I recently replaced my alternator fuse and ever since the air bag light to the left of the temperature and fuel gauges stays on. What is the relationship? Did something happen to the sensor?
I am assuming there is a connection between replacing the alternator fuse and the air bag light staying on since it happened immediately afterward. In general though what would cause the air bag light to stay on, a bad sensor, etc.?
Let's go back to your alternator first. What do you mean, replaced your alternator fuse? What specific fuse did you actually replace (where located, and what number)? Do you know why this fuse went out?
It's possible your alternator diodes are going out, which causes the alternator to put to much AC voltage out, above and beyond the DC voltage it should (called ripple voltage). That could cause sensor failures, like the center airbag sensor assembly located under and just forward of your shift handle in your console. You can run your car over to a place that tests electrical systems on cars for free (Advane Auto Parts, for example), and their tester should check for AC ripple voltage coming from the alternator. If it's too high, I will tell you of an alternator problem.
Even if this is the case, and you replace your alternator, the center airbag sensor assembly may have been damaged, requiring replacement. You can get them from a salvage yard (but you need to get one from a car that did not have its airbag deployed).
I replaced the 100 amp alternator fuse in the junction block next to the battery. I also replaced the alternator because it was intermittently charging and then stopped charging altogether. That's when I found the blown fuse.
I am assuming the reason the fuse blew was because the alternator was overcharging at some point. Isn't the fuse supposed to protect the electrical system, sensors, etc. from damage?
The fuse protects the against an overcurrent situation, as the fuse is rated in amps, not volts. A fuse doesn't know the voltage of the system, it just knows the amount of current passing through, and due to resistance in the fuse, if the current gets to high, the fuse element gets to hot and opens, protecting downstream items.
That said, your alternator should have been either a 70 amp unit (if you have a manual transmission) or an 80 amp unit (if you have an automatic). An alternator should be able to put out 100% of its rated current in the event the battery is nearly dead, and the fuse allows that. Because your fuse blew, it's almost a sure sign (assuming there were no other major electrical defects in your system) that the ripple voltage from your alternator was too high, so you were putting out too much AC voltage in the system (assume the alternator put out 14VDC steady, but had a +/- 3VAC ripple, so your electrical system was seeing 11V on the low side, 17V on the high side, and that alternated 2 or 3 times for every rotation of your alternator. So if your alternator could put out 80 amps at 14VDC, then with a ripple voltage of +/- 3VAC, at 17V, you would be surging 97 amps through your system momentarily. That's pretty close to your 100 amp fuse limit. A normal alternator should have a ripple voltage around 50-100mV (0.05-0.10V). When alternator diodes start failing, bad things like what you experienced can happen.
If this is what happened, you should have been hearing a relatively loud whinning noise from the alternator, especially at lower engine speeds, and with it replaced now, it should be silent. With your old alternator, your battery should have been getting hot (constantly cycling between getting a charge & then discharging). This condition is very hard on batteries. Your battery tries to act as a big buffer to a bad alternator, but it can't completely eliminate ripple voltage. So I'm willing to bet your your diodes in the alternator were failing, you got ripple voltage, and you should consider yourself luck only 1 component failed before your fuse blew.
Now I don't know if this is what happened, but if you had your alternator checked after you removed it, or if you still can, you can confirm what exactly is wrong with it.
But to answer your question, is there a relationship to experiencing a blown main fuse and having your air bag light on, the answer is yes, because of what likely was going on with the old alternator as its diodes (voltage regulation system) was failing. There aren't too many vehicle sensors that can handle an alternating current from 11V to 17V multiple times a second and survive.
I will change the air bag sensor when I get a chance.
I was fortunate not to have any other problems with the electrical system when the alternator failed and the fuse melted. I never noticed a whining noise or overcharging indicated at the gauge but who knows. Incidentally the alternator was the original from 1990 with 186,000 miles on it.
Nah, just make one of these little guys with stuff you can grab at Radioshack. I have 3 Celicas, and this not only resets their airbag light when I work on electrical. It also gives me a code readout for other problems. $20 buys you the parts needed.
My airbag light had been on for over a year, and I finally got around to making one. If you follow the directions, it works like a charm.
There was an older technique that essentially recreated what you are doing with the button/switch. You had to move the wires quickly from port to port like you were drumming. It was really ridiculous. This technique is so much nicer.
I have a 1990 Celica parts car that never deployed the air bag so I will try the sensor from that when I get a chance. Are there any special considerations when removing it?
Replacing the sensor is a waste of time. The airbag light is on because there is a code stored in the ECU. When power is lost to the system, it trips this code warning you something happened, and the airbag might be compromised.
If all you did was unplug the battery, alternator or blow the 100 amp fuse, the airbag is fine. The sensor is fine. You just need to reset the warning code.
Use the device above, or locate a dealer that somehow still has their OBD-1 computer to do it for you for a marginal cost.
Replacing the sensor and tampering with your airbag safety system might be a crime in your area if you are not a licensed and certified professional. Worst case scenario you could accidentally set off the airbag while replacing the sensor, or it could go off sometime after. Like, when you are driving. Leave the airbag system alone and just reset the code.
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