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During this spell of heavy rain my LS400 mkIV has developed an electrical fault.

I noticed a drip from the roof above the steering wheel.

The Alarm would sound for no apparent reason.

The remote key system failed a couple of weeks ago, so I've been locking it manually.

However, it now appears that the ignition remains on even after removing the key.

Initially all the dash lights would flicker, but now they stay solidly ON.

The roof, and the windows will open without the key in the ignition.

Obviously this has run down the Battery, and the 6 minute jump wont reset the immobiliser.....

Anyone have any suggestions?

heres hoping!

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Don't know about mk.4's, on series two there is a fuse box on the right side of the footwell.

If you have this check if any water got in here.....

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If you haven't already dealt with the possible route cause of this electrical mayhem I would do so quickly .... the drain holes for the sunroof might be bunged with muck and dirt !

Just a thought.


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Had an auto electric guy look at it, he wasn't hopeful!

He initially thought the alternator may have shorted, but quickly discounted that...

It appears that the ignition is permanently on, even with the key removed.....

He removed the ECU B fuse but this didn't kill the dashboard illuminations.

Obviously the immobiliser has kicked in, and although it turns over fine, the security light doesn't extinguish.

The alarm went off in the early morning a few weeks ago, and since then the remote lock and boot release haven't worked. The wash liquid low light came on, and after topping up, wouldn't then go out. Then front left indicator bulb blew...

As I said I had a few drops of water come down above the steering wheel a few weeks ago and close inspection shows the windscreen seal very slightly raised above the driving position. Could this have tracked down to the area above the throttle pedal where I presume the ECU is?

The fuse box under the bonnet seems dry, although the bolt at the back left rail is pretty corroded, the tech didn't want to force it as he thought it might shear.

Where exactly are the drains?


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Drain holes are in each of the four corners with the sunroof open ............ a gentle poke with something thin and unbreakable should push through any muck !


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OK, after a day of no rain, the short which meant the ignition was permanently on (ACC) has gone.

I jumped it from another car, 6 minutes in ACC then OFF, the security light extinguished and she started, a little lumpy for the first minute, but then fine.

I still suspect the windscreen. On closer inspection there are quite large gaps in the upper corners, and the plastic seal is slightly raised above both seats. This must lead to some rust/decay in the bodywork surrounding the screen. I have installed some rubber tape temporarily to see if this confirms my theory.

I wonder wether it is worth having the windscreen removed and re-installed with new rubber.

Having inspected the area in the drivers footwell, everything has been dry throughout. The remote lock/boot release is still inop so this adds to the belief in leaky screen as I believe the alarm/immobiliser ECU is behind the console at the top centre of the dash, and water could easily be tracking into that area.

I bought a complete set of ECU's from eBay for £20, but realise it is not an easy job to simply swap out the ECU for key/security.

Clearing out the sunroof drains is todays task....

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The remote lock / boot release might be as simple as turning the key in the lock alongside it ! I had that problem on mine and someone here advised me that the lock adjacent probably needed turning ! Twas as simple as that !

Thank heavens.

And the drip from the roof lining couldn't really be the screen leaking ... it's above that you say ?

Very likely just the sunroof drain holes. I had that same problem..


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  • 2 months later...

Problem now solved thanks to Stuart at Toyoda Tech, Hampshire, and Maurice (Benfur).

The electrical faults have all been caused by water ingress into a connector deep within the dashboard loom. Removing/repairing this section has cleared all the shorts, immobiliser/remote problems, and also the front right indicator/head lamp faults. When I collect the car I will discover if the steering wheel positioning, seat memory and wing mirror niggles are also corrected.

It appears that the body ECU was getting signals that the doors were momentarily closed then open, This was also causing the key barrel to transmit fluctuating signals.

The windscreen has been replaced by a previous owner with a non Lexus screen and no windshield moulding was fitted. This is the likely cause of the water getting behind the dash.

There is an 8" crack in the tax disc area, so I will need to replace the screen soon, this time I'll make sure of a proper job with new moulding! She sailed through an MOT yesterday with no advisories. Very happy!

I have all the ECU's and relays from a MkIII/MkV if anyone is interested.

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