Lucas CEI query for 4.2 SIII, no spark at king lead

[Dieselman]

@Frank_Andersen

@Dieselman

An ignition coil works by the negative side being shunted momentarily to Gnd by the points, or ignition module, then returning to open circuit, and repeating.

Not so, Dieselman. The igniter (here AB14) grounds coil negative; allowing current to flow through the coil - setting up a magnetic field around the coil primary. At the weak signal from the pick-up the igniter breaks ground, stopping current flow; the magnetic field collapses and generate a high tension voltage in the coil secondary - which is guided to the appropriate spark plug, brief spark results .Later, the igniter reconnects coil neg ground and the process is repeated…

The xk set-up differs from the V12 in as far as the signal generation and igniters differ. But as you say; there is no principal difference between EI and points ignition - it’s all about ‘make’ and ‘break’…

Frank
xj6 85 Sov Europe (UK/NZ)

Hi Frank,

I’m a bit confused by your post.
You firstly say I’m not correct that the electronic ignition amplifier performs the role of making and breaking the Gnd connection, then say that is how it works.

I am aware a coil creates a magnetic flux, which once the Gnd connection is removed creates an induced back EMF which (If working correctly) only has one exit path…the king lead.
Perhaps you are saying that the amplifier creates the Gnd shunt until the rotor point passes the pickup, at which point the Gnd shunt goes open circuit thus makes the coil fire, which I agree with.

All one needs to know is that there should be a direct Gnd connection made and broken for each ignition firing event. A test lamp connected between coil negative and Gnd will alternately flash on/off as each event happens, or inversely if connected between Batt+ and ignition coil negative.

Rapidly touching a connected wire to a Gnd surface should create a make/break Gnd so should fire the coil. I don’t think we need to worry about dwell time here…just see if the coil can produce a spark.
Remember the Gnd connections are as important as the Batt +ve ones.

I found what may be a useful post on this topic, which gives the various resistance and gap values for the Ignition components.

Poor starting on 4.2 ser III xj6

I’m lazy and didn’t read the entire thread; and I’m quite sure the battery topic was covered. I know Frank is pretty much on top of this one. He’s helped me with my '86 - ignition issues. I just finished a video about a “no start” on my '85 Series III. Put the spark back in your Jaguar to get right to it, jump to 2:28sec. I stopped at the ignition module because was the issue… I will be following up on a tune up with my next video. ANYWAY back to your hard starting. You’re gonna need a multi…

To me, the important parts of the linked post are:

5. Ignition pick up resistance should be no less be between 2.2 and 4.8ohms. Test with the ignition off and the two amplifier leads disconnected from the coil, or disconnect the pick leads from the distributor to the the amplifier. this is your best bet to isolate the magnetic pick up. Testing from the coil is really not conclusive because it could be wire related to the ignition amplifier, the amplifier itself…
6. check the gap between the pickup should be .008-.014 in
7. The angry pixies shooting out the top of the coil should measure at no less than 25k volts and at the plug should be no less than 10k volts.

In the O/p’s situation, either the coil is u/s, but passing voltage, the amplifier is failing to provide a Gnd connection, or the pickup is failing to trigger, either due to the coil being broken/damaged, the air gap being incorrect, or the rotor failing to turn, or a wiring break.
It appears to be a Gnd shunt issue, so look at the amplifier, pickup coil, rotor and wiring.

As a rule of thumb I consider a 25Kv HT spark to be able to jump at least 15mm, and the plug HT voltage needs to be at least 17Kv, or 8mm spark in air.
If one doesn’t have a tester don’t do as I do and just jam a screwdriver shaft up the HT cap and hold it near the engine…
I used to have a really neat caliper style tester, but it went awol. Great for finding ignition system failures before they become apparent.
Remember Kids, cars used to be nowhere near as reliable as they are today. Lack of a decent HT voltage used to be a major reason for failure to proceed.

Hang on, this is an old Jag forum…check your spark voltage…