GMC Troubleshooting Questions? Ask a Mechanic for Answers
how long since the fluid was flushed ?
has anything been done yet to correct this problem ?
did this problem start out of the blue one day ?
no codes at all ?
did the transmission flush help at all ?
was the pan pulled yet ?
any access to a scan tool to view live computer and transmission data ?
with NO codes the odds of the transmission loosing its power for the solenoids is very slim
are you sure its 4th gear or is it overdrive or lock up that goes out first ?
if we had a wiring issue or even a solenoid issue it would set codes
with no related codes the computer is shifting it and has the correct resistance and feed back from the transmission as it would trigger codes
this issue is going to be more so a problem internally with a forward srag one way clutch which will cause this type of issue and they love to act up when warm and be fine cold
the best action with no codes and a good flush done and the pan removed and checked for debris would be to drive it with a scan tool and shift the transmission with the scan tool if the scan tool says its in the gear but the transmission physically is not then that is more than likely our issue
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usually those would set a code
if the speedometer works fine and with a scan tool the TPS shows a full smooth range from idle to wide open throttle its not likely those are not the issue
not usually that would set a code or cause the transmission to slip