Allison Transmission with DTC P0700 & P1779

Allison Transmission

P0700 & P1779

If you happen to come across a DTC Code P0700 & P1779 ( You will only get a P0700 code (if you use any scanner other than a Laptop with the Allison Doc software you won't get the P1779 code, only a P0700 with a messed up shift pattern will show up using most other scanners such as Snap On or Genisys Scanners) take a few minutes to quiz the driver of the truck to see if they have experienced the motor stall out or quitting for no apparent reason & then starting later after the vehicle sets for a while. Only the 8.1L Gas engine has this problem. If they have you need to replace the crank sensor which is mounted in the bell housing of the transmission at aprox 10 o'clock position if you are standing at the rear of the transmission looking towards the front of the truck with the truck in the air.  

8.1L Crank Sensor

 

Gray TCM C1 Plug

Wire terminal #16 as indicated below with the seal pick carry's the Torque Management signal to the PCM from the Gray C1 connector of the TCM. If you look real close you will see that the terminal has backed itself out of the plug to where the wire was unable to make a connection. This triggered a DTC P1779 Torque Management code which in turn set a DTC P0700 code also. These codes will prevent proper shifting of the trans assembly.    

Gray-TCM-closeup 

This wire terminal is known to back out of the plug after a water pump or radiator change. Any other type of engine repair that requires moving the fan shroud or moving either the TCM or PCM out of the way can cause this code to appear. If you are dealing with this code, take a few minutes and check the Gray C1 connector, wire #16 & see if it'd backed it's way out of the plug. After correcting the problem, You may have to clear the codes from memory using the Allison software or have your local GM dealer do it but some say a Snap On scanner will clear codes IF you are scanning in Generic mode.

 

  Allison-1000-pic

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