The 90 is infamous for this light staying on. It seems that there are two sensors, one on the frame underneath the battery box, and one underneath the windshield washer container. Both bolt directly to the frame. These little electronic wizards detect a crash via vibration in the frame, and then set off the air bag.
The problem is two fold. First, their location promotes getting water on them. Secondly, the water causes a corrosion problem between the sensor and the frame. These little wizards are so sensitive to corrosion, 1/2 of one ohm of resistance that develops between it and the frame will send a fault light code to the computer, and trigger the trouble light. Most of the time, the air bags are still functional, just the light is on.
How to fix the problem.
First, let's check the codes in your computer to make sure this is the problem. Using a long jumper wire, jumper pin # K on the computer link connector to ground, like the seat frame. Pin K is the fouth slot from the left on the bottom row, or third from the right. (The connector is under the drivers side dash). Turn on the ignition, but don't start the car. Via flashing engine light, you will be able to read the codes in your computer. It will first flash a code 12 three times. The light will flash like morse code, a long lit bulb, followed by two short lit bulbs will be a 12. Once the three code 12's flash, then you will have another code, likely it will be a 25 or 26, but could also be a 15 or 16. ( , Mine (Jay) was a code 16, Indicating right side sensor) If it is not one of these, you have a different problem, so let me know if your's is not one of these. Code 26 is most common, right front sensor fault.
Assuming you have a 15, 16, 25, 26 code, you will be following the service bulletin #90-436-9J. To fix, you have to unhook the battery. Next, remove the side panel and inner fender well cover from the front tires. (Note, I also had to remove the tire to get to the bottom 2 torx screws) Then you can unbolt the sensors. Next sand the frame and back of the sensors with sandpaper, and then install di-electric grease (I found permatex tune-up, di-elecrical grease at Pep Boys) on the back of the sensor, then re-install it.
After this is done, you can then reset the computer. Now this is where the dealers have all of the problems. Since it is very difficult to reset, they usually replace the sensors with new ones, so they dont have to go through this. But if you can accomplish this, it will save you bunches of green paper!!! :)
Taking a jumper wire hooked to terminal K, ground it to the seat frame, and count one thousand one. one thousand two, one thousand three. Then unhook it from ground, count one thousand one, one thousand two, one thousand three. Then reground it once more, and count one thousand one, one thousand two, one thousand three. The code will disappear, leaving only a code 12 if you are successful. If not, retry it again and again. A friend of mine did it 10 times before it was successful. But, think of the money you would have to pay a mechanic to do THIS!!! :( ( , this is where I had the most trouble, until I got an email from Dan's friend and late-model guru, Ed.
Here is the secret....With the ignition off. Place the jumper wire in the K terminal and ground it to the seat frame...Switch the ignition on and AS SOON as the light begins to flash, remove the jumper from the frame and count 1001, 1002, 1003. Then reground it and hopefully you will only see code 12's flashing. I spent an hour yesterday doing it wrong and tonight I got it the first try using Ed's method. Anyway, on with Dan's instructions) The A/C condensor is what drops water on this sensor, and causes the problem.
In 91 they used di-electric grease, and the problem was fixed. As a precautionary note, do not use a volt ohm meter to do any troubleshooting with this system. The battery in the ohm meter will set off the air bag system. This is how mechanics get really in deep s@#* with their boss! :) As long as you can refrain from using a meter, you will be ok.
Author: Jay Meeks