Greetings,
My first post here in the XJS forum.


My 1986 XJS with Lucas "Constant Energy" ignition system is severely acting up. Sometimes it won't start at all. Other times it will idle fine for a few seconds and then go erratic before stalling.


Suffice to say that I have, on my own, investigated and researched to the point of capitulation. Now I seek the wisdom of experts here.


What seems to be the problem is this:


Yesterday the car ran erratically for a time. But right now, the car won't start. I pulled the center lead (from the main coil) off the distributor. I put a spare spark plug in the end of that wire and grounded the plug body to the chassis using a jumper cable. Cranking the engine I see a somewhat inconsistent yellowish-orange spark. Not a strong blue spark as it supposedly should be.


With the ignition on (run position) I checked the power lead to the (+) coil terminal and it showed 11.83 volts. That seems ok. I tried swapping the main coil with a different one and I still observed a seemingly weak spark (with or without the auxiliary coil connected). I changed the GM HEI module in the black Lucas ignition box and that made no difference. Unplugging the condenser in that same box made no difference either.


I am left with the possibility that the magnetic pickup in the distributor is bad (although I would think that if it was bad there would be no spark at all). Or another possibility is that the zener diode in the black Lucas ignition is bad, but I don't know what it does, how to test it, or what the symptoms of a bad one would be.


Would a bad magnetic pickup in the distributor or a bad zener diode cause these symptoms ?


And now for something really strange:
Earlier today while the car was actually running (roughly), I pulled a wire off the (+) terminal of the main coil. This is the wire that connects to the (+) terminal of the auxiliary coil. When I disconnected that wire (while car idling roughly), I had propped myself up by placing one hand on the black-painted grate in front of the base of the windshield. With my other had I pulled that wire off the main coil. While holding that unconnected wire by the boot (but not touching the metal part of it) I could feel electric shocks on my other hand that was touching the grate. While that was going on, I plugged and unplugged the coil wire to see if that changed the idle quality. It was hard to tell do to the erratic nature of it. But after unplugging and plugging that wire a couple times, suddenly the shocking went away and the car idled much better - but only for a little while and then the idle speed began to drop and then became erratic and then stalled. But no matter what I did to replicate that again, I never could get it to shock my hand again.

 

Unless you have already done this, I would, to start with:
Change the plugs and carefully gap them to 25 thou
Change the HT leads for top quality (eg Magnacores)
Remove the dizzy cap and inspect it carefully, if at all worn looking, or if the carbon brush in the centre of the cap is worn, change the cap
Remove the rotor and fit a new one
Check that the rotor shaft is not stuck, it should turn about 15 degrees on its shaft and snap back.
Check the advance/retard capsule holds vacuum. If not replace it (requires careful removal of bits of the dizzy
The coil must be a low resistance one (not more than 1.2 Ohms) check that it is the correct one. A higher resistance will fry the Lucas box.


Now see what happens. All these things are important if you want the ignition to be correct, so none of it is wasted effort.
Greg

 

All good suggestions, thanks.
However, all have been checked already.
Just changed all the spark plugs to Champion 304, 0.025" gap. Made no difference.
Spark plug wires are relatively new. And my weak spark test involved only the short wire from coil to distributor.
Distributor cap and rotor were changed. No difference.
Distributor centrifugal advance mechanism is not seized.
Vacuum advance pod functions and holds vacuum.
Both the primary and secondary ignition coils are Lucas brand (original equipment). A test using a different coil still yielded a weak spark. Ignition amplifier module was changed - no difference.

 

That magnetic coil thingy rarely goes erratic.

The 2 wires from it that travel thru the rubber bung at the front of the distributor casing are known to break/partially break, INSIDE that rubber bung. This would give a crappy signal to that module, and would give an erratic spark, NOT a weak half hearted spark.

Look down at the LH engine mount. You SHOULD see 2 earth braided straps. ONE goes engine TO cradle, the SECOND goes cradle to chassis, DUMB AS IT GETS.

You mentioned you have a jumper cable. Connect it from the engine to a good known earth point. See what happens.

That 11.83v at the ignition +ve of the coil is NOT going to cut it with that engine. You USA guys get 11.5:1 comp ratio, we got 12.5:1 comp ratio, and a FAT Blue CRACK of a spark is needed. I would start also checking the +ve cables from the battery to the 2 +ve posts in the engine bay, AND the electrical section of the ignition switch, as age is now reeking havoc with the contacts inside that switch.

Unplug that ignition supply wire at the coil, and do a volt test without it connected to anything, and you MUST see battery voltage (or very, very close). If so, then a shorting main, or secondary coil is high on my list.

After all that, I am still thinking EARTHS. Your hand supplied an earth path for some bizzare reason, but stranger things have happened with V12's.

 

I had a very similar issue with my car after I switched to a single coil. The car was running very rich, had a surging idle between about 300-800 rpm, and would not rev over 5000 rpm. After I had gone back over everything on both the ignition side and fuel side, I still wasn't getting the fat, blue spark Grant describes. I finally swapped my single coil for the two old ones and everything was fine. Turns out the single coil had a primary resistance of about 0.7 ohms (including the multimeter resistance). I measured the dual coil setup together in parallel and it was about 0.5 ohms. I thought a single 0.6 ohm coil would replace the dual 1.2 ohm coils just fine. I was wrong, they actually measure 1.1 ohms indivually. Thus my 0.6 ohm coil that actually measures 0.6 ohms wasn't sufficient (just to be clear, the multimeter adds about 0.1 ohms to the reading).

Low voltage at the coil will also result in a rough idle. I don't think the magnetic pickup is at fault in this case since it only sends a signal to the amp. However, you can check the resistance between the two leads of the pickup. There's a range of resistance specified in the ROM which I can't recall at the moment. You can also "bump" the starter to slightly turn the distributor shaft to see if it's wobbling. The resistance should stay within spec all the way around.

As someone who spent many, many days battling a very similar issue with hours and hours reading very thread on this forum related to rough idles, I'd start at the coil. Make sure the primary resistance is correct and it's being fed the proper voltage (which usually boils down to a bad ground).

 

Thanks for the help and suggestions so far.
Here is what I have done since last night:


I figured that the battery was somewhat worn down form repeated cranking and trunk lights and interior lights being on for an extended period. I put a trickle charger on the battery overnight. This morning the voltage at the battery terminals (with battery connected and trunk lights on, but everything else off) was 12.4 volts.


I then switched the ignition to the RUN position and tested the voltage in the power lead to the coil (while that lead was disconnected). This time it was 12.3 volts. It appears the overnight battery charged improved this some.


However, I tested the spark out of the coil and it was still anemic.


Both coils are the original Lucas type. With all wires to the coil disconnected, the main coil primary (+ to - terminals) measures 1.2 ohms. The secondary winding (+ terminal to the high-voltage output) measures 4.92 ohms. The auxiliary coil measures 1.2 ohms (primary winding) and 5.18 ohms (secondary winding).


So as to eliminate wiring and switching issues in the power lead to the main coil, I ran jumper cables directly from the battery (+) terminal in the trunk to the (+) terminal on the main coil. I tested the voltage reaching the coil with this setup and it measured 12.3 volts. So then I tried cranking the engine and still had a weak spark coming out of the coil.


I unplugged the two-prong black connector (with red and blue wires) from the black Lucas AB14 ignition box. Those two wires lead to the magnetic pickup in the distributor. The resistance across those two wires measures 3.2 ohms. I cranked the engine a couple times briefly and the reading remained consistent at 3.2 ohms. That seems like an appropriate reading, from what I remember.


So I am still at a loss for an explanation. The magnetic pickup in the distributor might still be bad, but that is unlikely. Now I'm thinking the problem is somewhere in the black Lucas AB14 ignition box - something other than the actual ignition module (which I have swapped a couple times to no avail) ?


.

 

Busy boy, GOOD.

Very methodical, even better.

The module is really just a switching device that replaces contact points, so if its switching, the coil sparks, (you got that) and thats is all it is required to do.

The quality of the spark is coil related.

Couple of things:

Unplug the secondary coil, it will NOT stop the engine starting, it will ONLY limit the revs to about 3500. Try again.

Take your SPARE spark plug, sit it "upside down" directly into the snout of the coil, try again. My reasons here are that although the HT leads are NEW, they may have too high resistance readings, thus blocking the energy.

I DO NOT run resistor sparkers in any of my V12's, I run NGK BP6EF, and Magnecor HT leads. NO issues after 20 years+++.

BBQ is ready, back later.

 

I replaced both coils with "MSD Blaster 2" units (advertised as having low-impedance windings), and I also "rebuilt" the black Lucas AB14 ignition box, cleaning all the electrical contacts, etc. I'm not sure how strong the spark should be. It doesn't look super strong, but it is now bluish-purple while before it was yellowish-orange. I suppose the appearance of the spark is affected by the gap in the test. I used an old spark plug gapped to about 0.05" for the test (of spark quality coming out of the main coil). So the spark seems to be better now and the car will at least start (but not run well).


The idle is rough and erratic. I'm going to start a new thread since my problems no longer seem to be an ignition issue, which was the subject of this thread.


Thanks for all the replies and info.