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View Full Version : Problem with a 4L80 that no mechanic or automatic transmission tech can fix


terrainman1
06-11-2011, 09:53 AM
https://www.pro-touring.com/images/icons/icon1.gif Shifting when it isn't supposed to


I have a 1991 GMC 2500 Sierra. With a 4L80 Automatic Transmission.
The problem I have is that at 110km/hr the Transmission starts downshifting from overdrive then right back in and over and over. I had it recently into the transmission shop and after a full scan they said it was because the TPS is sending the driver demand downshift signal to the computer to shift the transmission. So I replaced both the TPS and the Computer and the problem persists.
This truck has been lifted 6 inches and has 35" tires on it now. Additionally the Speedometer was adjusted to match proper speed after the lift and tires were done.
Anyone have any suggestions what could be done to finally fix this problem. I am at my wits end and am tired of spending money with 0 results.
terrainman1
06-13-2011, 01:00 PM
Recently adjusted the Throttle Position Sensor by reaming out the bolt holes to move it over a bit towards a higher throttle position.
Problem persists exactly at 110km per hour. Seems as though it will only go into the lowest overdrive gear for a short time and always shift up just slightly over and over again.
yfz450
07-07-2011, 06:47 PM
is this a gas or diesel?
terrainman1
07-11-2011, 09:13 PM
is this a gas or diesel?
Its a gas.
350 with a Injector throttle spacer. Recently took a trip up to a family reunion and my cousin (Automotive Tech) said to slightly depress the brake pedal when above 110km/hr to see if it would fool the ECM into taking the transmission out of lockup mode. It worked as long as my foot was slightly depressing the brake. Looked at a schematic for the transmission and ecm to see if there was a way to disable lockup but there is no specific tourque lockup selenoid only a TCC Selenoid which seemingly works to lockup the torque converter clutch at all speeds below 110km/hr, when crusing. It just won't stay locked up when crusing after that speed. That is what is responsible for the RPM shifting up and down constantly. Have any idea what can be done to get it to stay locked up?