Mark 2 Jaguar 3.8 ran good up until last week.
I put the key into the ignition, pressed the starter button and … nothing.
no lights, no fuel pump engaging, no indicators, nothing but the sound of silence…
Where should I start looking to isolate the problem?
Electricals is not my strong suit…
First off, clean the battery terminals.
At minimum, you need a DVOM, and a test light.
Cleaning battery terminals always the primary check.
Do the headlights come on?
Lower your instrument panel and check for power to and from the ignition switch.
If power to the push button, is there power from it to the solenoid activation terminal.
You didnt mention if its automatic or manual gearbox.
Aside from cleaning the terminals I’ve found ground (earth) connections to be a common problem. Electrical devices require both a positive and negative connection. Rather than running two wires to a component Jaguar uses the car’s frame for the earth connection which would be positive for our cars. The other side of the component is connected via a wire to a switched circuit. On our cars the positive battery terminal is connected to the frame with a heavy cable bolted to the frame. Hmmm how good do you think that connection is after 50 years of mud and rain. Unbolt the cable from the frame and clean all to shiny metal. Reconnect and paint over the connection. Since you are seeing numerous components not working you need to look for the common fault. I had the same thing happen to me. Another case in point: My fuel pump stopped working on a club drive. One common fix is to wrap on the pump to free the points. I did that and the pump started running for about one mile. I must have stopped to do that 10 times before I got home. It wasn’t the points! The problem was the pump ground wire connection to the frame. By wrapping on the pump I must have been wiggling the wire just enough to make a tiny connection - enough for about a mile’s worth. Sorry for the long explanation but many don’t understand this grounding problem and hope this helps.
Pat H
The electricals are so simple in these cars. Usually a simple dirty connection that needs attention after many years of exposure to elements and neglect. Wire Usually doesn’t go bad unless insulation gets brittle from excessive heat or cut from a missing grommet.
Likely need to clean several connections. See if headlights work if so not a battery. I would get a good diagram check fuses first.
Perhaps start with putting a battery charger on without altering anything. Check for a light bulb (headlights, interior light, glovebox, …) or ignition left on which would drain battery. Observe amperage on charger and compare to normal charging amperage and reduction of charging amps over time. Many times I have seen dead batteries when an interior light was left on.
First thing I would do is to use the manual button on the starter solenoid on the bulkhead near the battery. If that turns the engine you have no problem with the battery or the big thick heavy wires that attach to the battery and connect the engine to the body and the fault is in the smaller wires that run all over the place.
If the engine turns and it’s passed that test I’m told (haven’t needed to go there myself) that everything else is fed from one of the terminals on this solenoid (3 or 4 small wires attached to one of it’s terminals) so that connection would be the next thing to check. I think it’s very likely that your problem will be here because this is the only place that feeds all the circuits - if it was ok here you would expect some things to work. If it’s not here then the job gets more difficult but if everything is dead then I’m confident it will be
BE CAREFUL! Remember that if you press the manual button on the starter solenoid and the car is in gear it will move !!! So make sure it’s not in gear and the handbrake is firmly on and maybe even chock the wheels.
My car is negative earth not positive earth and yours might be as well depending on its age and whether a PO has converted it but it doesn’t matter which way it is for this type of fault finding.
No headlights… brand new battery… (manual gearbox)
Definitely helps, thank you (to all so far)…
Many thanks… Looks like I need to learn electrics (one way or another…LOL)
I thought that too…but everything is off and brand-new battery…still the same problem…
If it is the connection on the solenoid all you need to do is to undo it, clean it & do it up again 
Be careful with the battery wire to the solenoid and don’t let it touch ground. It may be best to disconnect the battery cable before you do anything.
Pat H
Thanks Pat–duly noted
Start by charging the battery overnight.
And after charging, if everything is still dead, checking for a good ground is definitely the next step. Adding a temporary second ground from the battery to the body would tell you quickly
I will reiterate, for the benefit of the OP: one must start with a test light and a good DVOM.
All the rest is kinda like shooting in the dark.
Headlights don’t go through the solenoid, although the main lead is connected to one terminal of it. Then goes to rest of system via the ammeter. I’ve never heard of an ammeter failing, so check for corrosion on solenoid terminals and main earth strap, as others have suggested. If NOTHING is working, solenoid, per se, will not be the issue.
Correct. My daughter is writing her Honours thesis. She gets far flung doing research. Messaged that “car had broken down”. I took jumper leads and a spare battery. The incumbent battery proved to be open circuit per miltimeter - no coming back from that. In with the spare I brought with me - out of my Mk2. Paul.
As Paul said, a multimeter and test light is as essential as screwdrivers for working on any vehicle
Does not need to be expensive, $30 should get you basic gear from the auto parts store or Ebay
Google “basic auto electrical fault finding”
We can guide you, its really very simple in most cases, just a process of elimination
You will be proud of yourself once its fixed