1987 XJ6...The Spark Is Gone


With help from many of the good folks who share knowledge in this forum, and proper maintenance, my family and I have enjoyed almost 14 years of delightful, worry-free driving in this car. Until a few weeks ago. My son and I decided to take it to run an errand on a Saturday morning, and as always, it started, idled and ran perfectly. One mile later, we were idling at a traffic light. On green, I touched the accelerator, and it stumbled and shut off. No spark. I ran through a diagnostic list I believe I found in the archives. The results pointed to a deceased Amplifier. I replaced the GM HEI module inside the Lucas AB14 box with a 5596 module from MSD. I did not disturb the condenser nor over-voltage diode. Still no spark. I ran through the checklist again. The results were very similar, and condemned the new module. Since a faulty new part is not unheard of, I ordered a replacement-replacement HEI module. I installed it and arrived at the same result, no spark and readings telling me that the amplifier is suspect. This time, though, I noticed that the coil got awfully hot to the touch while I was going through the tests with the ignition key in the on position and/or cranking. Since it was in its teens, and not the most expensive part to replace, I did so with a Lucas DLB 198. Still no spark, and you guessed it, fingers still point at the amplifier. I am embarrassed to report that rather than reach out to people who know a whole, whole lot more than me, I convinced myself that it might be the pick up base wreaking havoc. I ordered a new one, installed it and, yes, we are still without spark and the checklist still points to the amplifier. The only thing I noticed when cranking after installing the new pick up is that the tachometer jumped around a bit. Prior to that, it had not moved off zero. It has since gone back to not moving. The wiring all appears to be in good shape, but I have not checked continuity on anything to verify.

I am discouraged, but not ready to give up! If anybody can shed some light on what I have undoubtedly overlooked, I would be grateful for the assistance! Here are the current results with the second new HEI module, new pick up module and new coil:

The test assumes a fully charged battery 13.05V IGN OFF 12.20V IGN ON

  1. Measure voltage at coil “+” terminal with key “on”. It should be within
    one volt of battery voltage. If not suspect a problem with the wiring to the
    ignition switch, or the switch itself. 11.24V

  2. Measure voltage at the coil “-” terminal. Result should be the same as at
    the “+” terminal. If Ok, go to step 3. If not… Not… It’s 3.84V
    Disconnect the wire from the amplifier from the “-” post of the coil and
    measure voltage again. Less than 2 volts means the coil is faulty. More than
    2 volts means the amplifier is faulty. 12.95V

  3. Disconnect distributor pickup coil from the amplifier (this is the
    harness from the distributor that plugs into the amp). Measure resistance
    across the terminals. It should be 2.2k to 4.8k ohms. If Ok, go to step 4.
    If not, replace the pickup. 2.75K ohms - (The original unit measured 3.47K ohms resistance)

  4. Reconnect the pickup to the amplifier. Measure voltage at coil “-” post
    while cranking engine. The voltage should drop. If OK, go to step 5. If
    not, the amplifier is faulty. Dropped from approximately 10V to 6V

  5. Check distributor cap and wires, distributor rotor arm, spark plugs, coil
    wire Everything appears to be proper. I replaced the cap and rotor after the second HEI module did not bring spark back. By the way, I bench tested the original GM module that was in the AB14 box and it failed. The first replacement bench-tested good, though.

Another observation is that with both of the amplifier leads disconnected, the coil ‘+’ and ‘-’ read the same voltage. Probably useless trivia, but perhaps worth mentioning.

As you’ve figured out by now, I know only enough to have a beautiful, disabled car in the garage with winter coming. Boy, would I like to drive it to storage! Thanks for taking the time to read my essay. I look forward to reading your insights.


With everything hooked up, Brian; connect a test lamp between coil negative and ground. With ign ‘on’ the lamp should be fully lit - in ‘crank’ the lamp should dim and flicker. This, or indeed the drop in voltage you noted, both signifies that the amp is triggering…

Does that mean that with the ign ‘on’ you have ‘12V’ on coil ‘+’ and 3,85V on coil neg?? If so; current is flowing through the coil with ign ‘on’ - which it should not. The ign amp should block coil current until you crank…

Disconnect amp wire from coil ‘-’ and connect a test lamp between wire and coil ‘-’. With ign ‘on’ there should be no light - back up with an amperemeter connected the same way. There should be no current flowing…

If current is flowing the ign amp is not blocking as it should. As a secondary test; disconnect the Zener diode in the amp - if it has failed it may leak to ground, which would impair/disable sparking. With a failed diode; the ignition should then work…

xj6 85 Sov Europe (UK/NZ)


Thank you for your insight. I will certainly follow your suggestions, and report back.

Ugh, another incomplete story !!!

The tach is a great clue. If it bounces on crank, the amp is making and breaking as it should.

Old insulation and frayed wires can raise havoc.