This design allows tranny fluid to seep into the sensor and damage it.
The input speed sensor (and output speed sensor, mainly used for vehicle speed calculations, both of which carry the same part number) is suspected of failure, according to a GM bulletin, due to its poor open-magnet design.
4L80E TRANSMISSION DRIVER
Your transmission uses two main sensors to gather information on when to shift, which is the TPS (throttle position sensor), located on the throttle body, and the input speed sensor, located on the driver side gear case about 8 inches up from the pan, to the rear of the bellhousing. In addition to erratic shifting, the trans control program will default to backup shifting mode if you drive it long enough with this bad sensor, and as a result line pressure through the valve body will be boosted to maximum value, creating harsh shifting. P-R-N-D work, upshifts are erratic and occur at random engine speeds, will not upshift from first gear (no upshifts at all): This problem seems very serious in nature but in fact this is something very simple to fix. The following is a list of problems and possible solutions, should your 4L80E suddenly start to give you trouble.
This article deals with fixing your factory 4L80E to make it as durable and reliable as it should have come from the factory. Most of the problems are electronic in nature and are relatively easy, but expensive, to fix. As such, most of the 1991-1992 454SS trucks came with 4L80E transmissions that had faulty parts from the factory. But while the 4L80E is a strong, heavy-duty transmission, during the first few years of production the 4L80E was not as durable as its design permitted due mainly to faulty parts. The 4L80E, an electronic, overdrive-equipped version of the venerable TH-400, is tough enough to handle just about whatever hell you put it through when built properly. The 4L80E is being found in more and more high performance, race, and tow vehicles over the last few years. Heres some info on the early 4L80E transmission problems.
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