![]() Some have tried using a park-neutral switch from a 4L60E. If you work on these units, you should be in the habit of having one of these components on the shelf. If you have access to an Allison dealer, it may be a bit cheaper. The switch is costly if purchased from a GM dealer. It has redundant circuitry to let the TCM know if a wire or contact has failed. Any minor discrepancy in the electronic flow of this switch can and will throw the unit into limp mode. That TCM that’s on the fan shroud is very protective of its transmission. It’s been around for a long time and for those who haven’t seen too many of them you’ll want to pay close attention.īesides keeping the engine from starting, the NSBU can keep the truck from moving after it’s started. ![]() One such case id GM’s NSBU (Neutral-Start-Back-Up) switch used on the Allison LCT 1000 series. But a neutral condition? If you’re faced with this condition you might easily take off in the wrong direction. ![]() These concerns are simple to spot and we all know what to do if we see them. For the most part it prevents an engine start unless the shifter is in Park or neutral, it turns on the reverse lights and might interfere with upshifts, or prevent any upshifts at all. Some are on the outside of the unit, some are on the inside, either by itself, or it may be part of a MechTronic unit. View attachment 278779 View attachment 278780What we’ve known as the Neutral Safety Switch has changed over the years and has had various names such as the Park Neutral Position Switch, Internal Mode Switch, Range Sensor, and a host of others. I can't be without a truck for that long, so I am trying to do everything first. I spoke with the local Allision warranty shop who does not take appointments and he said to leave the truck with him for 4 days before they can look at it. The truck has 35000 miles on it and the transfluid level is ok with 79% oil life left. The trans-temp runs in the normal range - usually about 175 degrees unless we have a heavy load, then never higher that 225. Here might be another factor: the PTO has a leak (and has since been replaced) and we were having to add about 2 quarts of transmission fluid daily until it was fixed. If it's not the sensor, and it's not the wiring, and it's not the fluid level, then what? It's usually towards the end of the day, but not always. One day it will do it once, and the next day it will do it 8 times. So far, it looks like it might only be after 4 hours of driving. There doesn't seem to be any rhyme or reason to it. We checked the trans fluid and filled it, then it didn't seem to do it again. One day, we were able to make it repeatable by hard braking. It happens on the highway, or on city streets. Sometimes the driver thinks he hits a bump and then it happens, or sometimes it's while hard braking, or sometimes it's while gently accelerating, or sometimes it's just while moving. The truck could work for another few hours, or even a day before the problem comes back, or, it could happen 8 times in a row within a few minutes. The driver pulls over and turns the truck off, waits for a minute and turns the truck back on. ![]() Here's what happens: While driving, a gear icon shows in the dash and the allison gear selector shows the selected gear on the right, and nothing on the left leaving the truck essentially in neutral (but it does not say "N"). The wires were slightly tight so we adjusted them to give them more slack. Still no problem - we checked the wiring all the way up to the transmission control module. The truck went 24 hours afterwards with no issue, and then we still had the same problem. The error it gives me is an output speed sensor fault. The third (and newest) 2019 T880 has developed a problem where the transmission goes out of gear very sporadically. ![]()
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