Wiring Diagram For Mk 3 Ls400

[Colin M]

Does anyone have a wiring diagram for the Mk 3 LS400?

The tail lights have stopped working on my 1997 car. Both the bulbs are OK and light up when removed from the car and tested. When the light switch is turned on there is a live feed to the tail light fuse in the small fuse box with the white plastic cover above the driver's footwell. The fuse itself is OK and I have in any event replaced it with another one known to be good.

But there is no live feed to the bulbs in the tail lights so I am wondering if there is a tail light relay somewhere which has stopped working. If anyone knows if there is, and if so where it is located, I would be grateful to know. I have looked at the relays in the main fuse box in the engine bay but can see no mention of a tail light relay.

A wiring diagram and any other help anyone might be able to offer would be much appreciated.

Many thanks.

Colin

[Rupert Dubreaux]

Colin hi , it might be worth checking the rear light sensing unit ( not sure if that is the right name for it ) On the Mk3 it is located behind the cardboard/carpet trim just under the left side boot lid hinge . Mine is a small green plastic box with a white plug ,I have seen them in blue and red as well, inside is a small circuit board that might have a problem .

Mine had a bit of a burn up on some wire links in the unit after I had fitted some dodgy LEDS to the rear lights .

I am sure Steve 2006 will probably be able to help if you need the wiring diagram ,good luck with finding your problem .

[Colin M]

Hi Rupert

Thanks for your response.

I will have a look for the plastic box and white plug.

I am guessing that the circuit board is inside the box and that there will be feed wires to the box, probably through the white plug. Hopefully, I will be able to check whether there is a live feed coming through the wiring.

I may be able to take the box off my other Mk 3 and insert the white plug into that to see whether the lights then work. If, they do, this should indicate that the fault lies in the box. If I do this, hopefully the lights will still work on the other Mk 3 when I put the box back!!

Here's hoping ...

Regards

Colin

[Rupert Dubreaux]

Colin hi again , it is very handy to have a spare car in these situations to swap known working parts . Looking at my Mk2 diagram which seems to use a similar setup the supply for the tail lights runs through the light sensor unit circuit .

I think the rear light relay is somewhere near to the tail light fuse on the panel under the steering column .

[ambermarine]

The green taillight failure box which detects the failure of the bulb or bulbs should send a signal to the instrument cluster lighting up a warning icon.

If this is not happening and your bulbs and fuses are ok it is the ecu in the box that is probably suspect.The same unit was fitted to the full range of Ls400s and is as already been stated is located behind the petrol tank boot cover.

They come up on eBay regular.

[Colin M]

Thanks for the last couple of responses. I have not got any warning light on the dash.

I will take the unit from the car with working lights to try it on the other one.

I am a little uncertain on which side of the boot I am looking and would be grateful if you could clarify.

Is the box on the passenger/near side of the car or the driver/off side?

Thanks a lot.

[ambermarine]

On the Mark 4 the unit is located just inside the left hand hinge of the boot (nearside) and can be accesed by pushing the flaps of the cover to one side.Hopefully it is the same on the mark 3 but was'nt on the mark 1 and required the cover removal.

[ambermarine]

Couple of pics to help you locate

[Colin M]

Many thanks, ambermarine, the photos are very helpful.

I am going to investigate this tomorrow and will report back on how things go.

Cheers

Colin

[Colin M]

I am pleased to say it is the green box that is at fault.

I have swapped the boxes on the two cars and the rear lights on the car that were not working do now work. The rear lights previously working on the other car do not now work so clearly the fault is in the box.

I have just bought a replacement box off eBay and should have both sets of lights working when this arrives and is fitted.

Many thanks for your help on this. Not only do we have great cars but also great support when we need some help to fix them!

Thanks again.

Colin

[Colin M]

Replacement green box bought from jmc.autosalvage on eBay has now arrived, I've fitted it and the tail lights are now working.

At £9.89 including free delivery, this has been a cheap fix.

If only all fixes were this cheap!!

[Rupert Dubreaux]

Glad you have sorted the problem Colin , did you by chance open up the non working box to see if there was any noticeable problem ,as a matter of interest .

[Colin M]

No, Rupert, I have not done so yet but plan to do so at some point. I have kept the relay as it might be repairable.

I have made a note of which coloured wire operates the tail lights (green with bands of light purple or light brown - hard to tell the colour) so that I have an idea of whereabouts to start looking when I open the unit.

I will report back if I find anything.

Thanks again for your help.

Colin

[Colin M]

I have finally got round to opening up the non-working relay unit but cannot see anything obviously amiss with it.

There does not seem to be any break in the soldering on the PCB, so it is hard to know how it might be repaired.

I will probably still keep it anyway, since an auto electrician may know how to repair it in the event that I needed another unit and could not source one.