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Jaguar Repair/72 E Type V-12, opus failure


QUESTION: The opus unit on my Jag has failed, I have tried to install a pertronix unit and a pertronix resisted coil, the car ran for 30 to 45 minutes.  I turned the car off, and came back to it the next day to set the timing, car would not start, no spark.  Pulled the cap off of the distributor and smelled a burnt, electrical smell( the pickup).
This is not the first time, I returned both the coil and pick up unit for warranty and re installed.  I removed all of the opus unit including the ballast, trying to give a clean install.  Where is the voltage coming from to fry the pick up, is it from the red wire to the coil that is 12 Volts.  I am scratching my head and just want the car to run properly.  Any help would be appreciated.  Randy

ANSWER: Hi Randy,

There are several things that can fry an ignition system and all need to be looked at because there is no test that can be run on the fried unit.

Here are several things I found that can fry an electronic ignition system.

Electric current always takes the easiest rout. So when an alternator is charging and there is some high resistance in it's circuit it will seek an easier rout to the battery or to a ground.
Often an easier rout when it can't send it's current when charging to the battery it will look for an easier path and if that is the ignition system then that is where it goes. So you need to test the out put of the charging system first to see that it is not pushing too high a voltage. (At 1500 RPM with a volt meter across the battery posts it must be from 13.8v to 14.5 volts and not more.

Next you need to run a "voltage drop" test on the ground circuit and the power circuit. The method is to connect you (+) test lead of the volt meter to a bare metal part of the head and the negative test lead to the negative post of the battery (not the battery cable end) The post itself. Put the voltmeter on a low volt scale and start the engine and note the reading when cranking and when running. You want to see less then .5 volt.

If you see more then that you need to check the connections of the engine ground strap and the body to battery ground cable.

Now you need to test the power cables as best you can which is difficult due to the starter solenoid position.

Another major cause of fried ignition systems is high resistance in the coil wire to the distributor or any plug wire. The reason is that current takes the easiest rout and if a secondary current (spark) finds it difficult to get to a spark plug or to the distributor cap or even to jump the gap in the rotor to the cap then that high energy looks for an easier rout to ground and that often is down the coil tower to the one of the primary windings pins on the coil which is directly connected to the new pick-up / igniter which burns it up. This can even happen inside a defective coil.

Another possible is the wrong coil or it can be wired up wrong. Look at the wiring directions that came with the aftermarket system you bought to see that you wired it up correctly and used the correct kind of coil and if it required any resistors or not.

So look closely at.

1. Charge rate.
2. High resistance in the main cables (engine to frame and frame to battery especially)
3. High resistance in any plug wire or coil wire. (usually 5K ohms per foot of wire)
4. Confirm the wiring, resistors and type of coil used on the new system.


---------- FOLLOW-UP ----------

QUESTION: Thanks for the insight, however I have had the pick up fry with out the engine being running, meaning no charging system.  I do have a question in reference to the starting relay, as to its function.  Does it supply momentary power while the switch is in the start position? And it looks, if you look at the wiring diagram that I am suppose to have power coming from the switch.  I have three wires going to the coil, switch, starter relay and tach. I get power full time from the starter relay and no power from the switch.  So I am inclined to replace the relay and of course trace the wire back to the switch.
I still don't understand where I am getting greater than 13 volts to damage the pick up.  Thanks again.  Randy

Randy, it is physically impossible for you to have more then 13v to the ignition system if it never ran. But you originally said it ran for 30 to 45 minutes and if the charge rate was too high or you had a poor ground system it could have fried it then. The only other thing that could have happened is that you left the key on or power was applied to the igniter after the key was turned off and that power was on for a long time. This is usually noted if the coil were hot to the touch.

If however you have poor grounds as I told you, all you need to do is to try to spin the starter motor over with a bad ground and the very high current (amperage) (not voltage) will seek a easier path and that could have been the ignition system (which is a ground system)and that will fry an igniter.

You are wasting your time trying to surmise what is wrong. You need to run the tests I outlined especially the "Voltage Drop" tests on the ground system and the power system.

The only reason you should have a wire from the starter relay to the coil would be if you had a "Bypass" system that used a 6v coil and a resistor. In that case the key in the run position would supply power to a resistor and on to the coil (6v coil) and while in the "Start" position it would supply full 12v to the 6v coil only while the starter was engaged, There would not be a full 12v in this case because a starter draws so much amperage that it drops the voltage down to as low as 9v thus it does not damage the coil nor the igniter. As soon as the engine starts and the key is returned to the "Run" position the switch supplies the power to a resistor then on to the 6v coil, thus protecting the coil and the igniter.

I don't have a wiring diagram of the 72 E-type V-12 but I have been working in Jaguar dealerships from 1960 on to the mid 70's and I never seen any red wires used by Lucas on any British car that had anything to do with the ignition system. So someone has done some wiring on that car. You need to look at the diagram of the new aftermarket system you bought and compare that to the wiring diagram you have of your car to see how to wire it.

But it still does not matter how it is wired as far as getting more then 12v to the ignition system as that is impossible. The ONLY way you can get more voltage to the ignition system is by the alternator while it is running. But amperage can also burn up an igniter when a starter is engaged with a bad ground strap from engine to frame or a bad ground cable connection from the body to the battery.

The ONLY way you are going to know is to run the tests I told you to run.



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Howard M. Fitzcharles III


Jaguar from the XK 120 to XJ-6 ser. 3 1987


Dealership line mechanic on MG, Triumph, Jaguar for 15 years, Instructor in commercial mechanics school 2 yr. Product information manager for piston and valve manufacture, Instructor & hotline answer man for import car parts importer 15 yrs.

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