Ignition system

My XJC came with a series 3 ignition system when I bought it and it all worked fine until I had the spark plug leeds maid longer to go through the back of the engine to the plugs because I don’t like the way they go over the cam covers and because of that, No 5 and 6 were barely getting any spark.
I now installed a new ignition system in my XJC and I am having a couple of issues which I can probably live with but I rather not.
I installed an MSD ignition model 6AL Analog and although the engine is running perfect, it won’t stop running when I turn off the ignition key, unless I pulled out the ignition wire. I actually had to fit a globe in line of the ignition wire supply in other words putting some load and that made it turn off with the key.
The other issue is that the tacko is working fine and normal but as soon as the thermo fans come on, the tacko goes crazy.
Any idia’s?

When you connected up the MSD, Frank - did you copy the S3 CE wiring, comparing it to the MSD diagram?

The white to coil ‘+’ is powered from the ign key, and coil ‘+’ is also used to connect power to the ign amp - or MSD in your case. Coil ‘-’ is connected is connected likewise - but in both cases the MSD wire colors is likely different, as their instruction sheet will show…

The first step is of course to measure voltage at coil ‘+’; with ign ‘on’ you should have 12V - with ignition off you should have 0V, of course. If you read 12V with ign ‘off’; either the ignition system gets power from elsewhere - and/or the ignition key is faulty or misconnected. In either case; ignition is not cut - and the engine will keep running. What ‘globe’ did you put inline to increase the load?

The logic behind the rerouting of the plug leads escapes me! To clarify; on the xk engine the #6 is the frontmost cylinder, but the routing of the leads has nothing to do with spark energy. Ignition cannot fail/deteriorate purely because of the routing - and there has been no reports such problems with the original set-up. Did you actually do any other remedial actions to identify other causes?

Depending on the model year of your car; early versions fed the ignition through the tacho. This is ‘inappropriate’ for the CE - and likely also for the MSD. In late versions the ignition and tacho had separate power feeds - and the tacho was controlled by a wire connected to coil ‘-’. Which is a clue to the original set-up…

Obviously the PO did some modifications, including possibly adding electric fans? Which may explain the disturbance of the tacho with the fans running - they are connected to a circuit also used by the tacho…?

But the main reason the engine keeps running with ign ‘off’ is that ign is powered with ign ‘off’…so…?

xj6 85 Sov Europe (UK/NZ)

Hi Frank
Wow that is a lot of ifs and buts.
My son and I went through every wire compèred the s3 wiring and the msd diagram and followed everything to the letter.
There’s only two wires coming out of the dissy and the MSD diagram is simple and although the colours are deferent it tell you what their suppose to do.
As for the Leads the longest Lead is 1.2 m that goes to number 6 and the others are a 100 mm shorter as we go back to number 1. And with the S3 ignition system having 5 and 6 on or off the plug made no change the engine running where with the MSD ignition I can hold it 25mm away from number 6 plug and still sparking.
Well with that being said. I’m still at a loss.
Thanks for your input
Frank Sandham

Does the engine keep running if you switch it off before the fan(s) start? Some fans can act as generators and back-feed the ignition after the power goes off.

I ran the same ‘hidden’ XK120-style plug leads with an S3 XJ ignition on my 69 E-type. No problems.

Hi Peter
Yes the engine keeps running even before the fans come in. The only time the engine stops running by turning the key off is in the first 10 to 15 seconds when the engine is cold.

Again, for list clarity, Frank - #6 is the frontmost cylinder, with the longest lead…

The rerouting of the plug leads is irrelevant for the engine running on. But just to have said that; the original set-up with the CE or whatever is not a cause of any sparking problems. From your description of the original ‘#5 and #6’ misfiring was crossed wires - ie incorrect ignition sequence. Which was rectified when you installed the MSD - as a suggestion. There is nothing inherently wrong with the original lead routing…

The running on, with ign ‘off’, can only be because the ignition system is still powered - particularly since the engine stops if the coil ‘+’ wire is pulled. So measure coil voltage with ign ‘off’ to verify. Or connect a test lamp between coil ‘+’ and ground; lit with ign ‘on’ and dark with ign ‘off’…

Or to put it bluntly (frankly?) you stop the engine by cutting ignition power - and vice versa; if you cut ignition power, the engine stops. The reason of the anomaly is in the wiring - but exactly where it is wrong require some checking.

The white wire to the coil ‘+’ comes from the ign key. The only other wire on the coil ‘+’ is the power lead to the MSD…right? On coil ‘-’ there is either one wire, to the MSD - indicating you have the ‘old’ tacho. Or two wires; one to the MSD and one to the tacho - indicating you have the ‘newer’ tacho…so…?

All this have possible bearing on a wiring anomaly leading to engine run-on. And may be relevant for the tach behavior…

Check and report…?

xj6 85 Sov Europe (UK/NZ)