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Makes 🢒 Chevrolet 🢒 1999 🢒 Chevy P32 RV/Van Chassis 🢒 Transmission/Transaxle 🢒 Automatic Transmission - 4L80-E 🢒 DTC 74 AT ISS Sensor Circuit

DTC 74 AT ISS Sensor Circuit L57 MFI


Object Number: 372646 Size: MF
Automatic Transmission Components Diesel
Automatic Transmission Controls Schematics TCM Control
Handling ESD Sensitive Parts Notice

Circuit Description

The automatic transmission input (shaft) speed sensor (A/T ISS) provides transmission input speed to the transmission control module (TCM). The A/T ISS is a permanent magnet (PM) generator. The sensor mounts into the transmission case and maintains a slight air gap between the sensor and the forward clutch housing. The PM generator produces an AC voltage as the forward clutch housing rotor teeth pass through the sensor's magnetic field. The AC voltage level increases as the turbine shaft speed increases. The TCM converts the AC voltage into a digital signal. The TCM determines actual turbine speed using the digital signal. The TCM uses the input speed to calculate torque converter slip speed, and gear ratios.

When the (TCM) detects a low, or no input speed during high vehicle and high engine speeds, then DTC 74 sets.

Conditions for Running the DTC
    • No OSS DTC 24.
    • No TFP manual valve position switch DTC 28.
    • No CMP sensor DTC 71.
    • The OSS is greater than 200 RPM.
    • The engine speed is greater than 300 RPM.

Conditions for Setting the DTC
    • The A/T ISS is less than 50 RPM.
    • All the above conditions are met for 2 seconds.

Action Taken When the DTC Sets
    • The TCM does not illuminate the malfunction indicator lamp (MIL).
    • The TCM inhibits TCC.
    • The TCM stores DTC 74 in TCM history.

Conditions for Clearing the DTC
    • A scan tool clears the DTC from TCM history.
    • The TCM clears the DTC from TCM history if the vehicle completes 40 consecutive key cycles without a diagnostic fault occurring.
    • The TCM cancels the DTC default actions when the fault no longer exists and the ignition switch is OFF long enough in order to power down the TCM.

Diagnostic Aids
    • Inspect the wiring at the TCM, the output shaft speed (OSS) sensor connector and all other circuit connecting points for the following conditions:
       - A backed out terminal
       - A damaged terminal
       - Reduced terminal tension
       - A chafed wire
       - A broken wire inside the insulation
       - Moisture intrusion
       - Corrosion
       - The A/T ISS sensor harness being near the DIS components or the ignition wires.
    • When diagnosing for an intermittent short or open, massage the wiring harness while watching the test equipment for a change.
    • Inspect for any transmission DTCs that may have reset.

Test Description

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

  1. This step tests for proper circuit operation up to the TCM connections.

    2.

  2. This step tests for proper operation of the input (shaft) speed (ISS) sensor.

DTC 74

Step

Action

Value(s)

Yes

No

1

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

--

Go to Step 2

Go to Powertrain On Board Diagnostic (OBD) System Check

2
  1. Install the Scan Tool .
  2. With the engine OFF, turn the ignition switch to the RUN position.

    3. Important:: Before clearing the DTCs, use the scan tool in order to record the Failure Records. Using the Clear Info function erases the Failure Records from the TCM.

  3. Record the Failure Records.
  4. Clear the DTC.
  5. With the vehicle in PARK, start the engine and idle greater than 500 RPM.
  6. Select Scan Tool Transmission ISS and Engine RPM.

Is the Transmission ISS greater than 25 RPM?

--

Go to Diagnostic Aids

Go to Step 3

3

  1. Turn the ignition switch to the OFF position.
  2. Disconnect TCM connector C1. (Additional DTCs may set.)
  3. Probe TCM connector terminal C1-D4 and terminal C1-D3 with J 39200 digital multimeter (DMM) on AC voltage.
  4. With the vehicle in PARK, crank the engine.

Is the voltage above the specified value?

0.5 volts AC

Go to Step 12

Go to Step 4

4
  1. Turn the ignition switch to the OFF position.
  2. Remove the connector from the A/T ISS sensor.
  3. Using the J 39200 DMM and J 35616-A connector test adapter kit, measure the resistance between terminal A and terminal B of the A/T ISS sensor.

Is the sensor's resistance within the specified value?

1042-2088ohms

Go to Step 5

Go to Step 11

5

  1. Select AC volts on the J 39200 DMM.
  2. With the vehicle in PARK, crank the engine.

Is the J 39200 DMM voltage greater than the specified value?

0.5 volts AC

Go to Step 7

Go to Step 6

6
  1. Remove the A/T ISS sensor from the transmission.
  2. Inspect the A/T ISS sensor.
  3. Inspect the forward clutch housing for damage (rotor teeth).

Was a condition found?

--

Go to Step 12

Go to Step 11

7

Inspect circuit 1230 (RED/BLK) for an open or short to ground.

Refer to General Electrical Diagnosis in Wiring Systems.

Was a condition found?

--

Go to Step 8

Go to Step 9

8

Repair the open or short to ground in circuit 1230 (RED/BLK).

Refer to Wiring Repairs in Wiring Systems.

Was a condition found?

--

Go to Step 14

--

9

Inspect circuit 1231 (BLU/WHT) for an open or short to ground.

Refer to General Electrical Diagnosis in Wiring Systems.

Was a condition found?

--

Go to Step 10

--

10

Repair the open or short to ground in circuit 1231 (BLU/WHT).

Refer to Wiring Repairs in Wiring Systems.

Is the repair complete?

--

Go to Step 14

--

11

Replace the input shaft speed sensor.

Refer to Vehicle Speed Sensor Replacement .

Is the replacement complete?

--

Go to Step 14

--

12

Replace the forward clutch housing.

Refer to Unit Repair.

Is the repair complete?

--

Go to Step 14

--

13

Replace the TCM.

Refer to Powertrain Control Module Replacement/Programming in Engine Controls.

Is the replacement complete?

--

Go to Step 14

--

14

Perform the following procedure in order to verify the repair:

  1. Select DTC.
  2. Select Clear Info.
  3. Operate the vehicle under the following conditions:
    4. 3.1. The vehicle in drive range and the engine must be running.
    3.2. Vehicle speed greater than 12 MPH.

Is the Scan Tool A/T ISS greater than the specified value?

200 RPM

System OK

Go to Step 1