1999 GMC Sierra Repair Question
What could be the cause of my speedometer and my fuel guage needles periodically ceasing to maintain the current reading whiole driving?
Older trucks used to have a board to smooth out the pulsing of the fuel gauge due to sloshing in the tank; in fact, they called it the "anti-slosh" module. That won't affect the speedometer.
Instead, based on your observation of the ABS and battery lights, you might consider having the charging system tested, but that has to be done while the problem is occurring. The generator is well-known for developing huge voltage spikes and that will get much worse when one of the six diodes fails. One third of its current rating is all you'll be able to get, and when that can't keep up with demand, "ripple" will be extremely high. That is rapid fluctuations in voltage caused by one of the three output phases of current missing. Many computers are intolerant of low voltage, and the ABS Computer always seems to be one of the first to be affected, hence the yellow warning light.
Diodes inside the generator are usually not intermittent, but they could be. A failed or intermittent voltage regulator would be more likely, but regardless, the charging system load test will measure output current and ripple voltage. If the test does point to a generator problem, replace the battery at the same time. As they age they lose their ability to dampen and absorb those harmful voltage spikes. The old battery will work fine in a 1986 or older model with the better generator design.
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