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Here is a bulletin for the p0700 trouble code. Check this out and let me know what you find.
#PIP3095A: Allison LCT1000 Transmission PRNDL Inoperative and No Communication with the Tech 2 Scan Tool - keywords 261 263 ABS DTC case control EBTCM information module P0500 P0700 TCM transfer - (Mar 16, 2005)
The following diagnosis might be helpful if the vehicle exhibits the symptom(s) described in the PI.
Chevrolet Silverado and GMC Sierra C/K2500HD and C/K3500 trucks equipped with the Allison LCT1000 transmission may experience a condition in which the PRNDL display is inoperative and the Tech 2 scan tool is unable to communicate with the TCM. There may or may not be any transmission performance issues noted with this concern. Trucks equipped with the Allison LCT1000 Automatic Transmission may exhibit any or all of the following conditions:
Circuit 2470 should be checked for an open connection. If C2470 develops an open while the PCM/TCM are powered down, there will not be a code set and the vehicle will exhibit the described symptom. If C2470 develops an open while the PCM/TCM are powered up, it will set a DTC P0700 in the PCM and there will not be any communication with the TCM nor any PRNDL display.
One possibility is C2470 (yellow wire) may be backed out at terminal "R" of the C100 connector, see SI Document ID #(NNN) NNN-NNNNfor clarity. C100 is located in the I/P harness to engine harness, under the fuse block - underhood, see SI Document ID #799333 for clarity. Terminal "R" should be seated and the harness positioned to prevent further strain on the connector.
Other possibilities are:
Ground G110 should be removed, remove frame wax and secured. G110 is located on the left frame rail front cab mounting bracket.
Ground G102 (diesel engine) should be checked for being secure. G102 is located on the lower right side of the engine.
Ground G103 (8.1 gas engine) should be checked for being secure. G103 is located at the left rear of the engine, near the cylinder head.
Please follow this diagnosis process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed.
#02-07-30-011C: Slow or No Engagement of Automatic Transmission, No Start, Blank PRNDL Display, Flashing PRNDL, DTC P0708, P0872, P0847, P0875, P1711, P1713 or U1000 Set (Diagnose Condition, Repair) - (Jun 21, 2004)
This bulletin is being revised to add model years and additional model information and add graphics. Please discard Corporate Bulletin Number 02-07-30-011B (Section 07 -- Transmission/Transaxle).
Some customers may comment on one or more of the following conditions.
Moisture contamination within the Park Neutral Position (PNP) Switch may cause this condition. This is sometimes referred to as the Neutral Start Back-up (NSBU) Switch. On a left turn, water spray from the left front tire is directed directly at this switch. Proper installation of the "Front Shield" and "Rear Shield" are essential for proper repairs.
This bulletin ONLY applies to vehicles equipped with a two connector PNP switch as shown above.
A complete and through diagnosis must be performed by a qualified electrical technician to determine the cause and avoid needless replacement of good components.
Important: The Transmission Control Module (TCM) and or the Pressure Switch Manifold (PSM) should NOT be replaced unless properly diagnosed. They are probably not the cause of the concern. It is often assumed the module is the cause of the concern when, in fact, it is another component or the wiring between the components that causes the concern.
Important: Remember, everything starts with a properly operating Neutral Start Back Up (NSBU) switch. The NSBU switch has been incorporated into what is now called the Park Neutral Position (PNP) switch. If the PNP switch does not provide the proper voltage signal to the module indicating the transmission is in either Park or Neutral, the module will not allow the starter operation. After the engine is started, and the shift lever is moved to either Reverse or one of the drive positions, the range selected portion of the PNP switch portion provides a voltage input to the module as to what range the driver has manually selected for transmission operation. The module then controls pressures, shift points and other transmission functions based on the voltage input received from the range portion of the PNP switch.
The technician may find one or more of the following conditions. The list of these conditions does not imply any type of frequency or order. They are random events and may occur in any order.
A blank PRNDL display may indicate a condition with the PNP switch and/or one or more of the four signal circuits from the PNP to the TCM.
Or, a blank PRNDL display may also occur if an out of line condition exists with the Serial Data Circuit (CKT 2470, yellow wire) from the TCM to Splice Pack 205.
If the Gear Selected display does not match the shifter position in the vehicle, diagnose using one of the following methods.
The PNP switch, as referenced in SI, is known as the NSBU switch in the Parts information. It is now serviced with a Park Neutral Position Switch Kit, P/N 29540479.
A flashing PRNDL display may be the result of incorrect sensor inputs or transmission abuse protection modes being received by the TCM. The following are some of these conditions:
Transmission abuse protection modes are encountered when a customer performs one of the following types of maneuvers:
When the TCM encounters any one of the above conditions, the TCM will flash the PRNDL and default the transmission to Neutral. A flashing PRNDL indicates something is/was incorrect. The transmission may need the attention of a qualified service technician.
Important: If the old PNP switch will not slide from the selector shaft without the effort, the selector shaft may need to be filed to remove flaring on the end of the selector shaft. This is important, installing the new switch onto a selector shaft that has damage will cause damage to the new PNP switch.
Important: If the new PNP switch will not slide onto the selector shaft easily, the selector shaft may need to be filed to remove flaring on the end of the selector shaft. This is important, installing the new switch onto a selector shaft that has damage will cause damage to the new PNP switch.
Tighten Tighten the bolts to 27 N·m (20 lb ft).
Important: Installation of the front shield and rear shields are important. Without proper installation of the shield, a return repair is probable. Follow all directions as listed on the installation instructions sheet included in the Switch kit, P/N 29540479.
Tighten. Tighten the nut to 24 N·m (18 lb ft).
Important: Clear all DTCs before returning the vehicle to the customer.
Switch kit, Park Neutral Position
Parts are currently available from GMSPO.