to a old style radio… but rather spartan…
there is “something” that inside a dense foam-rubber sleeve…
it appears to be made of two zinc coated steel plates
(1/2" x2") next to three copper plates, a black “insulator”
plate and two more copper plates. There is a 400V.D.C.
“Type 3224” .25Mfd capasiter, a rotary switch and a six
amp fuse… That is the sum total of parts inside this
coil/condenser tester…
Charles #677556.----- Original Message -----
From: “Rob Reilly”

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?

XK120 FHC, Mark V saloon, XJ12L Series II, S-Type 3.0

25

In reply to a message from JagWaugh sent Thu 15 Sep 2016:

Jaguar warned of a ‘‘gap’’ possible at the center coil wire
(in that case due to a suppressor being placed there) that
would quickly cause the coil to heat up and be damaged…so
be sure there is no ‘‘gap’’ when the coil wire is attached.

filing points…been ‘‘dressing’’ points for years…Dad showed
me how…he had for years too…as said…very fine file, then
fine emory…to take any cupping or slanted shape out. New is
better…but flat smooth re-dress will be better than burned
slanted. Pipecleaner works to clean up… Rotor can be re-
dressed only a little…fine emory, maybe one time. Distrib
cap tho is a ‘‘toss it’’ IMHO…tho it can also be done maybe
once…if it is bad and you need to get home.
Well I have the cam covers off the 120…first time ever in
my 34 yr ownership…and breather cover too. Looking and
measuring, assessing…and checking TDC and flywheel marks,
…considering guide hold downs…??
Nick–
Nick53XK120S
Spokane WA, United States
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26

In reply to a message from Nick53XK120S sent Thu 15 Sep 2016:

All my HT wires are well attached.
I used a very fine points file and 800 and 1500 grit
sandpaper. The way I do them they are better than new, more
rounded which is what you want to avoid spark jump.

George, I was burned with Billy mostly because I was burned,
and it hurt. As Chief Engineer Heynes would have had final
approval of the coil location. Plus there was some clown
nearby saying ‘‘Wrenches in his hands, must be a Jaguar,
Lucas prince of darkness strikes again.’’
The Mark V (concurrent model) had the coil on the scuttle
which can be seen in its Service Manual if you are
interested. It also used the side entry dist cap with the
plug wires going around the back.

Charles, the Snap-On testers on ebay run $80 to $200 but
none are guaranteed. From fixing my Fender guitar amp I
learned that old wax filled capacitors can leak and change
value or die, but new caps can be found at Frye’s and online
from antique amp/radio specialists.

And here’s a curiosity…
Bernard Viart says on page 191 ‘‘On some Coupes, the coil was
fitted to the right inner wing.’’
Where did he get that information? Is it reliable?
My engine number is W3962-8 less than 100 before the coil
move and my coupe was made just about the time the coil was
moving. And I have two holes in my RH lower wing valence at
the right spacing for a coil.
So does anybody else have two holes there, or know anything
more?–
XK120 FHC, Mark V saloon, XJ12L Series II, S-Type 3.0
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27

In reply to a message from Rob Reilly sent Thu 15 Sep 2016:

Funny thing about those guys who know so much about Lucas, they
never think to lend a hand, or hold a flashlight, but when their
superior electrical system has a fault, even if you lend them a
Voltmeter they can’t do much with it.

Andrew–
The original message included these comments:

approval of the coil location. Plus there was some clown
nearby saying ‘‘Wrenches in his hands, must be a Jaguar,
Lucas prince of darkness strikes again.’’

–
1968 3.8S
Zurich, Switzerland
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28

In reply to a message from JagWaugh sent Thu 15 Sep 2016:

At the talk at the VSCC Dick commented about Luca
distributors,
When they rebuild the old ones now, they replace the
shaft and cam with Bosch. But he mentioned that Lucas and
Bosch cams have a different profile.

Lucas, being in the first instance intended for UK where
really cold starting was considered important have a cam
profile more suited to that.

Here, at least, where that isn’t an issue, the Bosch
profile gives better all round performance.–
Ed Nantes SS
Melbourne, Australia
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29

In reply to a message from Rob Reilly sent Thu 15 Sep 2016:

Rob,
My coupe 679080, Sept. 3, 1951 build date, has two holes
in the RH wing valence as you describe, although the coil
was relocated above the carbs at some point by a PO, using
an obviously home made mounting bracket. Interestingly
though, the car still had the side entry distributor cap
when I acquired it.

Check out the engine photos of 675083 on XKdata (an OTS
with a Feb. 25, 1954 build date) for an, admittedly poorly
depicted example, of a wing mounted coil.

Rob–
The original message included these comments:

And here’s a curiosity…
Bernard Viart says on page 191 ‘‘On some Coupes, the coil was
fitted to the right inner wing.’’
Where did he get that information? Is it reliable?
My engine number is W3962-8 less than 100 before the coil
move and my coupe was made just about the time the coil was
moving. And I have two holes in my RH lower wing valence at
the right spacing for a coil.

–
Rob Carne 679080
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30

In reply to a message from Rob Carne sent Thu 15 Sep 2016:

Thanks, Rob.
So on 675083 the coil is up high on the wing by the top
radiator hose.
My 679187 from Nov '51 has these two holes to the rear of
the two bends in the lower valence at the level of the starter.
Where are your two holes?–
The original message included these comments:

My coupe 679080, Sept. 3, 1951 build date, has two holes
in the RH wing valence as you describe, although the coil

–
XK120 FHC, Mark V saloon, XJ12L Series II, S-Type 3.0
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31

In reply to a message from Rob Reilly sent Thu 15 Sep 2016:

I sort of thought that was the real issue. It is amazing how hot
some items are when you are in a hurry! Will see if I have any
info on the coil move. Have a great weekend!–
The original message included these comments:

George, I was burned with Billy mostly because I was burned,
and it hurt. As Chief Engineer Heynes would have had final
approval of the coil location. Plus there was some clown
nearby saying ‘‘Wrenches in his hands, must be a Jaguar,
Lucas prince of darkness strikes again.’’

–
George Camp
Columbia SC, United States
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32

In reply to a message from Rob Carne sent Thu 15 Sep 2016:

The side entry dist cap may have been a replacement from a
MKIV or MKV which had side entry as standard.–
The original message included these comments:

though, the car still had the side entry distributor cap

–
Ed Nantes SS
Melbourne, Australia
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33

In reply to a message from Ed Nantes sent Sat 17 Sep 2016:

The side entry distributor cap (aka cover 415708) was used
on XK120 up to engine W6696, then the vertical outlet cap
(aka cover 407043) was used. I never heard any reason given
for the change.

According to the Mark VII parts book, either that model
always used the vertical outlet cap or they didn’t bother to
mention the horizontal outlet cap. Maybe they were all sold
out and figured nobody would want to special order one.

But it reminded me of something about the contact breaker
cam. From Dec ‘52 the XK120 received a new distributor
40199E with high lift cam, and the points gap was
accordingly changed from .010/.012’’ to .014/.016’’ in Service
Bulletin 116. The Mark VII got the same high lift cam in
March '53 ref SB 121.

I measured a few cams across the peaks and flats and got the
following:
40149E .746’’ and .703’’
40199B .748’’ and .701’’
40199D .746’’ and .690’’
40276A .746’’ and .701’’
40576A .808’’ and .741’’

Incidentally on that 40276A the cam lobes are unevenly worn,
something to check for when you have your distributor out.–
XK120 FHC, Mark V saloon, XJ12L Series II, S-Type 3.0
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34

In reply to a message from Rob Reilly sent Sat 17 Sep 2016:

great info…thank you,I was about to look that SB up…and
measuring the diff is super…thanks…Jaguar constantly fiddled
with the distributors, changing the mech advance, the vacuum
cans…and as noted here the distrib cam and pts gap, as well
as the static timing. A tiny bit of di-electric grease on the
cam lobes is ok…if thrown off a bit no harm done. Some on
rotor top too. (I think that is a 40199E…) The distributor
body DVXH6A indicated the early horizontal cap, tho the
vertical could be fitted. Later the distrib body had DVX6A.
Most any mech advance plate innards, and vac cans can be
fitted to any ‘‘body’’, but as you note the dizzy cams vary.
Nick–
The original message included these comments:

cam. From Dec ‘52 the XK120 received a new distributor
40199E with high lift cam, and the points gap was
accordingly changed from .010/.012’’ to .014/.016’’ in Service
Bulletin 116. The Mark VII got the same high lift cam in
March ‘53 ref SB 121.
I measured a few cams across the peaks and flats and got the
following:
40149E .746’’ and .703’’
40199B .748’’ and .701’’
40199D .746’’ and .690’’
40276A .746’’ and .701’’

–
Nick53XK120S
Spokane WA, United States
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35

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

Rob,

If you google ‘‘Ignition coil driver’’ you will find a great many
schematics for driving an ignition coil, many of which are quite
simple and use a 555 timer and other inexpensive components.

These appear to be quite popular in the Tesla community. I would
choose one with variable frequency and an output semiconductor that
can handle at least 8A at 12VDC. Some of the schematics are for 9V
coils, I would select one for 12V and use a ballast resistor if I
was testing a 9V coil.

Still on my first coffee, but I think that:
RPM = (Hz X 60)/(2 X Cyl. Count)

and for a reasonable test you would want to be able to sweep
between 10 and 110% of the redline value for the engine you are
simulating.

It would take an additional switch to heat the coil up (just apply
12v until ‘‘hot’’ coolant temperature is reached).

Not too sure about testing the condensor. My instinct says that
using a known good condensor in the box and the ability to switch
between the test and the car condensor while comparing the spark
would be a start, but I am at a loss as to how to artificially heat
the car condensor.

Probably an afternoon, a bit of fiddling, about twice the price of
a new plain Jane coil, and it wouldn’t necessarily allow you to
check if a coil was just barely marginal, but probably worth it as
it would allow you to confirm or eliminate a coil which is poor
enough to actually affect driveability, faster and less burn risk
than swapping to a plane Jane coil which may or may not be suitable
for the car you happen to be diagnosing.

Andrew–
The original message included these comments:

I wonder if there is any way to duplicate the tests of the
Sun machine with ordinary lab equipment.

–
1968 3.8S
Zurich, Switzerland
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36

You get capacitor testers which is all a condenser is. Apply heat with a hairdryer, it won’t take much.

DJOn 18 Sep 2016, at 08:12, JagWaugh andrew.waugh@bluewin.ch wrote:

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

Rob,

If you google ‘‘Ignition coil driver’’ you will find a great many
schematics for driving an ignition coil, many of which are quite
simple and use a 555 timer and other inexpensive components.

These appear to be quite popular in the Tesla community. I would
choose one with variable frequency and an output semiconductor that
can handle at least 8A at 12VDC. Some of the schematics are for 9V
coils, I would select one for 12V and use a ballast resistor if I
was testing a 9V coil.

Still on my first coffee, but I think that:
RPM = (Hz X 60)/(2 X Cyl. Count)

and for a reasonable test you would want to be able to sweep
between 10 and 110% of the redline value for the engine you are
simulating.

It would take an additional switch to heat the coil up (just apply
12v until ‘‘hot’’ coolant temperature is reached).

Not too sure about testing the condensor. My instinct says that
using a known good condensor in the box and the ability to switch
between the test and the car condensor while comparing the spark
would be a start, but I am at a loss as to how to artificially heat
the car condensor.

Probably an afternoon, a bit of fiddling, about twice the price of
a new plain Jane coil, and it wouldn’t necessarily allow you to
check if a coil was just barely marginal, but probably worth it as
it would allow you to confirm or eliminate a coil which is poor
enough to actually affect driveability, faster and less burn risk
than swapping to a plane Jane coil which may or may not be suitable
for the car you happen to be diagnosing.

Andrew

The original message included these comments:

I wonder if there is any way to duplicate the tests of the
Sun machine with ordinary lab equipment.

–
1968 3.8S
Zurich, Switzerland
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37

In reply to a message from Rob Reilly sent Thu 15 Sep 2016:

Rob,

My early 120FHC body 679080, built Sept. 3, 1951) is off the
chassis and stripped down, but I can offer some insight into
where the coil might have been mounted on the right wing
valence on early models.

I do not have the two holes you describe down near the
starter, but have a cluster of apparently factory drilled
holes midway along the upper valence.

http://www.jag-lovers.org/v.htm?1474306540

Photo 1 shows those holes and Photo 2 is a direct tracing
annotated with hole diameters and a letter designation for
reference.

Viart (page 363) labels my holes B and C as the location of
the coil bracket. I mounted my coil in that orientation and
Photo 3 shows that it will interfere with the bonnet
closing. Furthermore, Viart (page 366) also inconsistently
shows the coil mounted on holes C and E, which are too far
apart for my encircling bracket. Photo 4 shows the coil
mounted on holes A and B - a much more likely location.
Both holes A and B are uniquely 13/64’’ in diameter as well,
although my coil encircling bracket is sized for 1/4’’ bolts,
as Viart (page 191) shows on the later H6 coil mount fitted
to the front carb. Moreover, the Spare Parts Catalogue
lists the coil as being secured with 1/4’’ set screws (P.N.
NS.125/3D) for early engines W.1001 to W.4059, with the
later carb located mounting bracket used with 1/4’’ studs for
succeeding engines. 1/4’’ set screws won’t fit into the
13/16’’ A and B holes drilled in my valence (for early engine
W.3629) and there is no evidence of captive nuts of any size
have once been welded to the valence either - so I don’t
know where we go from there.

Viart (page 365, shows holes C and F (17/64’’ dia.) as used
for the water temperature capillary line clips and I have no
problem with that as there are an additional two similar
holes at the same ‘‘elevation’’ rearward along the upper
valence. Viart (page 366) labels holes G and H as mounting
holes for the air intake branch assembly. My air intake
assembly is long gone and I can’t verify the fit, but it
makes sense given their location and the fact that he does
not show them at all in the sketch (page 365) for the same
area on the OTS, which was issued with individual pancake
air filters for each carb. I have no idea what holes D and
E are used for and Viart offers no information in that
regard either.

I hope someone is keeping track of the glitches in Viart’s
Jaguar XK 120 Explored for a subsequent edition as I have
found it to be an invaluable source of info. I am winning
the battle slowly, piecing my incomplete car bought
completely disassembled with no notes or labels on the
buckets of nuts and bolts and misc. metal bits and the
Jaguar issued references are not as helpful.

Rob–
The original message included these comments:

So on 675083 the coil is up high on the wing by the top
radiator hose.
My 679187 from Nov '51 has these two holes to the rear of
the two bends in the lower valence at the level of the starter.
Where are your two holes?

–
Rob Carne 679080
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38

In reply to a message from Rob Carne sent Mon 19 Sep 2016:

Thanks, Rob.
One difference I see immediately is that I do not have the
hole you have labeled E.

Also I agree that the holes labeled A and B are too small
for a 1/4’’ screw and therefore not practical for mounting a
coil.

Plus the coil would really be in the way here for carb
adjustment work. I would not like it there.

The two 1/4’’ screws for early engines listed in the parts
catalogue are for mounting the coil on the block, see Viart
page 191. I burned my arm trying to get to those two.

The air duct plenum is attached by the four 5/16’’ holes, two
in front of the front big hole and two to the rear of the
rear big hole, see page 244.

I have omega clips for wiring, thermo capillary and bonnet
release cable in most of the other holes.

Another aspect of the hole identities is the fact that both
side valences have mostly identical holes, the reason being
that the punch press die set that punched all those holes
was set up to produce the pieces for both LH and RH sides.
Thus on mine I have holes for the brake fluid reservoir
bracket, horn relay and grease gun on both sides. But the 6
holes for the air cleaner plenum and intake hoses are only
on the right, so I conclude those 6 holes were added by a
different punch die set, or they could have added those 6
extra punch tools to the die set only when making RH side
pieces for FHC and DHC.

I read through the Service Bulletins and did not find
anything at all about changing the location of the coil. The
C-Type had two coils, one above each carb, but whether or
not that fact relates to the change in location on XK120 I
could not say.

Where do we go from here? Until new evidence surfaces, I
tentatively conclude that Bernard is wrong about that coil
location. He may have seen one car with it there and drawn a
hasty conclusion.

I am running with a new coil above the front carb and have
installed a spare coil on my lower valence below the intake
hose on my rear carb with wires ready to connect if my main
coil goes out. Not going to get burned again.–
XK120 FHC, Mark V saloon, XJ12L Series II, S-Type 3.0
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39

I recently spent a ridiculous amount of money on a 1950 Lucas HS12 Sports Coil (gold colour with small screwed-on alloy ID plate). It looks the business sitting on top of the XK engine! After reading all the above I’m a bit nervous of fitting it, as many people seem to have had issues with coil failure. I have also fitted an electronic trigger replacing the original points. I wonder if this arrangement might be easier on coils?
My 120 is an early one (Dec 1950) and would have had the side entry distributor cap with the plug wires routed round the back of the head, but I decided for ease of access to fit the the later top-entry distributor cap and also place the coil in the later position on a bracket above the front carb.
I tried several coils, thinking I might have a coil-related problem, as the car has always been a rather reluctant starter, especially when cold. The problem turned out to be very low compression (plus a blown head gasket) causing a very weak signal at the carbs. As I write this the engine is in the middle of a full rebuild - rebored to +.040" and light crank grind.

40

Relative to your original question, a couple of practical suggestions. I’ve had at least three failures identical to yours that I initially thought were due to the coil that were not.

First, try a coil from a known good car, or put one of your failing coils in something else and run it (must be compatible coil resistance). That will prove to you whether your unit is good. On my 120, I recently developed a stumbling and failure of spark problem after warmup that occurred with several coils, including one from a car with no problems. The problem was related to points with no visible flaw, resolved by converting the distributor to Pertronix. As you know, the 120 distributor is hell to work on without pulling it out, so converting to electronic keeps you out of there, and the setup pretty much eliminates those wiring related distributor problems related to arcing from broken insulation. You must use compatible components including coils and plug wires.

A previous e-type gave me fits over a high tension lead from coil to distributor that would fail after 20 minutes of driving, return to normal after cool-down. Failed with multiple coils! Resolved by replacing the lead.

The last one was a broken carbon center contact in the distributor cap. I agree with others that if you are having problems with multiple coils, you should consider that it may be something else. In my experience, the coil gets blamed too often for other maladies.