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PostPosted: Wed Jun 12, 2019 5:42 pm 
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Joined: Wed Jun 12, 2019 5:36 pm
Posts: 21
Location: Ireland
Car Model: 1992 Saab 900 s
Hi guys,

Just after buying a nice 3 door black 1992 saab 900 s, beautiful car but a few niggling electrical problems, one of which is that the fasten seat belt light along with the automatic heated seat stays on regardless if someone is sitting in them or not. The only way to put out the light is to clip in the seatbelt.

Where do i start diagnosing this problem? Sensor under the seat perhaps?


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PostPosted: Wed Jun 12, 2019 7:14 pm 
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Light Pressure Turbo

Joined: Mon Aug 22, 2016 11:23 am
Posts: 345
Location: Leicestershire
Car Model: 1992 900i, 1984 T16S “Snufkin”
Skooch under the seat for the belt light switch and the seat heater wiring. May need to pull some carpet back at the seat front as well.


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PostPosted: Wed Jun 12, 2019 9:27 pm 
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Joined: Wed Jun 12, 2019 5:36 pm
Posts: 21
Location: Ireland
Car Model: 1992 Saab 900 s
I had a look underneath the passenger seat (which i think is the culprit as when i put my seatbelt on in the drivers side the light doesn’t go out until i clip in the passenger seat belt) i see a few different wires, What am i look for here exactly? I’m assuming the fault must lie with the sensor pad in the seat itself maybe over sensitive? Oh and there are 2 strange wires with back plastic things with metal clips dangling from underneath the seat. What are they for? Thanks for your help!


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PostPosted: Thu Jun 13, 2019 8:12 am 
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Joined: Wed Jan 24, 2018 4:36 pm
Posts: 206
Location: Orpington, Kent
Car Model: 1987 SAAB 900i
Ah, the black plastic things with metal clips, those are your pressure switches! They are unlikely looking aren't they?

I have a spare passenger seat at work, I will take photos today when time permits. The plastic clips are connected to a chain either side which is clipped into the seat springs. The logic is, when the seat is unoccupied, the plastic clips stretch (I think they are spring loaded) and the circuit is opened. If someone (or a bag of shopping) gets in the seat the chains are relaxed and the contact closes, making the light come on if the seat beat buckle switch is also closed (i.e. no seat belt fitted). It's just two switches in series, both have to close to get the lamp on.
The seat heater wiring (if fitted) works the same way.
Often the plastic bits get stuck, or the chain comes loose. What catches me out every time is that the switch is 'illogical', a relaxed chain means the seat is occupied.

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PostPosted: Thu Jun 13, 2019 12:14 pm 
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Joined: Wed Jun 12, 2019 5:36 pm
Posts: 21
Location: Ireland
Car Model: 1992 Saab 900 s
CaptainBoom wrote:
Ah, the black plastic things with metal clips, those are your pressure switches! They are unlikely looking aren't they?

I have a spare passenger seat at work, I will take photos today when time permits. The plastic clips are connected to a chain either side which is clipped into the seat springs. The logic is, when the seat is unoccupied, the plastic clips stretch (I think they are spring loaded) and the circuit is opened. If someone (or a bag of shopping) gets in the seat the chains are relaxed and the contact closes, making the light come on if the seat beat buckle switch is also closed (i.e. no seat belt fitted). It's just two switches in series, both have to close to get the lamp on.
The seat heater wiring (if fitted) works the same way.
Often the plastic bits get stuck, or the chain comes loose. What catches me out every time is that the switch is 'illogical', a relaxed chain means the seat is occupied.


Talk about an Eureka moment! God i love this forum. That would be fantastic if you could send a photo of what way they are meant to be hooked on. Mine are literally dangling down and there is a hook at the end so i kind thought that it should be hooked into something! A bit hard to visualise exactly how they operate but if you could post a photo of what part of the seat it’s meant to hook into I’ll be forever indebted!


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PostPosted: Thu Jun 13, 2019 2:16 pm 
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Joined: Wed Jan 24, 2018 4:36 pm
Posts: 206
Location: Orpington, Kent
Car Model: 1987 SAAB 900i
Photos to follow shortly, I'm just about to have lunch at work (which is my chance to go to our storage area/SAAB storage.

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PostPosted: Thu Jun 13, 2019 2:29 pm 
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Joined: Wed Jun 12, 2019 5:36 pm
Posts: 21
Location: Ireland
Car Model: 1992 Saab 900 s
I’ve manged to get the light to go out by hooking back up the 2 plastic things to the seat underneath but now it’s not coming on when i sit in the passenger seat. I’d say i have to find a sweet spot but it’s completely illogical how to the spring operates. Surely when you sit down the spring should strech out and when you get off the seat it should relax in a bit?? I can’t get it right in my head??


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PostPosted: Thu Jun 13, 2019 11:21 pm 
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Joined: Wed Jan 24, 2018 4:36 pm
Posts: 206
Location: Orpington, Kent
Car Model: 1987 SAAB 900i
Quote:
it’s completely illogical how to the spring operates. Surely when you sit down the spring should strech out and when you get off the seat it should relax in a bit?? I can’t get it right in my head??


I know, I spent a couple of hours going round in circles when I had a similar issue! Sorry it's taken so long to get pictures. About two minutes into my lunch hour, it became apparent that I wasn't going to get any time to eat a sandwich, let alone go searching for my spare seats. I've come back in tonight to get the photos.
I'm guessing that as you have a 1992 model, you've go the newer seats that have the seat belt stalk attached to the seat frame, rather than the older ones that are in the centre console? This can easily be determined by moving the drivers or passengers seat forward or back, if the seat belt stalk moves with the seat, you've got the newer style. (I don't know which year that this design change happened.) Anyway, below are some pics of a passengers seat removed from the car:
Attachment:
Whole seat.jpg
Whole seat.jpg [ 261.54 KiB | Viewed 320 times ]

Attachment:
Seat Base.jpg
Seat Base.jpg [ 336.85 KiB | Viewed 320 times ]

Attachment:
Close up of switch and mountings.jpg
Close up of switch and mountings.jpg [ 366.51 KiB | Viewed 320 times ]

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PostPosted: Fri Jun 14, 2019 12:55 am 
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Joined: Wed Jan 24, 2018 4:36 pm
Posts: 206
Location: Orpington, Kent
Car Model: 1987 SAAB 900i
The next picture is with the seat empty (difficult to replicate with the seat upside down!)
Multimeter shows an open circuit.
Attachment:
Open circuit, seat unoccupied.jpg
Open circuit, seat unoccupied.jpg [ 248.14 KiB | Viewed 315 times ]


The next picture shows the seat occupied, (well, me pushing the seat from underneath) therefore pushing the seat springs 'down' towards the base, this shortens the distance for the chain and closes the circuit.
Attachment:
Closed circuit, seat occupied.jpg
Closed circuit, seat occupied.jpg [ 260.58 KiB | Viewed 315 times ]


The black plastic switch should hook onto a mounting point at the very front of the seat, next to a plug where the wiring goes to the rest of the car. The long hook with a U shape in it has a little divot in one of the seat springs to hook onto. Pictures above refer.

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PostPosted: Fri Jun 14, 2019 9:37 am 
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Joined: Tue Oct 30, 2007 9:26 pm
Posts: 6127
Location: North Essex
Car Model: 900Ruby,vert, 9000, 99, Sonett
Apologies for pinching one of Captain Broom's pic... :oops:

The springs on the seat stretch with age, this means that the switch 'senses' that there is someone sitting in the seat when there is not. The sensitivity of the switch can be adjusted by using some pliers to squeeze the U shape on the hook so that the switch opens when there is no one sitting on the seat.

Attachment:
Untitled.jpg
Untitled.jpg [ 360.02 KiB | Viewed 298 times ]

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PostPosted: Fri Jun 14, 2019 3:23 pm 
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Joined: Wed Jan 24, 2018 4:36 pm
Posts: 206
Location: Orpington, Kent
Car Model: 1987 SAAB 900i
Quote:
Apologies for pinching one of Captain Broom's pic... :oops:

Never a problem, if a picture is here, I'm happy for it to be used. There's not a lot of travel between an open and closed switch (about 8 mm).
Ah-ha! That explains what the U shape bit is for. 8)

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PostPosted: Fri Jun 14, 2019 5:43 pm 
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Joined: Wed Jun 12, 2019 5:36 pm
Posts: 21
Location: Ireland
Car Model: 1992 Saab 900 s
Guys ye are pure legends, great pictures captain boom it really, it was so hard to visualise what way to hook it up until i seen them. I was hooking it to the fabric!i actually have 2 little sensors in mine, I followed your instructions and now my seat light is working 100% again! This forum is awesome! And the best thing is the next person with this problem can see how to fix it because i literally searched for hours how to fix this and there was nothing on the web. Cheers guys!


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