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Gmc allison transmission

I have a 2001 GMC 2500HD with an 8.1 and an allison transmission. It has 82,000 mile on it. It was previously a plow/landscaping truck and I user it the same. Recently, while slowly plowing the entrance to a parking lot it seemed to slip either on ice or into gear. Either way the engine light came on and it turns out that it was two separate transmission codes. Since the code were cleared there has been no issue but my mechanic thinks its a sign its going. He says if it's slipping them a tranny flush might make matters worse. Another mechanic thought that the tranny codes came up due to a faulty speed sensor. any ideas? Suggestions?

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  • keith29717
    keith29717 Feb 11, 2014

    Hi jmservices18 , I want to help you with your problem, but I need more information from you. what were the codes?

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SOURCE: Slips between gears.

well I guess I will say change the fluid again and change the filter- you simply do not have enough miles to be transmission failure- so my guess wil be the shift solenoid as well - contact dealer to confirm.They will have to change with solenoid anyway.I sold my last honda with 275,000 miles and it had a hard life- no issues ever

Posted on Dec 18, 2008

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SOURCE: 1999 ford expedition - transmission slip?

the flashing O/D light usually means you have a bad speed sensor that sends speed info to the engine & trans computer.

Vehicle Speed Sensor The vehicle speed sensor (VSS) (Figure 42) is a variable reluctance or Hall-effect sensor that generates a waveform with a frequency that is proportional to the speed of the vehicle. If the vehicle is moving at a relatively low velocity, the sensor produces a signal with a low frequency. As the vehicle velocity increases, the sensor generates a signal with a higher frequency. The PCM uses the frequency signal generated by the VSS (and other inputs) to control such parameters as fuel injection, ignition control, transmission/transaxle shift scheduling and torque converter clutch scheduling.
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Vehicle Speed Sensor The vehicle speed sensor (VSS) (Figure 42) is a variable reluctance or Hall-effect sensor that generates a waveform with a frequency that is proportional to the speed of the vehicle. If the vehicle is moving at a relatively low velocity, the sensor produces a signal with a low frequency. As the vehicle velocity increases, the sensor generates a signal with a higher frequency. The PCM uses the frequency signal generated by the VSS (and other inputs) to control such parameters as fuel injection, ignition control, transmission/transaxle shift scheduling and torque converter clutch scheduling.

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Some trans use an OSS, Output Shaft Sensor instead of a VSS. Depends on which trans you have, 4R100 or 4R70W

Transmission Control Switch (TCS) and Transmission Control Indicator Lamp (TCIL)
The transmission control switch (TCS) is a momentary contact switch. When the switch is pressed, a signal is sent to the powertrain control module (PCM) to allow automatic shifts from first through fourth gears or first through third gears only. The PCM energizes the transmission control indicator lamp (TCIL) when the switch is off.

The TCIL indicates overdrive cancel mode activated (lamp on) and electronic pressure control (EPC) circuit shorted (lamp flashing) or monitored sensor failure.

Output Shaft Speed (OSS) Sensor
The output shaft speed (OSS) sensor is a magnetic pickup, located at the output shaft ring gear, that sends a signal to the powertrain control module (PCM) to indicate transmission output shaft speed. The OSS is used for torque converter clutch control, shift scheduling and to determine electronic pressure control.

Posted on Jun 01, 2009

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SOURCE: why wont my 2001 gmc sierra 2500hd go into 4th gear?

Check to make sure you don't have the TOW/HAUL switch on the end of the tranny stalk turned on

Posted on Jul 27, 2009

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Posted on Dec 11, 2009

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SOURCE: 2003 Chevy 2500HD 8.1 allison 4x4. Truck runs 100%

I have the same exact truck as you do and had a similar problem awhile ago when i installed the hypertech system. I had to uninstall the hypertech and then have a buddy (cause i did not know how) drain the residual power from the brain... apparenlty the brain houses a small amount of power and remembers these codes so after you turn off your truck...poof there they are again. This worked for me, but then I had other electrical problems so I the dealership had to replace the brain.

Posted on Apr 08, 2010

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