On my 87 series III I am measuring significantly lower voltage on the switched fuses vs the always-hot circuits. From looking at the wiring chart I am assuming that there must be resistance in the ignition switch (the plastic part, not the lock part). Does anyone have instructions on rebuilding the switch or do I have to replace it? Anyplace else I should be looking? I measured on both sides of the fuse so I know the fuse itself is not the culprit. Thanks for your help.

Jeff

the below pdf Music City Jag Club file shows how to refurbish the ignition switch if you want to tackle it.

It's doable - fixed mine a few years back.

Jeff - I tried to past a link of one of my posts here with no luck. So... search on "ignition switch re-assembly" for some good tips also. There are some really savvy helpful supportive people on this forum!! I am thankful to all of them.

Excellent! I found your post and have pasted the link below. Hopefully I am as good at fixing ignition switches as I am at pasting links. Thanks for the heads-up, I now feel armed with all the information I need to attempt this. A new one is available online for $100 and, of course our good friend David at everydayxj.com has used ones for a reasonable price, but there is something deeply satisfying when you fix something like this. I guess I like restoration over replacement .
www.jaguarforums.com/forum/xj6-xj12-series-i-ii-iii-16/ignition-switch-re-assembly-203206/
Jeff

Looks like the letters pasted but not as a link. Doing this on the tablet, maybe a PC would paste an actual link. Also, to add to what folks said in your post, if there is a loose or dirty connection causing a problem, remember that there are two connections, the hot wire and the ground. So the odds are 50/50 that the problem is simply a corroded or loose ground. Since I bought my first series III 5 years ago, whenever I have a little time I just go around cleaning and tightening electrical connections. That car is a 1979 which is regarded as the most unreliable of the series but I have had very few problems, maybe due to shiny electrical connections. And, cleaning and tightening doesn't cost anything.

Excellent practice. Whatever part of the car you're working on, take an extra 10-15 minutes to clean and connections and grounds in that area. You're preemptively fixing about 80% of the much ballyhooed "Lucas, The Prince Of Darkness" problems.

In defense of old Lucas I'll add that dirty and corroded connections and grounds are hardly exclusively to old English cars....although some people speaks as though that's the case. I work on all sorts of old cars and they're all similarly afflicted, believe me. There are some legitimate criticisms of Lucas electrics....I could name a few myself....but the whole "Lucas" thing is way overblown in my opinion.

Cheers
DD

Fiddly but some can. Not I. David to the rescue. muah better...