Invest a few bucks. Far less than the 700!!!
Three meters, two patch cords, a cigar lighter adapter, a power test probe. and an inductance meter. Oh, a carbon pile load tester.
Should you choose this option, the next episode will describe them and their uses
As to the isolater. I do not like them. Just another thing to go wrong and mislead us.
Clue: I just used my digital meter to check out a few household batteries , . Most OK, a couple dead or below the 1.5 V I expect of them!!
Carl
Richard,
it sounds like if you do not even own/know how to operate a voltmeter/ampmeter, then you will not be able to give the forum any useful information in helping you diagnose your problem. Our advice is a huge guessing game, and it sounds like some of the advice you cannot do by yourself.
Your mechanic seems to be your best source. But this car will bleed you dry, it is a high maintenance car, especially at 30+ years old. But as long as you’re ok shelling out the $$$, that’s fine.
What is this directed at me and Greg?
Thanks for your advise.
Everyone has an opinion, and you are correct, I know little about mechanics Greg.
I am 72 years of age. My wife and I retired at 55 to rescue homeless and abused animals. We take the worst of the lot and bring them back to life, and spend a lot of time and money doing just that. We do not rehome, what comes into our home stays. For example, I am including a picture of a cat we heard about which was located 95 miles away from our home. He had a tumour which we had removed through Cornell University School of Veterinary Medicine…not cheap. There is a picture of before…1 year ago, and today. Animals have no voice and someone needs to speak for them, which gave us the choice between rescuing and the multi-meter. I chose what I thought best and don’t regret it.
Broken cats - I guess that’s why we’re all here.
I’ve just managed to dry out the engine in the XJR-S so that I can get it back in the garage after a session with the pressure washer. I didn’t want to post “hey, I washed my, massively expensive to repair, V12 engine, and now it doesn’t work” - I knew I could fix it. Probably would have got a few “comments”. Meh, you ignore that after a while.
So, one cat fixed. But I salute your charitable work. Right now I have two other cats (the furry ones), both rescues from Battersea. In the past I did my own rescue of a stray that had a problem with one eye. Expensive to fix, but the result was so rewarding. Sadly, being a feral cat, he had FIV, and we only had two more years with him.
It seems Greg has opinion.
Thank you for understanding.
No, to Greg after telling me I had no idea of what I am doing, and he is correct. No and it seems he took out the part where he said my car will probably catch fire within two years and I won’t need to worry about it any longer. I thought we communicate on this forum not judge.
I think Greg was just stating some facts, in plain sight, which is often what is required in a technical discussion. And straight talking words seem like the most reasonable form of communication in such a forum, but there are nuances in the English language that can generate friction, however well intentioned the original sentiment. If we were all together in the pub, supping some nice, warm, English ale, any antagonism would have passed in a few seconds, once the context was clear.
Between us all, we’ll do what we can to get you to a place where your car is fixed.
Hi Richard-
A few new thoughts and summarizing some of what has been said above:
All of my statements below assume that you are SURE your battery is good. It’s been load tested (with a load tester like at autozone, not a volt meter) and has a full charge.
From the symptoms you are describing, it sounds like you have a parasitic draw. Simply put, when the key and everything else is “off,” something is still drawing current and eventually draining the battery. It could be a short (it seemed like that might be it when you said the problem was the starter, it is directly connected to the battery) or a faulty electronic component (e.g. trip computer, relay, etc.) The draw is “concealed” when the car is running because the alternator overcomes it by generating more power, just like it does when you turn on the headlights or AC.
Attempting to troubleshoot a parasitic draw on an XJS based on what happened on Other People’s cars is likely an exercise in frustration. There have been many different causes recorded over the years, from trip computers (me) to relays to the trunk light switch. Thus, a methodical approach is needed. It’s not difficult, just tedious. By using a methodical approach, your mechanic should be able to locate the draw with a very high degree of certainty. This will cost you money for time, but he should not be swapping out parts until he is sure he has it located. That’s just trial and error and expensive. Mark outlines the proper methodical approach on this other thread about as well as I’ve ever seen it done: Need input about battery voltage?
It seems like your car has a significant draw. Current is measured in amps, and most of us consider anything over 50mA (50/1000ths of an amp) to be too much draw. Even that will flatten the battery over the course of a few weeks.
Your measurement of the battery at 4.85V after a couple of days indicates a DEAD battery. Like really dead. Which means that IF the battery is GOOD, your draw is significant. Well over the 50mA threshold. Which means you’d get a pretty good spark when you hook up the last cable (positive or negative) to the battery.
Your mechanic will likely test for the draw using a multitester and the procedure Mark described. If he is patient and methodical, he’ll nail it down. Sometimes mechanics who are not patient or don’t understand electrics will just swap out the “likely suspect” parts, which is hard on your wallet and confidence.
Not sure that there is much new in the above, but maybe it helps you think through this as you work with your mechanic. I tend to question the experts, even when they are my friends. 
Good luck!
Bob
Being judgmental and assuming I know nothing about my vehicle is not acceptable. Even if I don’t, I am will to learn and not be chastised.
Bob,
Thank you for writing in a professional manner…it is appreciated. I am not a mechanic but that does not mean, like others allude to, that I should not own this vehicle.
All your points are valid. From what I understand, All items, the battery, starter, and alternator have been tested by my mechanic and pronounced healthy. Tomorrow I will measure the battery voyage in the morning and in early afternoon review the results and if down remove each fuse to find the general area from where the voyage is being robbed. I would think, regarding the battery, and since it has been fully charged on a professional machine, my mechanic with certainty could tell me if that was the culprit. He claims the tests prove all cells are fine and I need to take him at his word for now.
I really don’t want to break the relationship you have with your mechanic, but unless you know as much as they do, and are watching them all the way, I’m sorry to say but bad things do happen. There are only two guys I would trust to work on my cars, they work in a small local garage that I’ve used for over 20 years. They are not Jaguar experts (burned a couple of times by them critters) by any means, but they crucially have a lift and can get at the jobs I can’t. If I can’t do it myself, I tell them what I want done, give them printouts of the manual pages where necessary, even help out myself. I’ve seen everything in their shop from a 3 year old Peugeot SUV getting its first MOT, to an immaculate Aston Martin DB6. The cars and owners all get the same treatment.
I’m not saying you have a bad mechanic. It just seems like they are running the problem solving through in a strange order - going through the motions, unfortunately most expensive first. If you can do some basic tests yourself and confirm/reject the charging circuit, battery, or whatever you find, you will be in better position to ask them “why does this…”
One thing to ask is, can I have the old parts back please. This will get you either a bunch of crap that you have to dispose of (I put it on a shelf for later, whatever later may be) or an embarrassed look/twitch “oh we’ve already disposed of it”. Ask before they do the work, so they know you are serious.
Hi Richard-
I personally would not spend any time measuring battery voltage, it’s not a great indicator of battery health unless it is measured under load. Sure, if the voltage is about 12 or less the battery probably needs a charge, and if its 13 or more it’s probably charged, but even that’s not definitive.
If you follow Mark’s instructions (link above - note particularly step 3,) you can know instantly without waiting if you have a parasitic drain. You want to measure current (amperes) not voltage (volts.)
If you are pulling fuses expecting to see a change in voltage you won’t get anywhere.
Best Regards
Bob
Coming late to the party I can offer
.Richard, I hope your issue gets resolved. I am confident that between you, your mechanic, and ideas from this forum there will be success.
Best regards,
Jim
Thank you Bob! Nicely put. I’d like to consider ourselves Jag owners sharing our trials and tribulations with other Jag owners.
Thank you!
Mark
First off If you don’t have a proper set of manuals, then to go this link Jaguar XJ-S Manual get a proper set of manuals. If you prefer paper and don’t want to print out the online version, then Amazon has the XJ-S abridged manual available too. Personally I think it’s useless for anyone new to Jaguars.
The XJ-S factory manual is a 6 volume set - covering all years - which the OTPubs online / CD version is an electronic copy of. This manual is worth it’s weight in Gold. Word of caution about OTPubs the, install the reader software on a computer that you’ll always want to use for reading the manual; because OTPubs are real dicks about relocating the software. I got all excited over the manuals, and installed the software on my work computer, thinking I’d just uninstall it and install it on my garage laptop. NOPE! Tech support refused to inactive the license on my work computer and activate it on a different computer. Now they won’t even return my calls or email. 
If you need a mult-meter, let me know (personal message me) I’ve got a stack of them. Their like tape measures to me, I can’t ever find the damn thing when I need it. If you’re in the US, I’ll send you one of mine. The YouTube video is a real good hands on look on how to track down a parasitic draw. I forgot all about the post I did.
On a personal note
I joined this forum right after I purchased my second Jaguar, an '85 XJ6 for $500.00, The car had 45,000 original miles, no rust, a set of Dayton wire wheels, new tires, The downside was it didn’t run, the windows wouldn’t go up or down, the sun room didn’t work, the leather under the “seat covers” was torn, and the suspension and brakes were completely shot. The car was one tire away from being crushed, after my horrible experience with the XJ-S, summarized under the background heading, I decided to do the work myself.
I’m no stranger to wrenching. In one of my many career paths I was an ASC and GM Certified Service Technician. Worked at the local Chevy dealer for a bit and decided this wasn’t a career path as much as a dead end job for me.
So I’m thinking, I’ve got skills… It’s a car… How hard can it be? I went out to Amazon and purchased an XJ Series III (abridged) manual; and was stuck right out of the gate. I won’t bore you with the details…
It was with the help of @Frank_Andersen and the others alike, I got my XJ6 running. Total cost in parts? $50.00 - a coil. Time to figure it out? well it was more than the “manual says it should’ve taken” by a factor of weeks; but the car runs!! and is now my daily driver .
This experience gave me the confidence that I CAN repair my Jaguar; and If I ever get stuck, I’ll have a wonderful group of blokes to fall back on, who are more than willing to share their knowledge and experiences to help us of less (Jaguar) experience and knowledge on how to keep our Jaguars on the road.
My point is this. We are here to help.
Background
My first Jaguar is an '88 XJ-S I purchased 20 years ago. the brakes went out, and I was intimidated by the whole inboard brakes thing. At the time I didn’t have the money to spend on the car to get it fixed. The car sat - for years. years of scraping up what cash I could I had enough to send it off to get repaired. I finally did. The car was towed off to the not so local “respected” Jaguar centric repair facilities to have the car gone through so it could be driven again. Ten grand later, I still can’t drive the car.
After I got over the initial sticker shock, and kept telling myself I get to drive my Jag again!!! I’m heading home, tickled pink I’m actually driving my XJ-S! When I noticed the engine temp climbing. It’s in the dead of winter barely above freezing mind you. I pull over, and upon inspection, The radiator was pissing out coolant. The mechanic who’d been assigned to my car had punctured the radiator core while replacing one of the hoses coolant hoses.
I called the repair facility; they refused to accept liability and said to bring the car back and they’d gladly replace the radiator for an additional $2,500. I declined their offer, and decided if I’m going to own a Jaguar I’m gonna have to learn how to repair one. I’m so disgusted by that whole ordeal, I can hardly look at the car, let alone think about working on it… So there it sits up on the lift.
Back to the point.
Tracking down an electrical problem in a Jag is less than straight forward, it’s a time consuming process, and if you’re paying someone to do the work, it can be hugely expensive.
Jaguar’s done some pretty unusual things, like
Retired? then sound like you may have a bit of time to get it sorted.
We’re here to help.
Sincerely,
Mark
As I advised in a previous thread, perform a Parasitic draw test, and a Voltage Drop Test
These are the standard procedures for this problem
As you have made it clear you lack expertise to do a Voltage Drop Test, presumably you will need to speak to your mechanic.
Unfortunately, the correct person to do this is an Auto Electrician.
While most mechanics have good knowledge of this procedure, some would not know how to do a Voltage Drop Test
It will not absolutely find the issue, but it absolutely has to be done.
If it was not done, your mechanic let you down
I recently had to change a Starter motors several times, as I fought with a failed solenoid, then an improperly rebuilt Starter…they tested ok out of the car, but once installed and under load, would not turn the engine. Luckily in this vehicle, changing the starter was an easy job
Good morning.
The only ill I find is the battery. I ran the vehicle for 2 hours yesterday afternoon and last night measure the voltage at 12.5 before going to bed. This morning it is 11.9. This is a repeat performance of what transpired last week. The starter was installed on Thursday, the vehicle started on Friday, the weather here in Syracuse, New York was cold, snowy, and windy on Saturday thus I left it tarped, and on Sunday morning the battery was dead.
Rick