Gary Gable
Newbie
Hi Folks,
I have a '91 C3500 GMC Sierra with a 4L80E with 70K original miles on the truck. Bought surplus from a nearby municipality at 56K. About 6 months ago I began experiencing a loss of 4th gear. I took it to 2 different reputable transmission shops. Since it was intermittent and evidently not throwing a code, neither could adequately shed any light let alone fix the problem. Over time it got worse, up until the point where it happens every time it's driven. Although I originally thought it was just 4th that was lost, I soon realized it was all gears except 3rd and reverse that went away.
Faced with this, and not particularly delighted with the service provided by the aforementioned shops, I chose to get a kit that replaced the internal wiring harness (with the '93 & later spec) along with the internal solenoids, force motor, temp sensor, etc, as well as both external speed sensors. I upgraded the external plug to fit this harness and was very careful to correctly splice onto the old harness. BTW: The oil the pan was clean, no debris or bits of warn parts to be found.
I still have the exact same problem. I'm still trying to find an OBD1 code reader, but using the paperclip method the check engine light flashed 24. According to the internet that indicates an output speed sensor issue, which is more or less what my ATSG manual says as well.
I have not replaced the wire from the OSS back to the ECM, and further my speedo has always worked fine. I mention the speedo as evidently this often comes with the problem I'm experiencing as there is a DRAC unit integrated into the instrument cluster. Apparently if the speedo goes and you have shifting problems you've likely found your culprit.
Has anyone had this exact same issue? What was your resolution? My next step is to replace the OSS wire(s) and see if it makes a difference, but any ideas you all might have would be much appreciated.
Thanks,
Gary
I have a '91 C3500 GMC Sierra with a 4L80E with 70K original miles on the truck. Bought surplus from a nearby municipality at 56K. About 6 months ago I began experiencing a loss of 4th gear. I took it to 2 different reputable transmission shops. Since it was intermittent and evidently not throwing a code, neither could adequately shed any light let alone fix the problem. Over time it got worse, up until the point where it happens every time it's driven. Although I originally thought it was just 4th that was lost, I soon realized it was all gears except 3rd and reverse that went away.
Faced with this, and not particularly delighted with the service provided by the aforementioned shops, I chose to get a kit that replaced the internal wiring harness (with the '93 & later spec) along with the internal solenoids, force motor, temp sensor, etc, as well as both external speed sensors. I upgraded the external plug to fit this harness and was very careful to correctly splice onto the old harness. BTW: The oil the pan was clean, no debris or bits of warn parts to be found.
I still have the exact same problem. I'm still trying to find an OBD1 code reader, but using the paperclip method the check engine light flashed 24. According to the internet that indicates an output speed sensor issue, which is more or less what my ATSG manual says as well.
I have not replaced the wire from the OSS back to the ECM, and further my speedo has always worked fine. I mention the speedo as evidently this often comes with the problem I'm experiencing as there is a DRAC unit integrated into the instrument cluster. Apparently if the speedo goes and you have shifting problems you've likely found your culprit.
Has anyone had this exact same issue? What was your resolution? My next step is to replace the OSS wire(s) and see if it makes a difference, but any ideas you all might have would be much appreciated.
Thanks,
Gary