Just when I thought things were going so well, I’ve hit a snag.
For some reason, I now have none of the ignition-fed circuits working when I turn the key. It’s all dead - no ignition light, no wipers, no fuel pump, no heater fan etc. All fuses are good, and I’ve tried a different switch with no change. I have gone over the wiring with a test meter with the battery completely disconnected, and discovered that all the green wires attached to the fuseboxes were showing continuity with earth. I reasoned that this can’t be correct. After a long process of elimination, I have discovered that this can be traced to one of the green wires going to the petrol gauge. Three green cables go to one terminal on the gauge, and that terminal is showing on my meter as connected to ground. What’s confusing me is that there is no fuel in the tank, so the sender is right down. Also there is a low fuel light in this gauge, and I’m not sure where the bulb to operate it is wired. Could this bulb be acting as a connection to ground which my meter is picking up? Two days on this now and no result… any help very gratefully received. What do the connections do in this gauge, and are they working correctly with an empty tank, or do I have a wiring fault?
First thing I would do is disconnect the wire thats going to the gauge and see if you have regained any functions.
First check AMP meter, it should have 12V on both leads.
Then Check the key switch ( brown/blue from the AMP meter)for 12v , when on the White lead gets 12v, it feeds the Fuel pump and ignition.
Use the wiring diagram ( green should have 12V also).
Check the culprit of all Jaguars…bad ground connection
If there is no gas in the tank, the green/yellow wire on the tank sending unit will be touching ground inside it and will turn on the low level light.
I can’t remember but I think the green/black wire will also be touching ground when the tank is empty and the gauge will indicate so.
Roger it’s not easy to explain fault finding with txt but you should try using your test meter on DC volts and not continuity with this fault, given you are not blowing fuses. You have answered your own question, that the/a lamp will give you continuity to ground, hence why I suggest you use voltage. Put the negative lead from your voltmeter to ground then simply check for +12 volts from the battery thru the ammeter, ign switch etc until you locate where it disappears.
Thanks for all the very helpful suggestions. I’ll get back on it tomorrow and start with Phil’s suggestion - if that doesn’t throw anything up I’ll disconnect the gauge as Robin suggests.
I’m wondering if c.70 year-old switches are a good idea after all - I do have a new replacement light switch from Barratt’s, but the terminals are quite different from the originals so I’m not sure how it connects up.
Another bizarre symptom is that originally I had all the lights working bar one brake light. Now the sidelights have stopped working, and only the headlights and the foglights and the horn still work. Suspiciously, these are all run via a rack of modern relays. The sidelights aren’t, but now don’t work.
Roger,
In my thread detailing my 150 restoration, I detail a very similar problem:
One of the green wires to the fuel gauge was somehow shorting to ground. The strangest part about that was it was paired with another green wire and they shared a spade terminal at the gauge end. Once I separated them and used only the “good” wire, all was fine.
You can also read about all the troubleshooting advice I received and the steps we took to solve the issue.
VERY similar to yours…
Thanks David, I’ve just had a look through it and you obviously found the same connection to ground that I did. I should have mentioned that my car is a 140, and only has three green wires to the terminal on the fuel gauge - one from the fusebox, one to the heater fan motor and one to the wiper motor.
If you have removed the one that was connecting to ground, do you no longer have your low fuel warning light facility? Lots of excellent advice on your thread, and the car looks great.
Roger,
My early 150 has a similar wiring schema to your 140. There are three wires to one side of the petrol gauge; one to the fuse box, one to the heater fan, and one to the flasher. (the other side of the gauge runs to ground) However, the wire from the fuse box is actually a pair of green wires. They are separate at the fuse box end, but connected together at the gauge end by a spade lug. It was one of these wires from the fuse box that was grounding. Very strange because it has a very short run, entirely encased in the cotton loom.
As you read in my thread, it was a real bear to isolate. But once I separated the pair at the spade lug, it was easy to identify the shorting one. I capped both ends of the shorting wire and reconnected the “good” one. Problem solved. I assume they were paired to handle the current from the heater motor. Since my car is an OTS and never driven in anything but perfect SOCAL weather, the heater fan would only be used for momentary demonstration.
While the low fuel light also runs directly to the gauge, it is green/yellow and is directly connected to the bulb in the gauge, not the terminal with the three green wires. It may not be clear in my thread, but it turns out the low fuel problem was a separate issue and thereby caused a distraction when troubleshooting the short.
Hope this helps.
Thanks again for the clear-headed advice which was really helpful.
I’ve now discovered that my ignition switch is not working - and the spare original I had in a period box also does not work. Great - I’ll have to rely on the repro ones the usuals sell, which look very different but hopefully can be made to fit.
Also, my carefully rebuilt light switch has also failed. The brown/white wire going into the switch from the ammeter shows 12V (from a power source, not the car battery), and the one next to it going to the ignition switch shows 9.5V. So something inside the switch must be high resistance. I cannot get any voltage to register on the red wire feeding the sidelights now so something has failed internally.
With the ignition switched bypassed to check ignition circuits, I now have brake lights and indicators back, and the low fuel warning light is glowing, hence the green wire showing a connection to ground, see above. What I don’t have is wipers, now… see new thread.
Thanks for the help, all - it can be hard to see the problems when you’re right up to close to them.
Well, that was a weird one, not seen that before.
If you look at the two connector screws on your right, you will see that one is slightly dark, and there’s an odd appearance to the metal tie bar right next to it. This tie strip had burned through, meaning those two screws were no longer connected. This accounts for both problems, as these are where the two brown and white cables connect - one brings power from the control box, the other takes power on to the ignition switch. It also makes contact inside the switch with the sidelight circuit. Hence no sidelights, and no ignition circuits.
I’d never have found this without dismantling, and I have no idea how or why it happened - or why the master fuse in the car didn’t pop instead. It could have been seventy-year-old metal with some corrosion, I don’t know. I have a new, repro light switch, which is always a worry as you never know how well these things are made. So tentatively I prised the side depressions out to take a look inside - and for once I was pleasantly surprised. There are some hefty sliding brass contacts in there which look significantly heavier-duty than the curved spring contacts in the original switch. The repro switch also has much more substantial contacts on the back for the high-current connections - i.e. the brown/whites and the blue for headlamps. So I’ve fitted that which will hopefully get things working again. Just the wiper motor to sort out now.
I was going to say a lot of old switches and other Jag electrical components can be taken apart and repaired, so its good you could, new repros often cannot be
This is not repairable to any reliable extent. If you look at the photo, the connecting bar to this terminal has completely burned away. Repair would require drilling out the contact rivets, replacing the strip, then refixing rivets that could be threaded for the BA contact screws. Coupled with fragile 70-yr-old Bakelite that’s not a realistic proposition. Those two screws are taking the main power cables for most of the electrical system, in effect.
I’ve dismantled a few original Lucas 31524/A switches, and one of the modern replacements as supplied by Barratt’s. Barratt’s switch, in my opinion, is actually a better design and made of stronger components. That’s very unusual in repro parts but in this case I’d say the expense (they’re not cheap!) is well worthwhile.
This might be an opportune moment to present a couple of hand sketched wiring diagrams extracted and enlarged from Lucas W94602 XK120FHC to help anyone else who comes across this thread and needs a clarified picture of the circuits.
I don’t know why Lucas used so many greens for different applications, other than they ran out of colours?!
Control box green wiring.pdf (662.7 KB)
Petrol oil gauge wiring 679265.pdf (823.6 KB)
and timely as I’m just starting the wiring on my XK120 FHC (679314). Very useful. Thank you.
Put simply, it’s because British car wiring follows certain conventions, and plain green is the colour code for a wire that comes from a fused power source fed by the ignition switch. Thus, if you see a green wire anywhere on the car, it will carry 12V when the ignition is switched on and will be fused. This applies to any Lucas equipped car, of any make.
The American industry of the 1960s could have learned a thing or two from this, in my view - when restoring my 1968 Mustang, I found it mighty confusing that (for example) a plain black wire could be both a 12V supply or an earth, depending on the circuit.
Thanks for the thanks. I have some other diagrams so let me know if there are any more questions.
ps I am an enthusiastic amateur not an autoelectrician.
Do you live in Herefordshire or Hertfordshire?
Kind regards,
Simon Slaffer
Me? Cambridgeshire. Whereabouts are you?
It was because there was either an e or a t missing from DJ’s listed address.
I am in Jersey Channel Islands but have booked in to the XK75 stuff in September.
If you UK contributors are joining that we could have a UK JagLovers gathering?!