Jaguar Repair/coil


QUESTION: thank u Howard. Again did what u suggested. Got a hold of the Mallory people and  was told how to run the test. The new modular was bad . I get a new one and tested it. It seem to be ok. But the car still does not start.I noticed that my coil was a little warm,maybe more then a little. Is there a way to test the coil, and also would like to know what kind of power, voltage, not sure of the right word to use should be coming out of the coil to the distubutor. Don't mean to be a pain in the A, I am just at a loss. Car as always started,and ever sense I removed the tach I have not been able to get it started. Looking forward to your suggestion.
P.S. May battery is in bad shape, and have been using my charger on the battery,when trying to start the car,could that be a problem. I am going to go tomorrow to get a new one , just trying to tell u everything I can think of.


A coil can be bad or even the wrong coils because many electronic ignitions require a different coil then what the car came with. Again you need to look a the paper work that came with the Mallory ignition system or contact them and ask about the coil. Many electronic ignition systems use a low voltage coil and a ballast resistor. And a resistor to power the ignition unit. This would be in the paper work that came with the new Mallory system.

Look for the ohms resistance on the primary windings (across the two (+) and (-) terminals.) Also, with a known bad battery you have to know that some battery chargers produce pulsating DC current that can upset electronic ignition systems. And to add to your problems some chargers alter their charge rate according to load (bad battery) thus upsetting the electronic ignition system.

In the dealership we would also check the ohms of the coild wire. Then remove the coil wire from the cap and with an insulated pliers hold the end of the cold wire about 1/4 in form the cylinder head and spin the engine. You should see a thick blue spark (not in brightr sun light). If you have that then remove the distributor cap and hold the coil wire about 1/4 in from the center of the rotor and have someone spin the engine again but only briefly. If you can jump a spark to the center of the rotor it is bad. Then check the ohms of the plug wires and look for any carbon tracks insied and outside of the cap.

If all that checks ok either clean or replace the plugs and this time spray starting fluid into the intake when aomeone tries to start it.

Let me know the results,


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

QUESTION: Hello Howard Thank u again for all your help.
 I have had the electronic ignition in my car for quit a long time ,with the same coil.
 What should the ohms resistance be across the + and - terminals?Will that tell me if my coil is bad?
When u say upsetting the electronic ignition could u explain a little more.
 What should the ohms be off the coil wire.
U say hold the coil wire about 1/4 in from thecenter of the rotor and have some one spin the engine again but only briefly. If you get spark to the center of,  the rotor  it is bad,meaning that the rotor is bad?
I have cleaned the spark plugs.

What should the ohms be off the plug wires?
AGAIN Thank u
I hope I have asked the right questions!
Off to get a new battery

The company that made the electronic ignition system will quote the resistance of the primary windings of the coil. Most electronic ignitions that I seen, quoted down around 1.5 ohms and Kettering ignition (points) are usually above 2.5 ohms unless they are using a low volt coil and a ballist resistor in the circuit or a resistor wire. (this has nothing to do with the plug wires)

The primary resistance of the coils it extreamly important to the electronic ignition system and that is why you MUST refer to the manufacture of the ignition system as to what coil MUST be used. The wrong coil can damage the ignition unit. You MUST ask Mallory unless they supplied a Mallory coil with the system. Even if they did supply the coil you should measure the resistance in ohms across the two terminals (wires disconnected)and log the resistance.

The ohms of the secondary (coil tower) is different on each brand of coil so you would need to get that spec from the coil manufacture.

All automotive coils are just transformers. The primary windings builds an electro magnetic field when 12v is applies (or 6v if a ballist resistor is used). This magnetic field does nothing unless the voltage is cut off of the primary winding. (like when points open or like an electronic ignition circuit disconnects the voltage) Even though the current is gone, the magnetic field remains and looks for a place to go. Inside the coil is a second set of windings (secondary circuit) with a different number of windings. The magnetic field collapses and transferes to the secondary windings. But since the secondary has a different number of coils (windings) the power is not changed (WATTs) but the amps and volts are changed. 12 volts becomes 35,000 volts but it has lost a lot of it's amperage. (Amps X volts = WATTs)

Keep in mind that everything on earth conducts electricity even nothing. Electricity can jump across a gap if the voltage is high enough. Even a gap in a vacuum. So the resistor wire in the top of the coils and the carbon brush in the cap touching the rotor and the small gap between the rotor and the peg inside the cap going to each plug wire and the resistance of the plug wire and the sparkplug gap is nothing to stop the 35,000 volts from going home to ground.  

It is impossible to see "carbon tracking" through the center of a rotor so the attempt to jump a spark to it is a necessary test. If a spark will jump to the center of the rotor while spinning the engine, then it proves the rotor is bad and can not be used even though it may look good. "Carbon Tracking" on a distributor cap can be seen and can show up on the inside or outside of the cap.

Most plug wires are about 5 K ohms per foot and when testing plug wires if you have a connection it is ok even if the resistance is different. When the carbon in the string inside the plug wire burns, you will see no connection at all. When the voltage is high enough the insulation of the plug wire then becomes a conductor of electric.

So when you see people putting high energy ignition systems on cars, they are mostly doing more harm then good. IT WILL NEVER MAKE THEIR ENGINE RUN FASTER !!!!!


Jaguar Repair

All Answers

Answers by Expert:

Ask Experts


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.

Associate member SAE EAA member

Import Car magazine

ASE Master Auto with L-1 certification up to 2000

©2017 All rights reserved.