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Interior light switch.


muddychris300tdi
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Hi all, I have a commercial 300tdi Disco and the other night the interior lights stopped coming on when I open the rear door. The lights come on when I open the front doors. I thought it might be the switch so swapped it with one from the front and it worked fine. Would the rear switch be fused on its own, if so which fuse would it be? I carn't find my Haynes Manuel to have a look. And what other things should I look at? Thanks for any help, Chris.

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Ok, I have run a wire from the screw on the switch to the blue and purple wire on the boot light and the lights come on but don't go off when I close the switch. Thanks Chris.

That's normal because you are running an earth connection to a earth potential for another switch, the switches are grounded/earth via the screw to the body only, the illumination is controled by your MFU.

The wiring is common to the three doors. Just touch the spade connector to the body if the lamp light then you have a faulty switch.

There are NO fuses.

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To back up Teabag, he's correct. There's 12V to the lamp, through it, and to the switch. The switch, when the door opens, allow the 12V to complete the circuit, and make it's way to "earth" through the body of the vehicle.

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If you look inside the switch, you can see where the end of the metal that the terminal pushes onto, makes contact with the other "half" of the switch. The contact patch is tiny, and gets mucky/corroded very easily. If you can clean this up, you're highly likely to have a working switch, again.

A really half ar$ed way to do this, is push the switch full in, when it's still installed, and let it go, by sliding you thumb off it, sideways, so it fires out, rather than slowly moving out with the door. A couple of these is usually enough to break the layer of crud on the contacts for a few more weeks/months.

Your switch may of course be really knackered, in which case you need a new one.

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As was said, further up, you can test the rest of the circuit, by touching the wire that attaches to the door switch, to the body. If your light comes on, it's a problem with the door switch.

As you say the switch is very basic so a clean up and making sure the spring is doing it job , a dab of Vaseline and that's it.... rarely is the switch terminal and has to be replaced, but no doubt it has been known.

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As the wiring is common to all switches it could be the wire has come uncliped from one of the other door switches other than that there's a break in the wire. The wiring diagram shows wiring from switch to switch ithout any joints inbetween.

Bottom line is you may have to replace the feed to the rear door switch.

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Ok Blue and Purple doesn't appear on the diagram but Purple and Blue does (small difference, but it matters!). Looking at the diagram you have three interior lights which are switched so you can manually switch them on, or have them on auto, all fed from fuse B1 via a Purple and White wire. Do they work on manual?

If yes, then it seems power is getting to the door switch but not getting to earth. It seems the wiring is setup up to provide power to the lights, which have an internal connection to a switch which earths the light and so it comes on (in manual). In auto, the earth path is routed to the door switches, and if any door is opened then all 3 lights should come on. The diagram appears to show all the door switches closed for some reason.

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Ok Blue and Purple doesn't appear on the diagram but Purple and Blue does (small difference, but it matters!). Looking at the diagram you have three interior lights which are switched so you can manually switch them on, or have them on auto, all fed from fuse B1 via a Purple and White wire. Do they work on manual?

If yes, then it seems power is getting to the door switch but not getting to earth. It seems the wiring is setup up to provide power to the lights, which have an internal connection to a switch which earths the light and so it comes on (in manual). In auto, the earth path is routed to the door switches, and if any door is opened then all 3 lights should come on. The diagram appears to show all the door switches closed for some reason.

Your are on the wrong page, keep up, the OP has a 300 series disco.

Therefore no B1 fuse also the MFU (as I have already mentioned in post 8) switches on the interior lights on when it receives a signal via the door pin switch or the security ecu when unlocking.

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Your correct that I have three lights but only two have switches on them, the two in the roof. They both work manually. Could I possibly test it by running a wire from the rear door switch to the light in the roof? I really appreciate the help.

No, do you have 12v on the purple/brown when the other interior lamps if so that indicates that the supply circuit is ok. The purple/blue goes to the MFU.

As I have stated the door pin switch has common wiring if there is a break in that the MFU doesnt get it's signal so nothing happens, all logical stuff.

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Your are on the wrong page, keep up, the OP has a 300 series disco.

Therefore no B1 fuse also the MFU (as I have already mentioned in post 8) switches on the interior lights on when it receives a signal via the door pin switch or the security ecu when unlocking.

Yes I am...Post '93 legend over-ruled by pre '95 at bottom of page....

Manual wiring of the lights is still the same, 12v fed up Purple and Brown, and an internal connection to switch which goes to earth so the bulb lights when switch is in manual. Purple and Blue wire takes the earth path to the MFU for the door switchs. My diagram shows the tailgate switch as item 203, connected to the MFU with a P/W wire.

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