Jaguar Repair: Help Needed Diagnosing Ignition Problem -- 73 V12 XKE, voltage drop test, negative probe

Question
Hello,

Interesting site...my first time here.  I hope you can help me solve a problem.

My 73 V12 XKE died on the freeway over the weekend. Lost all power.  Only sign before it happened was the tach needle bouncing from 0 to redline.  Pulled over on the side of the road, with the engine not running, but ignition on, the tach needle continued to sporadically bounce from 0 to redline and back.  After a while, the car started up again.

Only known problem with the car is a bodgy ignition switch, which I have to back off of full start position about 1/8" to get it to turn over the starter.  Been that way about a year.

Things I've done to the car over the past 4 years:

Re-sealed engine
Replaced Opus with Re-Opus
Replaced starter relay
Replaced ballast
Replaced battery
Replaced SU fuel pump
Replaced starter
Replaced plugs and plug wires

So, any ideas how to go about figuring this out?  Nothing worse than a jaguar that breaks down on the side of the road...only confirms the reputation in the eyes of non-believers!

Answer
Hi Bill,

If it is dead and not starting it is easier to fix but the hardest one to fix is one that is running and only dies at times.

The only thing I can tell you from what you said is to do voltage drop tests on the positive cable and the negative cable. And double check for loose connections on grounds. Grounds from the engine to the frame and from the battery to the frame. To do a voltage drop test on the ground circuit connect a volt meter on a low volt scale if it is adjustable. Put the negative probe of the volt meter on the negative post (not the cable end) of the battery and the positive test lead probe on the engine itself (clean metal like a bolt or nut) and have someone start the engine while you watch the meter. You should not see any more then .5v on the meter. If it is a lot more, you have a poor ground either from the engine to the frame or frame to battery. There must be a load on the system for a drop test to work.

Do the positive the same way. It is difficult to get to the battery post on the starter motor so you need to put the test lead on another large power point. One is on the large post of the alternator. In this case you put the positive test lead on the positive post of the battery (not the cable end) and the negative test lead on the large brown wire connection on the alternator and start the engine. Again you should not see much voltage.

If all that is ok and the problem is intermittent, you may need to get several 12v LEDs from a electronics store like Radio Shack and make several long test leads and put the LEDs in a small plate or panel and run the test leads out to the ignition system. Connect one to the positive side of the coil and ground the other LED lead. Turn on the ignition and see that the LED lights up, if not reverse the leads on the LED. Now put a second one on the negative side of the coil and run it to another LED and and this time connect the other lead of the LED to a power supply. Now take the lead off of the negative side of the coil and touch it to a good ground and see if the LED lights up. If not reverse the leads on the LED.

What you now have is two test lights that don't draw hardly any current (about 20 or 30 mil amps# and give you a visual of power to the coil and a grounding signal of the coil when running. Then drive the car and see if you can make it fail. In the time it cuts off and the engine actually stops spinning if a LED goes out you have isolated the fault to either a loss of power to the coil or a failure of the Opus/Lucas system to ground the coil to make it operate.

Intermittent faults are the most difficult to locate because it is almost impossible to find a problem when the engine is running and the problem is not present. This is part of why they have put the code systems in the new cars.

Also test the charge rate as a faulty alternator can drive an ignition system crazy. Check battery voltage #12v +# and then start the engine and set it at a fast idle #1500 RPM# and read battery voltage. It should read #13.8v to 14.5v) but not more.

Let me know,
Howard