Strange electrical fault

(Alan4d) #1

Electrics are one of my specialities but I am stumped at the moment by a strange fault. Battery going flat overnight led to the discovery that there is an 11 Amp drain even when the car is stationary and the ignition is off. The XJ40 does have a small quiescent current associated with computers, radio etc. and that is typically around 60mA (no alarm fitted) which is normal. But clearly 11 Amps is excessive. Carrying out the normal isolation tests quickly revealed that the faulty circuit is that supplied by RH Fuse 1 (the RH door). With that fuse out, all was normal. Verified by placing an ammeter across that fuse socket. So I set about dismantling the door skin to look further. At which point the excessive current draw ceased. So far no amount of wiggling connections has brought it back. Anyone had this problem? Can’t be door light (off and too low a drain). Would not expect it to be door motor (although that drain might match) because that is only energised when ignition is on. Any ideas?

(Mike Stone) #2

Door lock heater (if so equipped)?

(Alan4d) #3

Thanks! That sounds feasible but I don’t think one is fitted. At least I don’t remember any wires to it. I’ll check. The drain would probably be about right, wouldn’t it?

(Alan4d) #4

OK So far don’t think there’s a heater connection but another odd thing happened about the same time. The window winder did not wind down. Just a relay click sound. But that’s only with the ignition on, and the drain is there with it off. Also, I am wondering about the central locking. But most of all, why did it clear when the door skin was dismantled?

(Robin O'Connor) #5

I wonder if the window relay fusesd itself causing the drain?

(David Jauch) #6

If neither heated mirrors nor heated lock:
On the Series III the Motor has both wires constantly powered and to move the window up or down the corresponding wire is cut from 12V and then earthed. A short?
Or the relay was stuck, but then again it did click. And it is probably not in the door so you didn’t reach it while moving around without the skin. Maybe the overload switch was stuck or shorted out, but that doesn’t seem to apply.
The only way out is that there could be a dedicated window main relay (for powering all the dedicated relays), stuck, that is powered through the main ignition relay. If it happens again the motor might be warm.

(Robin O'Connor) #7

With the ‘40 the relays are in the door.

(David Jauch) #8

Found this:



(Alan4d) #9

And a further thought looking at the wiring. The only time the unswitched earth is connected to the winder motor is when the door handle switch is operated. But I’ve had this problem before and have since changed door handles – but maybe not switches! The plot thickens!

(motorcarman) #10

Since there are 3 iterations of the XJ40 it would be helpful to know the MY.

The Passive Restraint motors can drain the battery if they are fitted to the car??


(Grooveman) #11

Bob …

I agree it would help if everyone added the year of their car in their post.

However if you click on the posters icon most have the their model under “MY CARS”. Alan’s is an 1987/88 model.

(motorcarman) #12

Sounds like more work than I care to perform. I’m out.


(Alan4d) #13

OK. Car is known as 1988 MY. I have the full wiring diagram, which I’ve now studied properly. The drain stops with Fuse 1 out. It is consistent with the winder motor being at the end of its travel and as it is there even when the ignition is off can only be via unswitched earth and Fuse 1. That seems to suggest possibly a problem with the the special feed via diode from the CPU if either door lock is fully advanced (closes the windows when key is held across, ignition off of course) and as it ceased when the door skin was removed it is likely to be driver’s door lock switch if it is the same switch I fitted to the new lock some time ago (rather than the CPU thank goodness). I’m following from there and will report back.

(Alan4d) #14

For those following along, I am looking at diagram No. 16. As the drain occurs when the ignition is off, the circuit causing the problem cannot be via connection 6, ignition switched earth. It can only be via LBG72 (always connected to earth) or via the logic line from the CPU (lb14-27), which is in turn fed by the key barrel switches (if you hold them over, the motors close the windows and of course this is when you are out of the car, ignition off, and are locking it). That leads me to suspect a fault on one of the key barrel switches, causing it to stick over. But why did the fault clear when I stripped the door… Could of course be the CPU (ouch!)but I have never heard of one failing! One thing to add. There’s almost certainly an error on diagram 16. The labelling on drivers door switch is probably reversed. It would not make sense to supply the down sense from the CPU.

(Alan4d) #15

I’ve narrowed this down and I think I have cleared the obscure fault. There was quite bit of logic to apply, and some red herrings. If anyone is interested I’ll show my reasoning sheet and diagram.

( Larry ) #16

No, whatever you did, keep it to yourself!

We love mysteries and unresolved issues on Jaglovers, that’s the whole point of the archive; keeping people in the dark, right?

(Alan4d) #17

Not sure what to make of that post! Obviously I’d like to share the info, but not at the risk of boring everyone stiff! I rather doubt that anyone else has had this problem, judging by the lack of interest…

(Mike Stone) #18

Alan, please don’t hesitate to share - as a community of Jag lovers we are here to learn and, when possible, teach by sharing our experiences. After all, it’s not like being stuck in a lecture class at university - we all know where our delete buttons are! :joy:

(Alan4d) #19

OK. Here goes! The notes are made are in Word format so as downloads are only JPGs I have appended the notes by copy and paste. Diag16 annotated attached and pic as well.

SLEUTHING THE BATTERY DRAIN — See Jag diagram 16 with legends
With ignition off (overnight), (new as well as old) battery draining 11 Amps.
Fault disappears when R footwell fuse 1 out (driver’s door). (11A verified via fuse)
When fault is there, driver’s DN switch (with ignition on) just causes click
Fault disappeared when driver’s door trim taken out.
(On Jaguar diagram, switch is mis-labelled. Left should be UP and right DN.)
Physical Investigations
Drain same as motor energised for UP connection even when already fully up. (Tested 11A)

  • Can simulate drain by manually pressing UP relay armature to contact on.

Ignition switched E (battery —) only affects door switches. Full E always on other stuff.
Battery + always on, via Fuse 1 (RH footwell).
Full E always applied to motor B terminal via H and Z (DN Relay) unless DN switch pressed
So must be batt + getting to motor A terminal. Only possible via relay armature actuating (if direct — see cause 2 — would cause dead short).
Possible routes
1: Y shorting to full Earth internally (eg right down to switch UP or diode inner),
holds UP relay on.
2: J shorting to X (relay armature not involved), same as M (DD1-1) shorting to N (DD1-2.),. ## But that would mean batt + on J, while J is already resting on W (always on Earth) and dead short! So rule out!
3: * UP Relay contacts mechanically stuck on (but spring is quite strong, and an intermittent fault is not very likely — weld more normal)
4: O (DD2-23 ex logic from door lock) shorting to E, via diode to Y, holds UP relay on).
5: Driver’s door handle override switches or wiring shorting (DD10-1 to DD10-2). ## But then power would be on ALL windows and the drain would be much more than 11A. So rule out.
6: Passenger door handle override switches or wiring shorting (PD14-1 to PD14-2). ##
But then power would be on ALL windows and the drain would be much more than
11A. Also, it was driver’s door skin off that cleared fault. So rule out.
1, (3), 4 only possible causes. No 4 most likely to have been disturbed when stripping driver’s door.

  • UP relay contacts cleaned and dressed re possible cause 3.
    Shorts in wiring could not be simulated, but now using cable ties to hold connectors in door void to avoid pulling or crushing same (causes 1, 4).

EDIT: Having posted this I see that the diagram is not very easy to read.
Looking at diagram 16, within and around the left hand rectangle (the switch and relay block), from the top:
M is the first right hand connector (DD1-1).
Y is the left of the Up relay coil.
J is the moving relay pole (armature).
X is the contact when relay energised.
W is the relay contact when at rest.
N is the second connector down (DD1-2).
A is the uppermost motor connector
O is the third connector down (DD2-23).
Next down is the DN relay.
H is the moving relay pole.
Z is the at rest relay contact.
B is the lowermost motor connector
Next down on the left is the erroneously marked driver’s door switch.
The left contact is for UP and the right for DOWN.
Verified because the CPU link (from door locks) must close windows and not open them!

(Mike Stone) #20

Alan thanks for taking the time to write this up and share it. I know just enough about electrical things to be dangerous, so all help is appreciated.