ALLISON 1000 AND 2000 PRODUCT FAMILIES ELECTRONIC CONTROLS TROUBLESHOOTING MANUAL

DIAGNOSTIC TROUBLE CODES (DTC)

DTC P0708 Transmission Range Sensor Circuit High Input

TCM

NSBU SWITCH !--'·....-----,

p J2-8
207
c J2-7
DIGITAL
206 INTERFACE
B J2-6
205
A J2-5
...._��-70 2��-
7 --�-2_ 0

\_ 12-PIN
J2-20
---
SINGLE CONNECTOR
NSBU
FORMER CURRENT
NSBU SWITCH NSBU SWITCH
4- AND 7-PIN 12-PIN

V06217.01.00

Circuit Description
The installation of a transmission-mounted Neutral Start/Back-Up (NSBU) switch is required. This switch mounts
directly onto the transmission housing from the outside and detects the angular position of the shift selector shaft.
This position is communicated to the Transmission Control Module (TCM) so that certain vehicle control functions
can be coordinated with the position of the shift controls. The NSBU switch has redundant circuitry to alert the TCM
in the event of a single wire or switch failure.

The neutral signal output of the NSBU switch is typically used as confirmation that the transmission is in
N (Neutral) before the engine starter is engaged. The NSBU switch is interfaced to the starter circuit with
weatherproof electrical connectors. The reverse signal provision may be used to activate vehicle back-up lights
and/or reverse warning devices.

Conditions for Running the DTC

• The components are powered and ignition voltage is greater than 9V and less than 18V.
• Engine speed is greater than 200 rpm and less than 7500 rpm for 5 seconds.

5-46 Copyright© 2004 General Motors Corp.

 ALLISON 1000 AND 2000 PRODUCT FAMILIES ELECTRONIC CONTROLS TROUBLESHOOTING MANUAL

DIAGNOSTIC TROUBLE CODES (DTC)

Conditions for Setting the DTC - Software Levels Prior To N11
OTC P0708 sets when the TCM detects an invalid condition or parity errors occurring over consecutive ignition
cycles.

Actions Taken When the DTC Sets - Software Levels Prior To N11
• The diagnostic response is to shut all solenoids off and allow the transmission to operate in hydraulic default
mode or "limp home". Shift selector and hydraulic state of logic valves determines range attained.
• DTC P0708 is stored in the TCM history.
• The CHECK TRANS light illuminates.
• The TCM inhibits TCC engagement.
• Due to this failure and associated responses, P0722 and P0845 may also set.

Action Taken When the DTC Sets - Software Level N11 And Later
• The CHECK TRANS light illuminates but no diagnostic response is implemented until after 50 warm-up cycles
occur when a NSBU switch parity error is active. A warm-up cycle is defined as a 20 degree change in
transmission temperature.
• The diagnostic response is to shut all solenoids off and allow the transmission to operate in hydraulic default
mode or "limp home." Shift selector and hydraulic state of logic valves determines range attained.
• OTC P0708 is stored in the TCM history.
• The TCM inhibits TCC engagement.
• Due to this failure and associated responses, P0722 and P0845 may also set.
• A shift to R (Reverse) will allow reverse range if a P0876 (previously P1714) is not active.

Conditions for Clearing the MIUDTC

A Scan Tool may be used to clear the code from the TCM history. The TCM automatically clears the OTC from the
TCM history if the vehicle completes 40 warm-up cycles without failure.

Diagnostic Aids
• Inspect the wiring for poor electrical connections at the TCM. Look for the following conditions:
A bent terminal.
A backed-out terminal.
A damaged terminal.
Poor terminal tension.
A chafed wire.
A broken wire inside the insulation.
• When diagnosing for an intermittent short or open, massage the wiring harness while watching the test
equipment for a change.
• You may have to drive the vehicle in order to experience a fault.
• When a P0708 is shown in failure records, this indicates that conditions to set a P0708 were present at one
time. If the condition was due to moisture intrusion, the moisture may no longer be present, and thus you may
not be able to reproduce the DTC. In such case, variables such as vehicle mileage, external physical condition
of the switch should be factored in when making a decision to replace the NSBU switch.

NOTE: Due to the TCM logic used to detect and set a P0708, this code can remain active even after a
NSBU switch replacement is complete and the ignition has been cycled. It is important to clear all
active DTCs from the TCM after servicing the NSBU switch.

Copyright© 2004 General Motors Corp. 5-47

 ALLISON 1000 AND 2000 PRODUCT FAMILIES ELECTRONIC CONTROLS TROUBLESHOOTING MANUAL

DIAGNOSTIC TROUBLE CODES (DTC)

Test Description

The numbers below refer to the step numbers on the diagnostic table.

3. This step tests NSBU switch status.

4. This step tests for proper TCM input response.

5. This step tests the wiring harness for opens or shorts.

DTC P0708 Transmission Range Sensor Circuit High Input

Step Action Value(s) Yes No
1 Was the Beginning The Troubleshooting - Go to Step 2 Go to
Process (Paragraph 5-4A) performed? Beginning The
Troubleshooting
Process
(Paragraph 5-4A)
2 Is the NSBU switch and shift linkage properly - Go to Step 3 Go to
adjusted? Mechanic's Tips
3 1. Install the Scan Tool. Refer to NSBU Go to Go to Step 4
2. Turn the ignition ON with the engine OFF. Switch Table Diagnostic Aids
3. Record the OTC Failure Record data. (Page 5-9)
4. Using the Scan Tool, monitor PRNDL A, B, C,
P status while moving the shift selector
through each range.
Does each switch toggle ON and OFF in the
correct sequence, and does the range selected
displayed on the Scan Tool match the actual
physically selected range?
4 1. Turn the ignition OFF. A=pin 5 Go to Step 5 Go to Step 10
2. Disconnect J2 connector from the TCM and B=pin 6
install J 39700 Breakout Box and J 43799 C=pin 7
Breakout Box Adapter. P=pin 8
3. Disconnect the NSBU 4-pin connector and
the NSBU 7-pin connector. Scan Tool Status
4. Turn the ignition ON. ON=Open circuit
5. In sequence, connect J2 connector pin 20 to OFF=Grounded
circuit
J2 pins 5, 6, 7, and 8.
6. Monitor the switch status on the Scan Tool.
Does each switch toggle ON and OFF in the
correct sequence as each pin is grounded?
5 Inspect the NSBU switch connector assembly. Go to Step 6 Go to Step 7
Does the NSBU have two connectors-a seven
pin and four pin connector?
6 1. Turn the ignition OFF. Scan Tool Status Go to Step 8 Go to Step 9
2. Reconnect J2 connector to the TCM. ON=Open circuit
3. In sequence, connect pins 4A, 48, 4C, and OFF=Grounded
40 at the 4-pin NSBU connector to pin 70** circuit
on the NSBU 7-pin connector. *
4. Monitor the switch status on the Scan Tool. 7D=wire 220
Does the Scan Tool indicate the proper switch
status for each switch when grounded?

5-48 Copyright© 2004 General Motors Corp.

 ALLISON 1000 AND 2000 PRODUCT FAMILIES ELECTRONIC CONTROLS TROUBLESHOOTING MANUAL

DIAGNOSTIC TROUBLE CODES (DTC)

DTC P0708 Transmission Range Sensor Circuit High Input (cont'd)

Step Action Value(s) Yes No
7 1. Turn the ignition OFF. Scan Tool Status Go to Step 8 Go to Step 9
2. Reconnect J2 connector to the TCM. ON=Open circuit
3. At the NSBU connector in sequence, OFF=Grounded
connect pins 4, 5, 6, and 8 to pin 7** at the circuit
12 pin connector.
4. Monitor the switch status on the Scan Tool. 7D=wire 220
Does the Scan Tool indicate the proper switch
status for each switch when grounded?
8 Replace the NSBU switch (refer to Mechanic's - Go to Step 11 -
Tips).
Is the replacement complete?
9 Repair the vehicle wiring harness. - Go to Step 11 -

NOTE: The vehicle OEM has responsibility

for all external wiring, wherefore the OEM
should perform vehicle harness repairs.
Harness repairs performed by Allison
Transmission distributors and dealers are
not covered by Allison Transmission
warranty.
Is the repair complete?
10 NOTE: In most cases, theTCM is not at fault. - Go to Step 11 -
Investigate thoroughly before replacing the
TCM. Refer to TCM Replacement Procedure
(Section 3-6).
Replace the TCM.
Is the replacement complete?
11 In order to verify your repair: Refer to NSBU Begin the System OK
1 . Clear the DTC. Switch Table diagnosis again.
2. Drive the vehicle under normal operating (Page 5-9) Go to Step 1
conditions.
3. Using the Scan Tool, monitor PRNDL switch
status while moving selector through all
positions.
Does each switch toggle according to switch
table?
* If J 39700 Breakout Box and J 44722 Transmission Harness are available, they may be used to perform this
operation.
** Use pin Hat the magnetic overlay to provide a ground source when J 44722 Harness is used.

Copyright© 2004 General Motors Corp. 5-49