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

Hello,

I have a problem with the Lucas CEI ignition system on my 1985 Daimler 4.2 Series III - mechanically, identical to an XJ6 4.2 of the same year.

I have no spark.

I have replaced the coil (Lucas DLB198), the king lead (NGK), the distributor cap (new old stock), the rotor arm (new old stock), and the HT leads (NGK).

I have also replaced the GM module in the Lucas AB14 unit.

I have 12v at the coil positive.
I have 12v at the coil negative, which drops to 9v when cranking. This suggests that at least part of the new GM module is working.

I have no spark at the king lead, as tested against an earth.

From memory when I tested the resistance across the wiring from the distributor to the AB14 I got 3.4k Ohms, which is within tolerance. I will double check this and update if necessary.

My suspicion at this stage is that the signal to break the negative voltage at the coil and force the spark isn’t being sent. This leads me to suspect that the pickup in the distributor isn’t issuing a frequency, or that the wiring between the distributor and the AB14 isn’t transmitting it to the AB14 unit to amplify and feed on.

Is there anything I might have missed that could be causing my issue? And are there any further tests I can carry out to confirm exactly where my problem is?

Thank you.

What do you get between coil negative and ground using a test light while cranking?

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Hello! Coil negative is 12v at IG2, but drops to 9v when you’re cranking. That aspect of the circuit appears to be working - it’s just that it’s a continual and unbroken 9v, so no spark is forced.

I have no test light to try with, so am reliant on the multimeter at present.

You need to see if the circuit is making and breaking. Rigging up a test light is not rocket science and should be in everyone’s toolkit.

What will I see with a test light, that the voltage setting on my multimeter hasn’t already told me?

I know I have 12v before cranking, that drops to a consistent and unbroken 9v when cranking.

Unless I am misunderstanding the purpose of a test light, all it will tell me is whether there is voltage, and whether that voltage drops while cranking. Am I missing another function?

Thank you.

You will see lots if you take my advice. Others will soon come on board and explain. Thanks

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Would it be more responsive to the changes in voltage? A DMV may not react quickly.

At the coil negative post you want to see a test light flicker on-and-off while cranking.

Cheers
DD

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As Doug says, Sam; connect a test lamp between coil neg and ground. With ign ‘on’ the lamp should be fully lit - while cranking; it should dim and flicker…

The sole purpose of the AB14 is to ‘make’ and ‘break’ coil neg connection to ground, like mechanical points do. The ‘make’ allows the current to flow through the coil ‘charging’ it - at ‘break’; the coil fires… spark.

The pick-up in dist triggers the AB14 actions; and the test lamp indicates overall function - first step.

If no flicker i ‘crank’; disconnect lead to pick-up and measure resistance between the two leads; it should be between 2,2 and 4,8. if it is; the pick-up coil is OK, but failure to trigger may have other causes.

If the lamp does not flicker in crank; the fault is either with the pick-up or the AB14 - to be pursued. If the lamp does flicker, but no spark from centre lead; check or replace centre lead…

Frank
xj6 85 Sov Europe UK/NZ)

I know I have 12v before cranking, that drops to a consistent and unbroken 9v when cranking.

Unless I am misunderstanding the purpose of a test light, all it will tell me is whether there is voltage, and whether that voltage drops while cranking. Am I missing another function?

Thank you.
[/quote]

I have procured a test light, I get a flicker but only a slight one.

I have tried three different king leads on the car so far to no avail. However, I shall go and try the original (which was on the car last time it worked) one more time.

“If no flicker i ‘crank’; disconnect lead to pick-up and measure resistance between the two leads; it should be between 2,2 and 4,8. if it is; the pick-up coil is OK, but failure to trigger may have other causes”

As per my original post,I’ve done this and got 3.4.

Right, this post sums up everything so far.

I have no spark.

I have replaced the coil (Lucas DLB198), the king lead (NGK), the distributor cap (new old stock), the rotor arm (new old stock), and the HT leads (NGK).
I have also replaced the GM module in the Lucas AB14 unit.

I have 12v at the coil positive.
I have 12v at the coil negative, which drops to 9v when cranking. This suggests that at least part of the new GM module is working.
I have used a test light and there is a faint flicker when cranking. I’m not sure how to upload the video.
I am using spark testers and none are lighting up at all when cranking.
I have no spark at the king lead, as tested against an earth. I have tested three different king leads on the car.

I have tested with and without the ballast resistor fitted to the coil, which I understand is an unnecessary component only fitted to some examples.

I tested the resistance across the wiring from the distributor to the AB14. I got 3.4k Ohms, which is within tolerance.

Flicker should very pronounced and definite with an old school incandescent test light. Problem more likely to be primary side.

If coil resistances are in order and primary wiring and ballast are good only the ignition module remains.

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That’s my flicker. Battery a shade down from previous attempts but having attached a jump pack afterwards (when my helper had gone inside) I had a fast crank and nothing sparking.

You have done all the right things, Sam - and should have gotten some decisive results…

But…

Replacing the GM module is only ‘valid’ if the unit is verified working - new components does not always work. It can be crudely tested…

The size of the test lamp is not immaterial; if available use an injector test lamp. While cranking; coil neg varies between 0 and 12V - an average of 9V seems too high. But it depends on dwell angle, coil resistance and battery cranking voltage - so is not conclusive…

The resistance in the pick-up only verifies its coil’s continuity. If the pick-up itself is loose or misaligned; the signal may be too weak to properly trigger the ign module. Inspect that the pick-up ‘rail’ is parallel with the pick-up tooth on the reluctor - and clearance is to spec; 0.008 and 0.014".

There is also a possible problem with the Zener diode in the AB14. It is meant to short high ‘ringing’ voltages to ground - it may have failed and allow some current leak through it to ground; which would grossly reduce spark voltages.
Disconnect it, crank and see what happens? Replacing the entire AB14 may also give a solution.

In the meantime we continue to think - the coil centre lead should produce inches of spark…:slight_smile:

Frank
xj6 85 Sov Europe (UK/NZ

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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.
What you are seeing the is coil input voltage drop due to cranking, but what you should be seeing is the test lamp completely go out and back on, then out and back on, as the coil negative terminal is shunted to Gnd On/Off, for each spark.
This indicates the trigger isn’t switching to Gnd, so either the electronic ignition module is duff, the trigger pickup is duff, or there is no Gnd connection.
You need a good Gnd connection for any coil system to function.

I think if the Zener diode had shorted to Gnd, you wouldn’t be seeing any illumination of the test lamp, as the coil negative terminal would already be at Gnd. The coil would become hot, the same as if the Ign was left on and the points closed on a points based system.
The HEI unit simply replaces the points, so think how that functions… :wink:

A quick test of the coil side is to connect a wire to the negative terminal and with Ign on, quickly drag the other end of the wire back and forth across the negative battery terminal, or other Gnd metal. this will create a rapidly switching Gnd shunt so should fire the coil.

I only have limited Jaaaag information, but does the 4.2 HEI work the same as the V12, if so, here is the info.

The hall effect sensor should create it’s own Ac voltage (it’s just a two wire single phase, alternator), the amplifier uses that to create the Gnd shunt.
Test the hall effect sensor with a Ac voltmeter.
You can also perform a resistance test just to see if the pickup coil is within range.

If my information isn’t correct for the 4.2, can someone please post up the correct info and I’ll do my best to assist.

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There has been such great advice and direction from so many experts here, I bet the XJ is already happily purring along.

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 plug fires at the break(!) of ground, and a crucial factor is that the faster the break, field collapse, the higher the voltage. With points ignition there is a spark over the points as they open delaying the break - the spark conduct electricity (and wear/pit the points). With EI, the break is cleaner and faster, and with the same coil gives higher ignition voltages. But any leak ‘bridging’ the break lowers ignition voltages

The test lamp alternates between 0 an 12V three times per revolution - at cranking speed around 5 times a second. The average voltage, and light intensity depends on the time in ‘make’ and in ‘break’, but numbers are not hard and fast; they depend on several factors, including variable dwell on the CE system. The ‘rest’ period between two applications of current flow reduces the heat load on the coil - in both cases…

More things may go wrong with EI than points, complicating fault finding - though the former requires less maintenance, but is usually very reliable.

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’…:slight_smile:

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.

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

  1. 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…
  2. check the gap between the pickup should be .008-.014 in
  3. 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… :smiley:
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… :wink:

I didn’t quite say that, Dieselman - I reacted to ‘the negative side being shunted momentarily to ground’.

Which is precisely what I meant to say - current flow until the pick-up signals the AB14 to ‘break’ - and fire the coil. Like with points there is a delay before the igniter again applies ground - to reduce coil heat load. The critical point is the ‘make’ lasts long enough to charge the coil - which the CE varies to maintain C(onstant ) E(nergy)

Arguably, rapid connecting/disconnecting coil neg with ground ‘manually’ is not quick enough to fire the coil to spark voltages - but would otherwise be an easy coil test…

Generally, while electronic ignition can produce very high voltages - the ‘old’ points system, producing less than 5000V produced perfectly adequate sparks. Certainly, testing an EI system to 25000V, or beyond, is commendable to verify peak performance - but is not mandatory. The voltages make the spark jump the plug gaps - not much is required to bridge 0,25 - 0,35", but it is the current that gives the spark energy (ideally blue) to ignite the mixture…:slight_smile:

Frank
xj6 85 Sov Europe (UK

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