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Transmission OD Problem Update


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A while back I posted a problem about my GM 4L80E Tranny refusing to shift into Overdrive (stayed in 3rd) after hill climbs (but that trip was near Smokies with lots of hills), so I had the tranny and torque converter power flushed and refiled with Synthetic Fluid BUT THAT DIDNT HELP. However I just returned from a 5000 mile trip during which I had the opportunity to "experiment" and have more data to tell the Transmission shop. Here's the deal: NOTE it turned out it is NOT due to hot day extreme long hard steep hill climbs (and possibly overheating) as I previously thought, I was NOT towing this trip and this can happen even on short easy grades on a cool day as long as its steep enough so it has to shift down to 3rd for any time at all, afterwhich it simply will NOT go back to OD. BUT GET THIS if I stop and turn off the engine as little as 30 seconds then restart and take off again IT WILL SHIFT INTO OD. My cheap code checker found one not seen before stored code "Manufacturers Control" or something like that??


I've done some looking on GM and 4L80E Forums and found it may be something electrical based and relatively cheap and simple like the TPS or VSS or it can be more mechanical and expensive such as the TCC Torque Converter Solenoid or OD Clutch pack.


Since a simple turn off the engine and restart seems to reboot or reset the computer/tranny allowing it to then shift to OD until a relatively long hill forces a downshift to 3rd, I'm hoping its an electrical problem (like TPS or VSS) versus OD clutches ???????????????


I don't have the patience to fix it myself or throw parts at it so its to the experts I'm headed and hoping for the best. When I was driving I sort of over rode the Cruise to get a down hill run for the hills then let her coast over the crest to avoid a downshift to 3rd, but when the little woman let Cruise do the work it would have to shift to 3rd and then maybe NOT go back to OD grrrrrrrrrrrrrrrrrr


Oh well as Jimmy Buffet says, "If you ever wonder why you ride the carousel, you do it for the stories you can tell"

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29 Ft Class C Chevy Motorhome with 454 Vortec and 4L80E OD Tranny. The possible problems I mentioned above are a few of what I found in a quick forum hunt, unfortunately, they may be wrong as rain and the repairs much more complex then an electronic sensor or control that needs replaced grrrrrrrrrrrrrrrrrr


Tomorrow my mechanic is going to scan it with his $4,000 Snap On Analyzer. He said my cheap code checker saying it was MANUFACTURER CONTROL may mean the tranny and engine computer are communicating and getting a (perhaps false high temp) signal its hot and to lock out overdrive which a new tranny temp sensor (if old one is at fault) just may cure KEEPING FINGERS CROSSED UNTIL TOMORROW



John T

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