DTC P1520 Transmission Range Switch Circuit. Cadillac DeVille 1997

For 1990-2009 cars only

Makes 🢒 Cadillac 🢒 1997 🢒 DeVille 🢒 DTC 🢒 DTC P1520 Transmission Range Switch Circuit

Engine Controls Components
IAC Valve and PSP/ Oil Level/ TR Switches
OBD II Symbol Description Notice
ESD Notice

Circuit Description

This vehicle uses an electronic gear indicator display. There are four inputs which are switches to ground that each have a unique switch pattern. All four of these inputs are housed and operate together in the Transaxle Range Switch. The PCM uses the combination of the switch patterns to determine the gear selected by the driver. This information is then sent by serial data to the IPC for gear indicator display. This diagnostic test monitors the transaxle range switch for false outputs. The DTC will set if the transaxle range switch is indicating a switch pattern that the PCM does not recognize.

Conditions for Setting the DTC

Test Conditions

•	Test continuously.

•	DTCs P0502 or P0503 not set.

Failure Condition

Transaxle range switch indicating an illegal combination for 3.3 seconds

Action Taken When the DTC Sets

•	PCM will substitute park/neutral status from transaxle pressure switch for transaxle range switch park/neutral status.

•	The Malfunction Indicator Lamp (MIL) will not illuminate.

•	The PCM will command a message to be displayed.

•	The PCM may record operating conditions at the time the diagnostic fails. This information will be stored in the Failure Records.

Conditions for Clearing the Message/DTC

•	The PCM will turn the message OFF after one run and pass of the diagnostic test.

•	A History DTC will clear after forty consecutive warm-up cycles with no failures of any non-emission related diagnostic test.

•	A Last Test Failed (current) DTC will clear when the diagnostic runs and does not fail.

•	Use a scan tool to clear DTCs.

•	Interrupting PCM battery voltage may or may not clear DTCs. This practice is not recommended. Refer to Clearing Diagnostic Trouble Codes in PCM Description and Operation.

Diagnostic Aids

Important: Check shifter for normal freedom of movement.

The gear select display will box all gear indications and flash on and off if an invalid or suspect gear position is received by the PCM. This will normally occur if the gear selector is not fully in a gear position. When a valid gear position is attained, the gear select display will immediately display the correct position. If the PRNDL on the IPC is flashing, this may be an indication that the shift cable and/or Transaxle Range Switch is out of adjustment.

•	The modes (circuits) are displayed in order A/B/C/P.

•	OFF indicates the circuit low (switched / shorted to ground).

•	ON indicates the circuit is high (open).

•

If the DTC sets and the PRNDL on the IPC is flashing while driving or when the driver in not moving the gear selector lever, record the transaxle range switch input and compare the values to the chart. For example, if the driver was driving in overdrive when the PRNDL on the IPC flashed and the input read ON, OFF, ON, ON, check for an intermittent open in circuit 773.

•	Manipulate related wiring and connectors while observing the transaxle range switch input. If an intermittent is induced, the digit corresponding to the circuit will change. If wiring and connectors check out OK, substitute a known good transaxle range switch and recheck.

Depending on the Scan Tool used, the transmission fluid pressure switch parameter and the gear select lever parameter may have two different titles.For example: when using a Tech 1 on a 1996 vehicle the gear select lever parameter would be named Sel. Lever Sw. and Sel. Lever Sw. A/B/C/P. The same vehicle and parameter using a Tech 2 would be named Transmission Range and T R Switch A/B/C/P. These same parameters may also call the state of the circuit differently. For example: the Tech 1 may indicate ON for an open circuit while the Tech 2 would indicate High for an open circuit. Proper verification of the parameter being observed can be indicated by the inability of the transmission fluid pressure switch to indicate any gear other than Drive 2 when the vehicle is not running (no pressure available).

Test Description

Number(s) below refer to the step number(s) on the Diagnostic Table.

The adjustment procedures must be done before diagnosis of the system. This step does a comparison of both the transaxle range switch and the transaxle pressure switches. If both displays match in all gear selections then the fault is not present or may have been repaired when the adjustment procedures were performed.

If Trans Range (or TFP Switch) display doesn't match the gear selected then the transaxle pressure switches are at fault.

Checking if the ground circuit to the switch may be at fault.

If a circuit stays ON (High) or OFF (Low) all the time, that circuit or switch is open or shorted to ground.

Step

Action

Value(s)

Yes

Was the Powertrain On-Board Diagnostic (OBD) System Check performed?

Go to Step 2

Go to A Powertrain On Board Diagnostic (OBD) System Check

Important: Perform the Park/Neutral Position Switch adjustment procedure and the Shift Cable/Lever adjustment procedure in Section 7 before proceeding with this chart.

Block the vehicle drive wheels to keep the vehicle from rolling forward or backward.
Connect the scan tool.
With the engine running, move the shift lever through each range and observe Trans. Range (or TFP Switch) and Sel. Lever Sw. (or Transmission Range) with a scan tool.

Do the scan tool displays match each other for the range that is selected?

Fault not present. Refer to Diagnostic Aids

Go to Step 3

Does Trans Range (TFP Switch) match each range selected?

Go to Step 4

Go to DTC P1810 in Section 7

With a scan tool observe the Sel. Lever Sw A/B/C/P (or Trans Range A/B/C/P) display.

Do all the modes display ON (or High).

Go to Step 13

Go to Step 5

Do any of the modes (A / B / C / P) stay ON (High) or OFF (Low) all of the time regardless of selector position?

Go to Step 6

Go to Step 15

Do the mode(s) that did not change always display ON (High)?

Go to Step 9

Go to Step 7

Record the fluid life indexes, turn the key to LOCK and wait 30 seconds.
Disconnect PCM connector C1.
Disconnect the Transaxle Range Switch.
Using DMM J 39200 measure the resistance to ground on the circuit(s) that were always OFF (Low).

•	Mode A is CKT 771.

•	Mode B is CKT 772.

•	Mode C is CKT 773.

•	Mode P is CKT 776.

Is the resistance on the tested CKT(s) less than the value specified?

5 ohms

Go to Step 8

Go to Step 11

Repair the short to ground in the CKT(s) that have low or no resistance to ground.

Is the repair complete?

Go to Powertrain Control Module Diagnosis for Verify Repair

Record the fluid life indexes, turn the key to LOCK and wait 30 seconds.
Disconnect PCM connector C1.
Disconnect the Transaxle Range Switch.
Using DMM J 39200 measure the resistance between the Transaxle Range Switch connector and the PCM connector C1 terminals on the circuit(s) that were always ON (High).

•	Mode A is CKT 771.

•	Mode B is CKT 772.

•	Mode C is CKT 773.

•	Mode P is CKT 776.

Is the resistance on the tested CKT(s) more than the value specified?

5 ohms

Go to Step 10

Go to Step 12

Repair the open in the CKT(s) that have high or infinite resistance.

Is the repair complete?

Go to Powertrain Control Module Diagnosis for Verify Repair

Reconnect PCM connector C1, leave the range switch disconnected.
Check the Sel. Lever Sw A/B/C/P (or Trans Range A/B/C/P) display with the key ON.

Do all modes indicate ON?

Go to Step 16

Go to Step 17

Reconnect PCM connector C1, leave the range switch disconnected.
Check the Sel. Lever Sw A/B/C/P (or Trans Range A/B/C/P) display with the key ON.
At the transaxle range switch connector (harness side), jumper to ground the CKT(s) that were always ON (High).

Does the display change from ON (High) to OFF (Low) when the circuit is jumpered to ground?

Go to Step 15

Go to Step 17