GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 1999 🢒 Blazer - 4WD - RHD 🢒 Engine 🢒 Engine Controls - 4.3L 🢒 VCM Controlled Tachometer Diagnosis
Table 1: Frequency Vs. RPM

Object Number: 345077 Size: MF
Handling ESD Sensitive Parts Notice
Engine Controls Components
Cell 20: Ignition Controls
OBD II Symbol Description Notice

Circuit Description

The IPC supplies 12 volts to the tachometer circuit. The VCM Pulse Width Modulates (PWM) the tachometer circuit. The PWM signal will vary with the speed of the engine.

Diagnostic Aids

Use the following table in order to verify correct VCM operation of the TACH circuit.

Frequency Vs. RPM

Commanded Output (RPM)

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

5500

6000

VCM PWM Signal (Hz)

16.4

32.9

49.4

66.7

83.1

99.7

116.1

133.6

150.1

166.6

183.1

200.4

Test Description

The number(s) below refer to the step number(s) on the diagnostic table.

  1. This step checks the IPC's ability to control the tachometer needle.

    2.

  2. This step checks for proper tachometer operation.

    3.

  3. This step checks the VCM's ability to send an accurate PWM signal to the IPC.

    If the Tachometer Output matches the Frequency Vs. RPM table the fault must be a poor connection at the IPC or an internal IPC failure.

Step

Action

Value(s)

Yes

No

1

Important: Before clearing the DTCs, record Freeze Frame and the Failure Records for reference with a scan tool. This data will be lost when the Clear DTC Information function is used.

Did you perform the Powertrain On-Board Diagnostic (OBD) System Check?

--

Go to Step 2

Go to Powertrain On Board Diagnostic (OBD) System Check

2

  1. Install the scan tool.
  2. Select the BODY (IPC) diagnosis tachometer sweep test function.
  3. Sweep the tachometer needle ON and OFF.

Does the tachometer needle sweep when commanded?

--

Go to Step 3

Go to Instrument Cluster System Check in Instrument Panel, Gauges and Console

3

  1. Select the POWERTRAIN Tachometer Control with a scan tool.
  2. Start and idle the engine.
  3. Command the TACH output to the first specified value, then to the second specified value.
  4. Compare the commanded TACH output to the actual tachometer value displayed by the IPC.

Does the tachometer value displayed by the IPC match the commanded TACH output?

600 RPM

6000 RPM

System OK

Go to Step 4

4

  1. Disconnect the IPC connector containing the TACH circuit.
  2. Using the scan tool select the POWERTRAIN Tachometer Control.
  3. Connect the J 39200 DMM between a good ground and the TACH circuit terminal (harness side).
  4. Measure the AC frequency (press HZ button) of the TACH circuit at the IPC connector with the J 39200 DMM.
  5. Start and idle the engine.
  6. Command the Tachometer Control from the first specified value to the second specified value.
  7. Compare the frequency output on the TACH circuit to the Diagnostic Aids Frequency Vs. RPM table.

Does the frequency output of the TACH circuit match the Frequency Vs. RPM table?

600 RPM

6000 RPM

Go to Instrument Cluster System Check in Instrument Panel, Gauges and Console

Go to Step 5

5

Check the Tachometer Output circuit for an open, short to ground or a short to voltage.

Did you find a problem?

--

Go to Step 6

Go to Step 7

6

Repair the circuit as necessary. Refer to Wiring Repairs in Wiring Systems.

Is the action complete?

--

Go to Step 3

--

7

Check for a poor connection at the VCM. Repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Did you find a problem?

Go to Step 3

Go to Step 8

8
  1. Replace the VCM.
  2. Program the new VCM. Refer to VCM Replacement/Programming .
  3. Perform the Passlock Reprogramming Procedure. Refer to PASSLOCK Reprogramming Seed and Key .
  4. Perform the CKP System Variation Learning Procedure. Refer to Crankshaft Position System Variation Learn .

Is the action complete?

--

Go to Step 3

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