My car loses all electrical power sometimes either on switch off or ignition - there is nothing anywhere, radio loses it's code, windows lose their settings, trip computer resets etc

I think I've narrowed it down to this cable running through the firewall. It usually happens at inconvenient times and as soon as I lift the battery cover the boot (trunk) lights cut back in again and all is good. However, I managed to catch it the other day whilst I was working on it.

I've lubricated and tighten both battery terminals and tightened the connector (fuse?) that this cable connects to but I think it's shorting in the firewall

Any idea where it goes, and is it easy to replace?




Thanks

 

Check the battery cables themselves. They sometimes fail internally, not just at the ends. Failing cable get warm too (added resistance). Double check what "lubricant" you used. Dielectric grease is used to isolate, not promote conductivity. To my knowledge, all these connections should be dry and clean. Sand paper is your friend here. Also use one of the cheap specialty battery brushes from the parts store. You can also disassemble that fuse block in the trunk that the battery cables go to. Check for anything loose/corroded/burned. The fact that it fixes itself by moving the battery cover means this is likely a "simple" problem. Depending on the age, it might be best to look at replacing the battery as well.

Best of luck, keep us posted.

 

Thanks for quick response - I originally suspected the battery cables (used vaseline on the terminals) but all seemed ok - I specifically targeted that are, especially the earth strap, when I had the failure.

It was only when I wiggled that cable as goes through the firewall that it cut back in - I'll strip the fuse block and have another look but there was nothing obvious when I checked tightness of terminals previously

Just hope that there isn't a break / weakness in that main cable as I dread to think what route it takes to the front of the vehicle

 

As Merts says,,, et al...

Believe it or not I actually have a spare cable - from your photo - laying around. My XK8 needs it. A while back at the megafuse relay I noticed that the cables coming off of it, running under the back seat, up to a connection point on the RH area of the tunnel, under the carpet, just south of the shifter - was getting really hot while the car was running. I cleaned it the best I could, vinegar, sand paper - all that business, cleaned all of the connections and redid up everything. Still gets warm, but not hot. It needs the full fix - replacing cable.

Recently, I went to the false bulkhead connector, RH side that houses the ECM, fuse block, with the removable cover needed a good cleaning as well. If you look on the outer wall of the false bulk head you'll see a heave gauge battery cable style cable bolted into a junction in the wall. Remove it. Clean it up.

Also, if you're feeling adventurous, remove the center console, pull the rug back a little on the right hand side and you'll see another junction (the one I mentioned above). Open that connection up too, clean the terminals... This junction (big ol heavy gauge wire/cable) feeds lots into fuse boxes all around the front end...

These cars are getting old and that 24oz coca-cola someone spilled in the 14yrs ago is finally taking it's toll, if ya know what I mean...

 

Lay you hand on that cable car running - see if it's running hot... It's only about 5 or 6 feet long (if it's the cable I'm thinking) and it junction is at the RH side of the tunnel.

You can see it here...

 

Brilliant - thanks for this info. So I'm assuming it's item 11 I'd need

 

You'll need it,,, if you need it..

I don't have a lot of money, so first I would go and check the connections and feel for heat (aka resistance) in the wire(s) while the car is running...

Check the starter connection (not as hard as all that).
"" The alternator connection
"" The false bulkhead connection
"" The connections at fuse boxes"" The connections at the mega fuse...

Clean them and feel a bit for heat. Take a voltage reading. They should be close to the same at all.

For these older cars checking things like this (they've been 'connected' for 20yrs) is "regular" maintenance. Especially for the higher milage cars. Check for signs of water in-gress as well. Haha, is that a word?

 

Was still getting the cut-outs, but more frequently. Took all the terminals off the mega fuse, cleaned and vaselined before re-installing

However, looking for replacements, just-in-case, and I think my set-up is wrong / previously played with

I've got 1 off Ford 175A and 1 off 500A MEGA - is that right?

 

While the car is running and you have access to all that, is/are the cables and fuse block warm? I believe it's supposed to be a 500 and a 250 mega fuses. But the fuse is not blown - heat does matter though.

There are a couple of other places that need to be checked. One is just to the RH of the center console. And an important one is the major connection on the false bulkhead under hood - then of course there is at the alternator and starter itself. You are on the right track with what you've checked. The center console seems like a real pain to remove (maybe check this last) but it's really not.

Then, as Mertz has said - because these cables (20yrs old) look good from the outside doesn't tell the whole story. A knick in a cable where water (and air) can get to em. Over time they loose conductivity is a spot - or the open to air major ground straps - one in the trunk and one connecting the engine to the body.

On my XJS (due to age and electrics) for example, it usual practice to just run a whole separate redundant earth from the engine to the body of the car at the alternator mount. Maybe it's time to start doing the same on these aging XKs...

Gotta keep digging!

 

That should be AFTER.

To get a good electrical joint & keep water etc out.

 

Ah, my bad

So far, it's worked though. The day before I cleaned the terminals it was pretty much failing on every switch-off. Yesterday, didn't let me down once, but only a few stops / starts

Any thoughts on the actual fuses? Are they right?

 

In my car (2002) it's a 500 and 250amp mega fuse.

Are you feeling HEAT at the mega fuse pack and/or ANY of the power cables coming off it?

 

No, nothing felt (but by the time I get there the power has usually been off for a minute or so). Standard use, no discernible heat, no sign of heat damage, wiggling the cable into a connector doesn't replicate the fault

Hopefully it was just dirty connections, but someone has been there before me (lid to the megafuse is pivoted on cable ties) so I'd like to confirm that the actual fuse ratings are right

 

Hey Man, I want to be careful and say that I can't know that all my jibber jabbering and suggestions absolutely ARE the issue... They are just suggestions. That said:

The heat will last. Things will not cool for several minutes +... Electrical heat due to resistance is impressive. What I'm suggesting is to have the tire out of the boot, that whole jumble of wires/cables accessible. Start the car, have it running and go start laying your hands on the cables. Check, clean and reconnect the jumble of black grounds in the boot right near the battery. Same for the main battery ground.

Then, the other junctions suggested (under hood - at fuse boxes etc) aught be checked.

But hey,,, at the same time, if youre feeling good about things, I ain't going to get ya all alarmed and upset. Go easy, I say... The world will keep spinning

Good luck

 

I like Jay's suggestions, give it a try!

John

 

+1 to Jay - it doesn't cost a penny to check things thoroughly. don't worry or pay money to fix things that aren't broke.

As said above aging big cables is common on old cars. In my ex-99 XK8 I too had replaced the big red cable from the battery to the junction box, upgraded the earth strap from the battery to the body, plus replaced the earth from the enginee to the body (while doing my exhaust and CATs replacement).
BTW I recently had to throw out my old jumper cables, even though there didn't look to be anything wrong, but they got really hot to touch so must have been an internal fault. As JayJagJay said, with these big cables, it could be an invisible fault but heat is the sign of a problem.

Note what JavV8 said above, I'm a bit concerned how you're using Vaseline as a Lube. First and foremost, the terminals should be clean and shiny (no lube - unless you have a purpose electric contact cleaner). Sandpaper is good. You can get silvering contact improvement paste but that really is V. expensive and IMHO not worth it for these connections (save silvering for the 5V electrical glitches). The dielectric grease used is put on after the connection is made to isolate and prevent corrosion.

 

I use this silver based conductive grease. At $38 for 7 grams it may seem expensive. I use it on all electric contacts and the same tube has lasted several years and is still more than half full. A “drop” of goes a long way. It’s the right tool for the job. Very good for mitigating voltage drop.

Z


MG Chemicals 8463-7G Silver Conductive Grease, Maximum Thermal Conductivity, 7 g Dispenser

 

OK, quick update on this

Changed the cable from battery to mega fuse box and replaced both megas with 250A versions. 6 months later no issues (guaranteed it'll be dead tomorrow now )

Next - sort out the intermittent rear parking sensors

 

Good stuff!
This is next on my list.
Then, one of the long ones from the mega pack to the post on the RH side of the console...