Interior electrics gremlins

Ok, I have tried for a whole day and failed. The interior electrics on my 420G have me foxed. Here are the symptoms:

  1. When inserting a fuse in fuse holder 7, all interior lights come on, despite doors being closed. Door switches in the front I could undo and check and they are OK, the rear ones are in the door hinge screws and I can’t undo those to check them

  2. The interior light (dim) switch does switch on in position one, but in position two blows the fuse (no 7). The purple/yellow wire at the rear of the switch was disconnected to prevent this from happening.

  3. The instrument lights do not work at all, despite headlights all working fine and all the remaining fuses being fine. Nor do the ignition and headlamps lights inside the speedo work… suspect same root cause, but can’t be sure.

  4. Rear reading lights do not work, bulbs and switches are fine, just no 12V feed.

I have tried looking at the wiring diagram, but can’t see where wires may be crossed over/unplugged… I suspect there is a 'grounding somewhere, but I don’t know where to start.

Any suggestions?


Haven’t looked at the wiring diagram, but I would have a look at the purple/yellow wire that blows fuse 7 when connected. Where does it go, and why does it blow the fuse? Just a thought.

It disappears into the back of the dashboard downwards. Can’t follow it.

However… interestingly enough, all the door switch wires are purple/yellow as well… could that be grounding somewhere?

What is the easiest way to have a look behind the speedo/tacho as that is where all wires seem to go to…


to search out a short like that, I suggest a test light and also a “special” tool

that is a 2 prong indicator flasher relay.

wire it up, so that is now your new fuse !

it will click on/off with 12V on one side, and earth on the other

shake or disconnect wires until it stops clicking = earth (short) is gone

a multimeter with “continuity” function that beeps is a big help

Instructions to remove the speedo & tacho are in the FSM

not quite as easy as it says, as large hands will struggle to fit, and the speedo cable knurled nut can be tight

the rear door light switches are integral with the door hinge screw (wacky I know!)…they can be removed by taking out the screw…an imapct driver will be best for that. I doubt they will be faulty

Thanks, I have a multimeter that has a ‘continuity’ indicator. I have been shaking wires whilst the fuse was in to see if the interior lights would flicker, but no, no flicker.

But I could not get behind the speedo/tacho so that will be the next thing to check…

I won’t bother checking the rear door switches as they don’t look tampered with so no reason really for them to go bad. The front ones have been replaced as they look brand new inside.


you probably have at least 2 issues…no 12V, and a short circuit…they may not be related,

need to be diagnosed separately, battery best disconnected for earth continuity tests

it will help you to magnify & print out the circuit diagram and highlight the relevant areas, if you have the electronic manual,

dont forget the correct circuit diagram is in Chapter 15 IPL4

these things are a bit of an auto electricians nightmare,

label any wires you disconnect & take pics

I only have a manual and that has but one diagram in it… hope that is the correct one.

Yeah, nightmare is the right word… for anyone! :woozy_face:

I was hoping this was a common occurrence and that someone could point me in the right direction… I’ll keep prodding and trying.



I dont want to be the one to dash your hopes, but doubt that there is anyone one that can help you with a MKX-420G electrics !

As an owner for 20yrs, and a qualified electronics technician

only took it to an auto electrician once, a mate of mine with 40+yrs experience, he could not work it out.

You will need to be very systematic in diagnostic tests.

I checked the FSM…seems fuse 6 is involved in the instrument lights

Start by checking for the presence of 12V on both sides of every fuse.

Use a test light, not a multimeter

The speedo comes out as per FSM…you will think it impossible, but just sneak your hand up there, if you have enormous hands, you would presumably have to drop down the steering column,but still need to reach in and get the knurled screws from behind

E-type share the same auto electrics as the same year MKX-420G

Going by my color wiring diagram for my 3.8S I can offer the following on your point #3, but your car may vary.

From the main headlamp switch there should be a red black wire to fuse #6. Then solid red to the two position (bright and dim) panel lamp switch. Then from there, red with white wires to various instrument lights, and a red blue to the panel edge lights to illuminate the toggle switches. Additional red wires from fuse #6 feed the LH and RH front side lights, LH and RH rear tail lights and the glove box light.

Thanks John, that helps. I have found your description on the 420G wiring diagram. looks like the same routing is taken on both our cars, All points to fuse 6 not working… I thought I checked all the fuses and indeed the tail lights come off fuse 6 and they work, but will check all the wires coming off it again this afternoon - with a test lamp.

On the grounding issue, I still suspect the old radio being faulty. There is a brown wire with an in-line fuse coming off fuse 7 that seems to feed the radio (optional in the wiring diagram). Of course the radio itself is grounded, but there may be an internal issue with it. I might disconnect it altogether and see what happens to the interior lights.


Paul, More info and some sections of my color wiring diagram. I am technologically challenged so these are just photographs of sections of my large wall poster of the diagram. Again this is 3.8S material. Fuse 7 feeds via a purple wire, the map/interior light switch which is a two position switch, from off the first position is the map light which is under the top rail of the dash. Second position is for all three of the interior lights, on my car one each above the B post and one central one above the rear back glass. This switch has a ground function that is used to turn on the map light and the interior lights when operated from this switch. Alternatively the interior lights have door switches that provide the ground when doors are opened. Any one door switch will create a ground that serves to turn on all three interior lights. I hope the diagram photo is readable and useful to you.
John Quilter
Eugene, Oregon USA

That is so cool! And very helpful as much clearer than the manual which is in B/W and way too small for comfort. If you are able to take a pic of the instrument lights section, then I can have a look at that as well!

The map light is not on all the time when the fuse is in, only when I throw the switch, so as the interior lights are on a separate circuit, I can only conclude the switch is grounding (if of course none of the two rear door switches are not at fault).

More to check!

Thank you!


ur gonna have to remove and check them

(further, if ur a long term owner, I removed all 20 door screws (one at a time) with an Impact driver and the correct bit (No4 Posi I think), and copper-slipped them…in fact every fastener)

You have a short to earth somewhere.

my 420G has “magical” wiring,
I gave an example some time ago, where my indicators sometimes behave very bizzarely

After 20yrs, I decided there was no answer, disconnected the offending wires, and ran my own wires/harness

OK, Paul, here’s a shot of my diagram of the instrument lights. A red/black wire comes from a terminal on the main light switch to fuse #6 then from the fuse, a red wire leads to the two position instrument lamp switch. Then from the this switch there are three red/blue wires to the panel edge lamps and there is a red/white wire that branches to the eight instrument lights. I’m guessing one lamp for each smaller gauge and two for the speedo and two for the rev counter. My disclaimer again: this is a 3.8S diagram. Good luck. Do report back.

I give up! :cry::angry:

I cannot find where the grounding take place that makes the interior lights stay on when fuse 7 is in. It is one of the four purple wires going into the fuse, but where they go, who knows. Looks like doors, but I have checked the door switches and they are fine. Must be a ground in one of the wires in the car somewhere.

Also, now fuse 8 has blown and keeps blowing. No idea why. That amongst other things like the water, oil pressure and fuel instruments, feeds the hisser solenoid, so I am back to difficult cold starting and having no idea about how much fuel I have. Not even sure if the switch operates the fuel tank switch-over!

Is there any way to test without having to keep inserting and blowing fuses?

I’m gonna have to find an auto electrician somewhere. Anyone know of one in the London area?


Put a bulb in series with the battery connection. Even a dead short will just make the bulb glow.

I have found shorts by rigging a bulb at the feed like this, then just wiggling and shaking the harness until the bulb started reacting to my moving the harness about - once you’ve narrowed the branch of the harness down it gets easier to find the fault.

Ok, so one end of the test light to positive battery terminal, the other to the negative end of the fuse holder?

Or can I simply replace the fuse with a test light in series, one to one end of the fuse holder and one to the other end?

Yes, exactly that.

It drops the voltage to the load, so some loads won’t work, but a 21W brakelight will still allow most of the lamps to glow visibly.

A bulb instead of the fuse works too, but what if your short moves to a different fuse circuit when you’re moving stuff around?

Paul, You mention that there are four purple wires coming from fuse 7. If these are attached with separate Lucar connectors (not all bound together in one connector) can you unplug one at a time and see if the interior lights go out and the fuse stops blowing (or the test lamp mentioned above stops glowing)? On my first diagram above there is a purple going to the map light and then to the map light/interior light switch on the fascia. Disconnect this from the circuit and check again. The cigar lighter is also fed from fuse 7 so disconnect this as a further test. Same with the clock but your clock may be different than an S Type that has one in the tachometer.

Hi John,

There are actually five purple wires going into fuse 7, two of them on one connector, the rest on their own individual connectors. I did try them all individually, but that didn’t help as even though at the fuse I know which wires are the lights/cigar lighter/clock, I don’t know which one of the five is grounding, or where. That can be anywhere, and because they are all connected to each other in the same circuit, a grounding anywhere in the circuit will light up the lights.

I tried disconnecting the cigar lighter (no result) and the clock is already disconnected as it has no battery… it must be in the lights wiring loom somewhere.

The fuse that keeps blowing is fuse 8. I have bought a test light today. Fuse 8 is now my priority as that also controls the brake lights!

From the manual there are three green wires that go to 1) the choke solenoid, which I think is the hisser, 2) the brake light switch and 3) the instrument voltage regulator. The latter controls all the instruments, but not the fuel pump or fuel tank switch-over as those are fed by fuse 6 and that seems to work (phew!).

On the greens, testing with a light, one is not lighting up the light, one does it very faintly and one result in a bright light. But of course not sure which is which…