1994 XJS V12.

I had a no-start recently with a flat battery (down to 3V). This was 24h after a run, and no previous issues.

I disconnected the battery and read 2.7 ohms across the terminals with the key out of the ignition. I pulled every fuse in the car (from the fuseboxes in the passenger and driver footwells and behind each taillight). With all those fuses unplugged, I still get that low resistance across the terminals. I even tried pulling all the relays I could get to easily.

I reconditioned the battery at home (I can recommend a charger brand called Autoxel, BTW) - now it holds 12.8V and the car starts again. Alternator clearly works, voltage across terminals is up to 14.4V with engine running.

But the battery voltage dropped by 0.5V overnight. And I still read 2.7 ohms across the terminals.

Is this normal? What do I need to look for?

 

after a full charge ,does it hold overnight With cables DISCONNECTED?

 

Yep. Sorry I was unclear. Battery holds charge when disconnected, charge drops when connected.

 

GOT a Parasitic drain, Jags are notorius for Electrical problems!

 

On your facelift car the problem is typically the trunk lights are staying on or the power antenna has internally shorted out causing the drain. I would start there

 

Do you have any aftermarket alarms, stereos or remote starters? Those can be common sources of a current drain.

 

No aftermarket bits. I'll leave the wife in the boot overnight and ask her if the lights stay on, good shout

 

As much as I'm sure she would appreciate your including her in on the diagnostics, I put my phone on video and shut the trunk. Let it sit for a few minutes then watched it. My light turned off within 10 seconds or so. If you're still getting the drain with all the fuses out, I would think it's a "hard wired" thing. One possible option might be to go to the engine bay and pull the leads off the bulkhead connector one by one and see if anyone of them kills the draw. That would at least give you a possible area to look harder at. It almost sounds like something is stuck open or grounding out

 

Lolololololololol,,,, hahahaha,,,, lolololololololol!!!
Good one!

I have a 2003 XKR Supercharged Portfolio Convertible (I love saying that name) that took a nice long swim in a Texas flood. Parasitic drain x 3...

Turned the car off, key out, (I'm not familiar with how car off systems work in the 94) let the car go to "sleep"... Trunk/boot lights disconnected... Remove the negative cable from the battery and put my DMM inline between the negative battery cable and the battery (little clamps)... Remember that unless you want to blow up your DMM don't touch the key. Be careful,,, any current that flows will now flow through the DMM. You'll have to rearrange the probe lead plug placement on the DMM to the fused area.

Slowly watch the DMM reading as you check all kinds of electrics. Disconnect the antenna, pull fuses, mess around with door switches and check the reading. Should read in the milli volts or tenth of volts - I forget. There are some great videos on YouTube. One that stands out is Eric the car guy...

Took a little while but I found and solved my problem.
Good luck...

 

sometimes when people are looking at my XJS consuol ask why, with multi switches, if a had a drain , i can turn any thing on and off, to help diagnose problem!

 

Which ones for the ejector seat? Lol😁

 

.
a secret Shh, its under the left side of driver seat!

 

Disconnect the battery neg terminal, put your meter on AMPS use the 10A range to start then move to the lower range if the reading is low, good meters have separate connectors for 10A range. Put your meter between the battery post and neg cable.

What is the current draw?

 

my jag has remote door openers, a relay in circuit, thats what has a draw on battery, if i disconnect the battery, doors wont open!
but no problem mine is open top , so i just reach inside and open them!

 

The alternator itself has a rectifying diode pack to rectify AC to DC. These diode packs can internally short and draw amps which will eventually kill the batt.
Try disconnecting the alternator wiring (+ cable) at the alternator to remove that suspected draw and see what you get.

 

Hey Ron. Curious. REALLY curious...

What did you have to tie into to Gian control of the fuel mixture? The rich lean control...

 

I agree, testing resistance across the terminals is not a valid test. Current draw, particularly on electronic components , may not be what Ohms Law would predict. I would suggest the additional step of connecting the battery, running the engine and operating all components, let the car sit for 10 minutes, then, without ever disconnecting the battery, connect your meter, set to measure a 10 amp draw, then remove the battery cable. Now read your draw. In many cases, disconnecting the battery before measuring will change the draw. Beware, that anything that causes draw to exceed 10 amps will blow the fuse in your meter, or damage the meter if it does not have a fuse. Draw of more than about 0.04 mA will run the battery down quickly. Now, start pulling fuses or disconnecting components to see where your excess draw comes from.

 

It's not so much Ohms law as components like diodes and other solid state devices in the power cct. These are like one way valves for current they measure low resistance one way but turn the meter probes around and they measure open circuit.

I think you mean 40mA 0.04amps.

 

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i use a standalone ECU , SDS EFI.com.

this is the ECU box,cover off, comes with plugs and harness! button is a reboot!

this the hand held programmer , hooks up with USB cord. its quite simple!
the knob has a 50% rich or lean from center ! or a 100% total on a full sweep!
YES it is a learning curve but with practice it gets easier!

 

i an still marveled with the New battery, must be an evulotion in Lead/acid batteries!
after i drive for 1/2 hr or more , i shut engine down(disconnect neg battery terminal), after 5 minutes bat measures 12.65V, considered normal OK!
next morning measures 12.67V , that fine too!
BUT left for a 10/15 days,in cold unheated garage, battery measures 12.97/12.98, close to 13V !!
now how does a battery gain voltage when NOT in use, that ever so slight increase makes my Jag V12 start quicker, gages come up quicker, response seems better?
i,m thinkin maybe more spark energy also!
then i drive away for some road time , come back it measures 12.65V, and the cycle starts over!
I LIKE IT , SO BE IT.
times they are a-changing??