I have an interesting electrical problem on my 2000 XJR. Why do you not look surprised? Every so often, when I turn the ignition to the ON position, the whole system goes black. If I open the hood and jump it on the front left side by the fuse boxes, it will recover and will even start on its own. This seems to cure it for a while (days or weeks), but I never know when it will happen again.
So it did it again this week, this time in the garage, so I just hooked up a 2 amp charger and tried to start it. Well, this time when I turned the key to ON (second position), the dash lights went berzerk instead of recovering like it normally does when I jump it with another car.
I tried this several times with the same result.
I then turned the charger up to 10 amps (or was it 55?) and when I turned the switch to ON, the lights behaved and I was able to then turn it to the START position and start the car. I then disconnected the charger and the car started normally again. So, now it will probably behave for a week or two before it tries to strand me somewhere in need of a quick jump.
My theory is that something is drawing a lot of power when I turn the ignition switch to ON (before I even try to turn it to START) and causing the high-power protection gadget to shut off the power. If I give it enough power on the other side of the power protection gadget, it can get past that hump and proceed normally. I found that I could turn the switch to the first position (where the radio and windows work) with no problem, but when I turned it to the second position, that's when it went wacky. I'm thinking that some motor (fuel pump?) or the engine stops in just the right position so that it draws a lot of power to start rotating again. Or maybe a stuck sensor or some other thing the computer is trying to do. Does anyone know what circuits get energized when the ignition is turned to the second position? What should I check and how? It would be easier if it was something that was fused and just blew the fuse, but it seems to be something that is able to draw a lot of power without blowing a fuse.
Thanks in advance.

 

Mr. Bern:

That's a nice theory BUT, unless someone has added something to your car, the "high-power protection gadget" is a fuse, so, no, it is not resetting. If your battery is any good at all, a high current draw after power on which would instantly drop the battery voltage would burn something or stink!

I bet you just have a loose bulkhead power stud or fuse box stud condition. Have a look, and I'll bet you see one that is loose, looks corroded or burned. Take the battery loose and disaaemble the studs, clean, and retighten.

If that does not do it, then a few minutes with a test lamp (or any test meter) and an assisitant will figure it out. Just start along the power distribution bus in the trunk, and follow the four or five taps along the way to the stud you jump to and you will find where the voltage drops. If you need the power path, just check back and someone will give it to you.


And GOOD LUCK!

 

Hi
Here are the 1999 XJ X308 Main Power Distribution Wiring Diagrams extracted from the JTIS Wiring Guide:

https://www.dropbox.com/s/fq0l1q6kai...stribution.jpg

https://www.dropbox.com/s/v3a2iis04w...%20Details.jpg

Click on the image when dropbox webpage opens to get a better view of image.

Jim Lombardi

 

The High Power Protection Module (thanks Jim) is what I was referring to. I can't figure anything else would be capable of killing all power so effectively. No dome lights, nothing. Total blackout and 0 volts at the front fuse box until I give it a jolt.
Once I restore the power with a little jolt to the front fuse box stud, the front fuse box studs show full voltage without any assistance, so I don't think it's a loose connection. Thing is, once I do that, it doesn't do have a problem anymore for a few days or weeks. If it'd just do it all the time it would be easier to locate. I don't really smell anything when it happens either, so I can't sniff out the problem.
A guy at work suggested that it might be a relay that's misbehaving. Trouble is, there are probably 30 relays on this dang thing all over the place. Not sure how to go about testing them.
The Ignition Positive relays (diagram 2; thanks again Jim) look like promising places to start to look. I'm thinking they are activated when the ignition is switched to ON. I'm just not sure which ones are energized in the ACC position also. Wish they had LEDs. :-)

 

Looks like you might need the entire Electrical guide for your 1999 XJ.

Here is the PDF file:
https://www.dropbox.com/s/glrsrylgyxltbxk/jxj1999en.pdf

There are cross references (bottom of certain pages) for the symbols that give you the Fig numbers - that the referrals lead to input and output from other diagrams.

Jim Lombardi

 

You know, I want to help you and I really don't want to come off as a butthole, BUT-

If you have zero volts to the main power stud on a fuse box, then it cannot be a relay. There just aren't any in the circuit to that stud. period. And let me be clear- The "Main Power Protection Module" is three fuses. Look at the diagram. Yes, really!
So, follow Jim's diagram for the electrical bus with a meter or test light, or keep asking the guy at work what's wrong!

 

Sparkenzap is correct-the power for the fuse box main studs does not come from any relays whatsoever-it comes from the main fuses inside the boot-mounted High Power Protection Module.

There are 2 big fuses inside this module-the circuit diagram shows 3 fuses, but there are 2 in parallel for the starter motor supply. The smaller fuse supplies the power to all the fuse boxes, starting at the rear & then working forwards via the rear seat heelboards to the engine bay fuseboxes.

I would check the cleanliness & tightness of all the fusebox studs for all 5 fuse boxes-I wrote a thread today, with many pictures, specifically on checking the whole vehicle main power connectors & grounds:-

https://www.jaguarforums.com/forum/x...ections-94978/


The circuit is not that complicated, but the supply to all the fuse boxes is a series arrangement starting in the boot & ending up in the engine bay. The "High Power Protection Module" is just a fancy way of describing what amounts to nothing more than a bloody great big fuse of several hundred Amps rating