Any help is appreciated
2 posts from another forum:
Have 99 XJ8 . Every now and then the car goes completely dead. No power to anything . The battery was checked and its good. The way I get it started again is to disconected both battery terminals. Then it starts. Also when this happens , I checked the large 12 volt power wire under hood on the right side by hood latch. It shows no power. But when I reset car by un hooking both battery leads and then check it, there is 12 volts of power and the car starts. So is there a relay somewhere on car that is making this happen or ECM problem or ???? Any help greatly appreciated
I also have a 1999 XJ8 with the same problem.
It always starts again after I move all of the battery cables in the trunk until power is restored. The cable connections in the trunk at the battery, junction box and fuse box are good. The cable connection under the hood on the right side is good. I replaced the starter and the battery. I suspect it is a loose connection under the center console but I haven't figured out how to remove that yet.
When I figure out why it's intermittently losing power I will post it. Good Luck.

 

Remember there has to be a ground strap of the thick starter cable size between engine (therefore starter motor) and car body, check here for corrosion/looseness. It forms the power return for starter motor current via the body back to the battery -ve. The only other way is to have TWO thick cables from battery, this is expensive, and most manuf. use the car's steel body as -ve. or ground return. Of course if your body is carbon fibre or glass fibre and there is no metal chassis, two leads are obligatory.
Leedsman.
p.s. Sometimes there is, or has been fitted, a battery disabling switch as a security measure. If you have a radio with no security code system (old-fashioned) check this disabling switch too. I assume you've cleaned any sulphation off the battery terminals with boiling water, and you've checked right at both battery terminals where the actual wire goes into the terminals there's no loosness there. Remember the SLIGHTEST looseness, corrosion, dirt etc. will stop this 400 to 800amp current for starting.

 

ckirby, the next time this happens, I want you to check something for me. Do not disconnect the battery, but when you loose all power, follow the wire coming off of the positive battery post to the next component up. This is known as the high power protection module. When you look at the top of this, you will see three posts. One post will have the battery lead attached, a smaller post will have a single wire coming off of it, the third post will have 2 wires coming off of it. Using a multimeter, check to make sure you have 12 VDC (measured between these posts and the metal chassis of the car) on all of these terminals. If you only have 12 VDC on the battery post, then you have an issue with the car thinking it is experiencing a high current fault, and is therefore "protecting" the car. If you are getting anything under 12.4 VDC on all the posts (assuming the battery is fully charged), then you have a high resistance connection on this block or the battery wire is starting to fail on you and you are simply helping to remake the broken wire inside the car.

If you find that you have 12 VDC at all the posts, then let me know and we will take it from there. Just for reference, the smaller post with the single wire feeds power to the starter and is the return line for the power coming out of the alternator. The post with the 2 wires on it, one of the wires feeds the fuse box in the trunk, the other feeds the fuse boxes down in the heelboards of your car and then finally on up to the fuse boxes under the hood.

 

Thanks for the replies. I have a voltmeter in the car and the next time it loses power I will check the connections in the trunk.
I will post what I find.
Thanks again, CWK

 

Thermo,

I checked those connections and the voltages and all are normal in the trunk.
With my son in the driver seat, I carefully moved one wire at a time until power was restored. The wire causing the loss of power goes forward to the heelboard fuse panel under the rear seat.
I assume the rear seat has to be removed to get to that connection. Is that difficult to remove?
Thanks, Bill K.

 

You need to check all the connections and all the leads for a good connection. We recently had a few cars that had a bad connection and a bad high power protection fuses and checking it with a volt meter is not always the answer. Remove the battery lead and clean all the connections, clean them and use di-electric on the connections. Check for loose connections and the not so obvious problems then reconnect the leads to the battery. Good Luck!

 

Kirby, removing the rear seat isn't that bad. Probably take you about 20 minutes to get it out (if not less). To remove the rear seat bottom, perform the following:
-disconnect the battery
-remove the heelboards at the base of the rear seats using the 1/4 turn fasteners, set the heelboards off to the side
-remove the screws holding the seat to the body of the car
-lift up on the rear of the cushion and disconnect the plug on each side (about in the center where a person would sit on each side).
-slide the cushion forward, making sure to not catch the seatbelts, remove the cushion from the car.

From here, you should be able to trace down the wire and see where you are having issues. Installing the seat is the opposite of what you did to remove the seat.