Heres some info on the early 4L80E transmission problems. I thought it may help someone out.
The 4L80E is being found in more and more high performance, race, and tow vehicles over the last few years. And it's no secret why. 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. 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. As such, most of the 1991-1992 454SS trucks came with 4L80E transmissions that had faulty parts from the factory. Most of the problems are electronic in nature and are relatively easy, but expensive, to fix. This article deals with fixing your factory 4L80E to make it as durable and reliable as it should have come from the factory. The following is a list of problems and possible solutions, should your 4L80E suddenly start to give you trouble.
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. 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. 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. 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. This design allows tranny fluid to seep into the sensor and damage it. The sensor with the updated, completely sealed design may be purchased from your local GM dealer for a cost in the mid-$30 range. When changing it, put a pan under the tranny to catch the fluid that will leak out when you remove the sensor. The bolt that retains the sensor is 10mm.
P-R-N-D work, upshifts mostly work, no downshift into 1 from 2 or to 2 from 3, "stuck" in a certain gear: Pull out the wallet and grease pan. You have to replace the solenoids that control the 1-2 and 2-3 shifts, the "A" and "B" solenoids respectively (the 3-OD shift is not controlled via a solenoid…more on that later). The problem with these are that the solenoid intake port has no screen, which means any debris floating around in the pan can get into the very small inlet hole and jam the solenoid open, forcing fluid flow, which prevents the plunger from sliding in the valve body and hence prevents a downshift to the next lower gear. The updated designs have screens over top of the inlet ports, preventing crud from clogging the solenoids. These solenoids are found on the front of the valve body and are easy to get to once you drop the oil pan. The bolt that secures them are Torx head bolts in metric thread, so be careful removing them, as they cannot be replaced with standard bolts. Don't worry about cross wiring or installing the wrong solenoid in the wrong shift circuit when you install the new solenoids. Both use different styles of clip-in connectors so the wiring cannot be crossed, and due to the mounting tab location, the A solenoid will not fit the B port and vice versa. Each solenoid will cost about $40.
No shift into OD under acceleration; OD will not hold under load and can hear or feel trans shift to OD when throttle is let off but slips into 3rd when throttle applied; sudden erratic lugging condition in lower gears when manually shifting: Bad news: this is going to involve a lot more time and money to fix this problem. The torque converter needs to be replaced, which requires between $300-$500 for a replacement, lockup, 4L80E converter. It is imperative that the replacement converter you choose is a lockup converter. The transmission will not hold in OD when there is a load on the drivetrain if the torque converter will not lock. The torque converter locks up (to make a solid, non-slip link between the engine and trans) in an effort to increase fuel efficiency and thereby reduce emissions on highway driving trips, when there is no heavy load on the vehicle (trailer or heavy weight in the bed). But, OD is a cruise gear, not a pulling gear, and as such to prevent the OD from burning out (from constant 3-OD up and downshifting and straining the OD with a 9,000 lb trailer) when towing something under load, the trans will force itself out of OD into 3rd when it detects that the converter is not locking up. The factory converter has an internal clutch that controls the lockup of the torque converter, and this clutch's torque capacity is minimal. Hence, if the clutch is going bad, just the weight of the truck and torque required to pull it will force the TCC to unlock the converter and a downshift to 3rd will occur. The lugging in lower gears (mostly first) is due to the converter locking up prematurely. Additionally, if left long enough, a look in the trans oil pan will reveal burnt fluid and black sediment. If there is black sediment in the pan before you notice drivability problems, it will soon be time to replace the converter. To replace the torque converter, you must either get the truck to a shop or have at least 2 jacks handy, one of which will be a transmission or motorcycle jack. An extra person to help out is a great asset, as the transmission weighs close to 200 lbs. This repair will take you about 6-9 hours to complete if you know what you're doing. Remember to fill the converter with as much fluid as you can before you install it. Don't worry about it all spilling out upon turning it on its side during installation; the fluid is slow going in and is just as slow coming out. At the very least, this is a good excuse to upgrade to a new, high performance converter with a little more stall speed (and a valid justification to the wife to spend $500 on another performance item!).
There are other problems that I've not had the joy of experiencing first hand however. In addition to the shift solenoids, the TCC, PSM, and force motor solenoids are also suspect of being defective (see above). These are located on the valve body and are as easy to get to as the shift solenoids-but much more costly. The force motor solenoid alone is in the $200 range. So, leave them alone until you are sure those solenoids are defective.
Lastly, the direct clutch pack spring washer was suspect of poor manufacture. This lead to the washer breaking up into little pieces, which would jam the direct piston, thereby damaging it and the direct clutch. The direct clutch is located in the transmission main body. A diagram for you:
1992 GMC F 6.5L TD 2wd 4L80E 253,000 miles, manual torque converter lock up switch, rear air bags, 2.5" crossover, 3" down pipe, 4" straight piped, remove key while running mod.
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