Jaguar Repair: XJS V-12 no-start, electrical drain, light test

Question
Hi Howard,
I was driving my 1987 XJS-C today and noticed the batt. voltage suddenly dropping off, and things getting warm in the passenger compartment. Radio started going intermittently a few moments later. Parked as soon as I could (about 5 min. down the road) and stopped the engine. Voltage was down to the bottom of the gauge. Suspecting an electrical (starter relay?) fault I let things cool off first (there was a little white smoke and burning smell below the end of the fuel rails) and then tried re-starting a while later. No results; just a heavy click and no turnover.
What do you think the cause is? A blown relay is just a guess on my part and I really need an expert opinion before I start playing with pieces.
Many thanks!


Answer
Hi Paul,
I guaranty you will run out of money before fix it by plying with pieces (buying pieces).
First you need to charge the battery. (cable disconnected) Then check for any major electrical drain by putting a test light between the battery post and the battery cable with everything turned off. Then if there was no drain (the test light didn't light) attach the battery cable back up and put a volt meter across the posts of the battery and read battery voltage. (should be 12+ volts) If it is, start the engine and set the RPM at about 1500 RPM and read the volt meter again. (should read from 13.8 to 14.5 volts) If it does not, then you have either a alternator, regulator or wiring problem.

If after you charged the battery and made the test light test between the cable and the battery post the light was bright with everything turned off, then you need a wiring diagram and need to start tracing circuits. At this point the problem can be in a thousand possible places. So a test procedure must be used.

First unplug all of the fuses and unplug all the wires from the back of the alternator. At this point you run the test light test again between the battery cable and the battery post again. If the light does not light this time reconnect ONE wire at a time on the alternator checking the test light between the battery cable and post on each wire installed and continue on to one fuse at a time until you come to a connection that lights the light.

This is the procedure used in a shop to find an electrical short.

There is no magic fix to just go to when you have an electrical problem just testing and hard work.
Howard