On the 1990 4runner, the rear ABS light should be on ALL the time.
This indicates that it is working. If it just came on, their maybe a
problem with its wiring or the bulb. Can't say if this indicates a
problem with he ABS system though.
If this is like my Tacoma, the idiot light for the ABS system will come on
if one of the sensors at the wheels fails or the ABS system has been
disabled. The ABS will be in disable mode if the light is on. I had to take
my truck to a dealer so they could check the codes and see which sensor had
failed. I had to replace the sensor on the front, drivers side wheel. I
never could get used to ABS and had a couple of close calls because of it.
Rolled into the middle of intersections twice where I know I could have
stopped easily without ABS so I disabled the ABS by removing the fuse for
it a couple of years ago. My ABS light is always on so that drivers are
aware that it is not working.
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