We are all familiar with the reputation of earlier Jaguars as having maybe more than the usual electrical problems. This was in large part due to our friend Mr. Lucas. But I would like to address a few non-Lucas problems that I've encountered recently with the hope that it may save one of you some grief.
General connection problems
As many of our Jaguars are approaching 50 or more years old, it should be expected that there will be some loose or corroded connections showing up. Many of the early cars used bare twisted copper wire attached to switches or other devices by means of a screw clamping the wire (this is 30's vintage technology). Many times these will work loose due to vibration or handling over the years. In later cars, slip-on connections are used. These generally hold up well unless they have been removed several times again causing intermittent operation or allowing corrosion to develop. It's a lot of work to track down these types of problems so when they are fixed you want to make sure they never happen again. I have found a good product to help ease these problems. It is sold by Eastwood and called "Kopr-shield"(Part #25002). This is basically a copper loaded grease which when applied to an electrical connection improves the conductivity and also seals the joint from the environment eliminating corrosion. It is easy to use and a tiny little bottle lasts years. If you're doing a restoration every connection should get this treatment.
Series 3 XJ6 Intermittent Headlights
Ever have one or more of the four main headlamp bulbs fail to come on? Then, as you are traveling, it pops back on! I've found the cause of this is most likely the fuse box mounted on the left hand fender just under the bonnet. Usually wiggling these fuses will make the problem show up. It can be corrected by removing the guilty fuse and squeezing the contacts together. While you're in there clean the contact and apply some of the Kopr-shield. Then do all of them before another starts acting up.
Hard starting on and E-type or Mk2 (or others with unibody)
Recently while troubleshooting some starting difficulties in Mike Tate's beautiful E-type OTS, we found an interesting problem that may be overlooked by many. The starter would turn but only slowly and not enough to fire the engine. Since the E starter is such a bear to replace, every other possible cause had to be explored. A new battery was installed but that didn't help. Maybe a solenoid but that still meant removing the starter. A check of the wiring was in order because the E-type has a very long run from the battery around the engine to the starter. A good check of the wiring condition is the temperature of the various joints. The E-type, Mk2 and other unit body cars connect the "Earth" terminal to the chassis via a large bolt on the firewall (directly behind the battery on the E and nearly invisible). When touching this bolt on Mike's E after some cranking, it was red hot! That means resistance and voltage drop where there should be none. Ah-ha! Maybe this is the problem. Removing the bolt found the threads totally rusted. We wire brushed the bolt, threads and eyelet connections until all were shiny. Then applied a coating of Kopr-shield and re-assembled everything. Turned the key and - little change! But the connection on the firewall was cool as a cucumber. It turns out we really did need a new starter. But the new one really spins well now. And that connection should never cause another problem.
P.S. Another point to check for starting problems is the grounds straps. All cars had short ground straps connecting either the frame or body to the engine block in the area of the motor mounts. Many times these are left off during a repair(like a new clutch). If they are missing the starting current has to flow via other means including the carburetor linkage or other small wires. This can be dangerous as these items could get very hot. Check to see that the ground straps are installed properly.