False seat belt alarm

[brendangeorge]

Has anybody had the seat belt warning alarm sound when driving?  I don't mean because you did not plug in before setting off.  I don’t think the false alarm is a collision sensor fault – it is the seat belt alarm – I tested this.

I took everything off the seats and the alarm still kept going off intermittently.  This was annoying when on a motorway.

I stopped and plugged in all the seatbelts and the alarm stopped. I drove around for some time with all the belts plugged in.  I asked the dealer to investigate at a service (may have been last November).  The guys did not find any problems – however, the alarm stopped so I think they tinkered a bit – great outcome but no RCA. That is until two days ago – the seat belt alarm went off and stopped (after a few mins) a few more times. So the belts are all plugged in again and the alarm is quiet.  Anyone have any ideas?

[m4rkw]

Under the passenger seat there will be an occupancy sensor, these can sometimes go bad and cause this kind of problem. If you're lucky it'll just be a loose connector, not sure about Lexus but on BMWs I used to own this was very common. When the seat is moved back and forwards it gradually wears out the cabling and causes a loose connection.

[m4rkw]

Saying that, I'm not sure if it has one. Other Lexus models have an occupant detection computer listed on this diagram but the 450h doesn't seem to have one:

http://europe.toylexparts.com/lexus/521230/agl10r-awtgkw/430w/009/4/8421/84a243/89178A

[m4rkw]

It's there for US models but not the european ones. It must have some kind of occupancy sensor though or there wouldn't be anything to trigger the seatbelt alarm. You could try scanning it for codes with techstream to see if there's anything logged. My guess is a sensor or sensor connection has failed somewhere.

[cruisermark]

I can only add: my old LS430 had an intermittent airbag light problem when the passenger seat was used - when I was investigating this problem I found that it could be a faulty Occupancy Sensor - but apparently its buried deep in the seat somewhere.

On my Mark 4 LS400 if the passenger was sat without the seatbelt done up there was a slight IIRC

So it is a system definitely used on older LS

[m4rkw]

Yeah I found the occupancy sensor/computer part on toylexparts for various other lexus, including US variants of the 450h. Couldn't seem to find it for the euro ones but that doesn't necessarily mean it isn't there.

You can actually turn off the seatbelt alarm with techstream, I did this on my wife's rav4 because she was often driving around her allotment at low speeds where there's no point putting a seatbelt on and the chimes became annoying.