I've repaired and replaced many bits but I cannot determine what's causing my XJS to stop running.

Right now I have no spark. Inline spark tester not flashing. If I come back tomorrow she will start up and run for 30 seconds to a minute and then the spark tester will stop Flashing and she dies. At that point it will have no spark if I try to restart. If I wait a day I'll repeat this same pattern.

I've tried replacing the main coil, and replacing the amplifier's GM ignition module. The button and rotor and cap are all in good condition.

I've found a shade tree guide to testing the ingnition but it's confused me thoroughly. Can anyone tell me how to determine what is causing me to lose spark?

I have a multimeter and a stiff drink ready.

 

The pickup in the distributor, or its wires or plug connecting it to the ignition module can go bad. Check the plug in the connecting wires. You can check the harness from the ignition module for continuity with your multimeter.
Also, completely remove the large silver condenser in the Lucas box. It can also cause the problem you describe. It's supposedly for noise suppression, but really doesn't do anything but fail, causing the engine to quit.

 

Condenser removed. Confirmed pickup wiring from plug to amp. Don't know how to test pickup itself and / or it's molded pigtail.

I was pretty excited when it started up! Then it died.

 

As matter of course I'd check all of the wiring between the control box and coils. Sounds to me like you may have a connection that is bad, maybe between a wire and a push on connector end someplace. When cool, resistance is low, as current flows the bad connection heats up, resistance goes up.

Could also be a bad pickup in the distributor, or a broken wire in its connecting wiring and connector that you can't really test.

Are you getting power to the coils when the spark stops? Might be an intermittant power problem between ignition switch and coils/Lucas box.

 

You should have constant power on the white leads at the coils and Lucas box whenever the ignition is on. Do you?

 

You said you replaced the "main coil". Do you still have the second coil in front of the radiator? If so, it could be bad. If you have updated to the single, solid coil setup, disregard.

 

To fault find you MUST work methodically. Start at one end of the system and work your way through.

You have replaced the coil and HEI module so for the moment start after the ignition module, do you have spark on the HT lead TO the distributor.

To test the HALL sensor in the distributor put you meter on AC VOLTS and measure the output of the sensor while turning the motor over, (you'll need a buddy for this so make sure you have enough beer). You should read somewhere between 0.5 and 1.0volts AC.

 

Thank you all for the suggestions. I'll try to be thorough and methodical in testing.

As for the secondary coil in my system I assume replacement is the only option as I can only test the primary cool with the HT lead hole filled as it is. Perhaps it's time to go to the single coil upgrade.

 

The second coil has no effect at all on whether the car will start. You can disconnect it's leads from the engine bay coil for the time being. It comes into effect above about 2600 RPM when, without it, the car will misfire. So you can forget about it until the engine is running properly, and then see if she misses at high RPM.


As part of the very wise advice to work methodically, I would certainly change and renew all the push-on spade connectors to the main coil, often the copper wire cracks inside the connector cover, and this causes a problem.
Also, try to find a spare amplifier, and next time the car plays up, swap it over for the spare. I changed the HEI unit in mine, but was still plagued by problems which disappeared when I fitted my spare one. In t.he end I bought a new one, and kept the spare as a spare. No problems since.
Greg

 

Two ideas:


1. Save the stiff drink and/or beers til work is done. All faculties needed. Then, cheers.


2. Jump 12v+ direct from battery to primary coil + post. It might just start and remain running. If so, the electric issue is in connectors, wires or even the ignition switch itself.


Ditto on methodical testing. Not helter skelter...


Carl

 

When you write the methodical list add this somewhere.

Where the 2 wires exit the distributor base is a rubber bung. These 2 wires break INSIDE that bung. I have had 3, one was mine. Took years to find the first opne (mine), and now it is the first item on my list with any V12.

 

NO NO NO. You will fry the coil. A coil takes milliseconds to charge and if left with 12volt on it will get hot and in seconds be a dead coil.

 

Warren:


You probably know far more about these and other critters than I.


But, the coil gets 12 power from the ignition switch. The circuit is open until the distributor completes the ground in one manner or another.


But, I surely did not mean to apply the 12v to the + posat and leave it there. No, crank the engine. If no fire, remove it.


No better, no worse than leaving the key on....


And, yeah, I've been there. Way back in point and condenser days.Never lost a coil. Dead battery, burnt points, oh yeah...


Carl

 

Uh, warjon, the + terminal on the coil has 12v on it constantly anytime the ignition is on. It gets power directly from the ignition switch. The negative terminal is the one that is controlled by the Lucas amplifier box.

 

How would one test these wires since they are molded into the pickup? I could make a probe with a alligator clip and a needle, but I hesitate to do anything that would break through the insulation. These are not high voltage wires, would that be safe?

 

Sorry I thought you were planning on putting the battery across the coil.

 

I'd be looking for each end of the wire and test end to end with an ohm meter. Disconnect both ends first.

 

Testing that small amp loom.

Simple.

Unplug the connector that goes TO the amp, the one near the 3B spark plug. Some do not have that connector, mine did, so if yours goes direct to the amp from the distributor, unplug it at the amp.

Remove the distributor cap, look at the windings of the reluctor coil. You will see a solder "blob" where each of these wires attaches to that coil winding.

Probe the wire, one at a time, plug end to blob, then WIGGLE the loom that goes to the amp, and see if the rreadings zero out, indicating a broken wire.

PLAN B, we gotta have a PLAN B.

Cap off, amp loom unplugged as above.

Now a simple sewing pin from wifeys sewing basket, probe one of the wires INSIDE the distributor, and probe the other end at the plug as normal, and note the readings., then do the fiddle again.

Mine required me to actually get a little rough with those wires, right AT the grommet, and then I had a reading, then I lost it, and obviously BINGO uttered forth, along with much colourful verbalisation.

Addition, I do that.

When the beast dies, TOUCH NOTHING, but take a reading at the +ve terminal of the coil. If you still have battery voltage, move on. Anything less indicates the electrical section of the OLD ignition switch is giving issues. Easily removed, dismantled and cleaned.

 

Grant:


"easily removed, dismantled and cleaned". Oh, boy. With your obvious skills, yup. With mine???


I did have ignition switch issues, amongst others. But, it always ran, or mostly. It was in the crank circuit In the switch. Cranked the engine, even when running!!!


I wrestled the switch out. Fixing in situ, not even close to possible for me. On the bench. Oh h... parts flew around or were never there....


NOS switch via David Boger fixed so many other things as well.


Point:


One way or another check out available volts from the switch to the coil....


Added:


I like to use a test lamp to check continuity whenever feasible. A step up from touching to ground for a spark, but less sophisticated than an ohm meter. From age 16 to 87, so many things....


Carl

 

This past weekend I spent my days working on Jessie. Keeping in mind the consistent mantras shared on this list...

Work slowly, be patient, be methodical. Do it right, do it once.

I started off by reconnecting my exhaust system, having ensured that it was no longer restricted by melted cats and running my 500 cfm electric leaf blower through each assembly from the mating of the downpipe and the second cat.

Fuel pressure read 30psi on the rail. A new fuel filter, pickup filter and the removal of the A bank FPR made no apparent difference to the 1 US gallon/min flow I was getting out of the return line. I don't know a spec for this, but it seams reasonable. On to the ignition.

Battery charged.
Voltage to coil with ignition on :12.8V
I found a bad connection on the amp to (-) coil. Replaced.
Disassemble the amp again. Install original GM part. Use heat sink compound.
Meter the resistors in the amp. In spec.
Capacitor was previously removed from the amp. Left it out.
Meter wiring loom between amp and dizzy. Good.
Meter wiring from pickup to loom. Good.
Test for spark at coil. Good!
Test for spark at dizzy. Good!
(Getting pretty excited by now)
Test for spark at A1. Fail.
Open the dizzy and stare at it.
Order cap and rotor.
Drink. Good.

Later I received a package from Terry's Jaguar.
Installed rotor button.
Fired her up. Idled until warm. Test drive. All good.

So, three weeks without Jessie two weekends in on and under her. Many things fixed that were needed, a few that weren't. And it turns out I have a bad rotor button.

How does a piece of metal mounted in plastic fail in such an interesting way? Looks ok to me...



The only thing I regret is not starting at the end of the system. Sparky end first would have found the fault earlier but now I know the whole system is good. For now. Still going to order a single coil replacement.

Thanks for all the help!