On occasion, for no obvious reason, the electrical system on my 2002 XJ8 is dead as if the battery has been disconnected. It never happens once the car is running. If I disconnect the ground cable to the battery and then reconnect it, everything comes to life as if nothing was ever wrong. I’ve even found that just running a jumper wire (without disconnecting the ground cable) from the negative battery post to the exposed bare mesh on the ground cable restores the electrical function. I’m suspecting that I’ve got a defective ground cable although visually it looks perfect. Any other ideas to investigate?
Check the continuity of the negative cable when disconnected, should be a very low Ohms value. If that is ok then thoroughly clean the clamp, terminal post and ground point, reconnect and try again.
Make sure all connections are tight and very lightly greased with petroleum jelly.
I’ve done the cleaning of the posts and clamps a couple of times and always use dielectric grease on the connections. Good advice on continuity check on the cable. I’ll try that and report back.
I give up. When I checked the resistance of the ground cable, I got 0.1 Ohms, the same reading as I get when I touch the + and - probes together. The post and clamp were corrosion-free and had a light coating of dielectric grease on their surfaces. I reconnected the negative cable and checked the voltage; engine off, 12.63 V across the posts, on the clamp and at various points along the mesh cable to its attachment on the body. With the engine running, 14.03 V. at those same points. No changes found when I wiggled the negative cable. One other note of interest, last evening when the problem occurred, my radio antenna failed to retract and the radio was dead. No change when I drove to and from work today, but the radio and antenna returned to normal function after I disconnected the ground cable and then reattached it. I’m out of ideas; maybe the wiring smoke reservoir is getting low?
Now look at the ground points in the engine bay as there is nothing wrong with your main negative cable.
The voltage readings are also correct for your car.
There is a loose connection somewhere and unfortunately only methodical fault finding will track it down.
This manual will help you in your quest, http://www.jagrepair.com/images/AutoRepairPhotos/jagxj2002.pdf
Excellent! I will add that link to my documents file. I wish that Haynes or Chilton had an XJ8 repair manual. I guess that they figure that anyone that has enough in their bank account to buy one of these cars (when new) wouldn’t do any wrench turning. I had earlier checked the engine bay connections and found that they were all tight but I didn’t do any resistance or voltage testing. I just hope that this doesn’t turn out to be one of those electrical gremlins that only acts up on occasion; that can make for a diagnostic nightmare.
Don’t forget to check the two mega fuse connections in the trunk and also the main positive terminals to the fuse boxes.
and be really sure the battery is good. My X308 behaved just like yours when I first got it. I replaced the battery with a correct spec new one and the problem never recurred.
I had the same problem when I got mine in 2014, Justin. The “dealer” had installed an undersized battery and I had a number of problems that may have been related to the battery. I put in a proper battery a couple of years ago and it is in good shape.
I think you may have nailed it, Jimbov8. I checked the Positive cable in the engine bay and here’s what I found:
Then after a few minutes with steel wool and a mounted wire brush:
I coated everything with dielectric grease, cleaned and recoated the negative post on the battery and its cable clamp and then reassembled. Time will tell if that was the problem, but it does seem quite possible. Thanks for the tip.
Really hope that was the problem Mark. It’s something ive checked on my cars for years.
It’s these forgotten areas that will always cause problems in the end.
Yes that is a common failure point