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Well since I hinted at this about a week ago the response has been overwhelming, what with the innumerable PM's, suffocating media coverage, and herds of paparazzi camped on my front lawn I've decided to come clean and post a how-to.

The problem is the Passenger Seat Sensor in the 93SS can be too sensitive. I'm not sure how widespread the problem is, but the most annoying symptom is the warning activates and stays activated on hot days, at least until the seat cools. Some have also complained that something as light as a paperback book will activate the warning regardless of the weather.

Moving the sensor within the cushion helps, but not in all circumstances. It is also a labor intensive job.

Thanks to Griffin9K and the Saab WIS we have all the info we need on how this thing works. The only function appears to be occupant detection and seatbelt warning. The WIS doesn't say anything about occupant classification (by weight) or any impact on airbag deployment.
http://www.saabcentral.com/forums/showthread.php?t=75795

The sensor grid is a passive resistive device. When unloaded the resistance is unlimited. As load (weight) is added to the seat the resistance drops. The WIS says the system uses 47 ohm as a threshhold. Anything below that and the seat is considered occupied. Based on my measurements, I think they really mean 47K ohm. So adding resistance to the circuit is a way to raise the amount of weight you can place on the seat without activating the warning.

The kicker is the grid also contains a diode. At startup the car sends a burnoff current through the grid to clean the contacts. After 600ms that current is reversed and passes through the diode. The WIS doesn't tell you, but this reverse current is also used to verify that the sensor is actually plugged in. Otherwise you could disconnect it and never get any warnings.

I made a simple adapter cable which plugs in under the seat. Reach under the rear of the passenger seat. The lead for the sensor comes off the back middle of the cushion and plugs in under the seat. Very easy to locate.

The plugs to make your cable are the common two-lead connectors used throughout the car. Anyone who has upgraded the stereo will have extras connected to the stock speakers (provided you didn't throw them out). Each tweeter has one male and one female connector. That's all you need (as pictured below).

In my case I determined I needed an additional 20-30K ohm in the circuit. I used two 10K resistors and added another 10K trimmer (not in the picture). When finally tuned I ended up with 28K ohm.

The diode is the most important part. It gets added on the side of the male plug (left in the picture) with the cathode facing the #2 wire. I soldered and shrink wrapped the wires, then used electric tape to wrap around the whole thing to give it more strength. The adjustment screw for the trimmer sticks out to allow tuning.

The resistors and diode came from RadioShack. The resistors are 10K ohm 1/4 watt. The diode is a 50V rectifier diode (1 watt I think). The 10K trimmer is the rectangular kind, also rated for 1 Watt or so. This project ran me about $4.00 in parts.

I can now put rather large objects on the seat without activating the warning. The weight to activate the warning is probably about 30 pounds, but shape is also a factor.

If you want to completely deactivate the warning, you could just solder a diode to a male plug and be done with it. Just plug it in and tuck the dangling (unplugged) sensor cable under the seat.
 

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Gee, Ctrlz, you must be RadioShack's first customer in 25 years for resisters and diodes. I didn't even know they still carry that stuff. All I see in there is toys, oudated boom boxes, and ipod accessories.


Barry

'04 Nocturne Blue Arc SS
 

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Good job on figuring that out. Way over my head. I may have to try this some day. Luckily it doesn't go off in the heat or randomly, but my 2 lb lunchbag sets it off.
 

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Awesome work ctrlz and thank you for your hard work, we appreciate it.
 

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Discussion Starter #5
MarkJ said:
Awesome work ctrlz and thank you for your hard work, we appreciate it.
Thanks, Mark!

This is actually pretty easy to build. Took me less time than adding the AUX cable.
 

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This circuit works like a charm!

Thanks ctrlz for working this out. I finally got around to trying it out and it works like a charm!

To add my $0.02 to the thread I've done a quick and dirty schematic that might help anyone else wanting to try this out. I have laid out this drawing in exactly the same orientation as ctrlz's work-in-progress photo.

http://www.coolchameleon.com/saab/circuit.pdf
 

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Thanks for reminding me of this. I added it to the FAQ up top. Thanks again to ctrlz.
 

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As i don't happen to have any connectors on-hand... Anyone have spares? Or maybe the part numbers so we can get them in bulk and mass-produce the adapters for those less inclined.
 

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Discussion Starter #10
kitlaan said:
As i don't happen to have any connectors on-hand... Anyone have spares?
If you have the 13 speaker setup, I strongly recommend you replace the front left and right speakers.

That's easier than building this circuit, and you will have two sets of connectors to play with. Plus your stereo will be much improved!
 

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Discussion Starter #12 (Edited)
JonV said:
Any particular specs on the diode to disable it....my dog or laptop sets it off every time.

thanks,

jv
I used a 50 V rectifer diode from RadioShack. They are sold in packs of two for about $0.79.
I can post the part number later.
 

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ctrlz: Is there a web site that allows you to make simple schematics, or do you have a special application?
I just did a search on the internet and found Diagram Studio from a company called Gadwin. It has a 30-day free trial. It has a pretty good library of ready-made symbols and it is very easy to use.
 

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Found this old thread when I was searching on how to solve my passenger seat warning.

Wonder if this modification does solve the warning completely? and it does not leave any damages the fuse box or affecting other electrical component due to the not regulated power circuit.

Thanks.
 

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Gee, Ctrlz, you must be RadioShack's first customer in 25 years for resisters and diodes. I didn't even know they still carry that stuff. All I see in there is toys, oudated boom boxes, and ipod accessories.
Some Radio Track stores don't even carry basic electronics components. I don't even bother checking Radio Trash any more - I just go to digikey.com or jameco.com and order exact replacements (for example, caps matching physical dimensions and pin spacing; not just values).

Radio Trash's new motto: "You've got questions? We've got blank stares"
 

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have any of you taken your saab 9-3 to the dealer to get
the passenger seat belt fixed? if yes how much did they charge to fix it?

Mine sensor was going in and out now and then and now it thinks someone
is always seating in the passenger seat

and its very embarrassing to me and my Saab to have the seat belt
plugged in with no one in the passenger seat :(

Thanks
 

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Would you be interested in building and selling this to a fellow Saab owner? Not all of us have the electrical background to build and solder diods. It would be worth $20-$30 just buy this part from you and plug it in on my own.
 
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