GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 1999 🢒 GMC C-Series Medium Duty Truck B7 🢒 Engine 🢒 Engine Controls - 7.4L 🢒 Powertrain Control Module Controlled Tachometer Diagnosis

Object Number: 215316 Size: MF
Engine Controls Components
Cell 20: IP Cluster, Transmission Controls, Multifuction Alarm Module
Handling ESD Sensitive Parts Notice
Handling ESD Sensitive Parts Notice

Circuit Description

The tachometer supplies ignition positive voltage to the PCM by way of the tachometer control circuit. The Crankshaft Position (CKP) sensor signal indicates to the PCM that the crankshaft is rotating. The PCM controls the tachometer by momentarily grounding the tachometer control circuit and causing the voltage to pulse at the rate of two times per crank shaft revolution. The solid state circuits of the tachometer convert the voltage pulses in order to drive the tachometer pointer and display engine speed in RPM.

Diagnostic Aids

Check for the following conditions:

    • Poor connection at the PCM or instrument cluster. Inspect harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and poor terminal to wire connection.
    • Damaged harness. Inspect the wiring harness for damage. If the harness appears to be OK, disconnect the PCM, turn the ignition ON and observe a voltmeter connected to the tachometer control circuit at the PCM harness connector while moving connectors and wiring harnesses related to the tachometer control circuit. A change in voltage will indicate the location of the fault.

Test Description

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

  1. The tachometer diagnosic table will direct you back to this diagnostic table if a problem is not found elsewhere in the tachometer circuitry.

    2.

  2. This test may help identify a Crankshaft Position (CKP) sensor problem by indicating a CKP DTC.

    3.

  3. Normally, ignition positive voltage is present on the tachometer control circuit with the PCM disconnected and the ignition turned ON.

Tachometer Control Circuit Diagnosis

Step

Action

Value(s)

Yes

No

1

Did you perform the tachometer diagnosis?

--

Go to Step 2

Go to Tachometer Inoperative in Instrument Panel, Gauges and Console

2

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

--

Go to Step 3

Go to Powertrain On Board Diagnostic (OBD) System Check

3

Important: Reconnect any components that where disconnected in any previous diagnostic procedures.

  1. Turn OFF the ignition.
  2. Disconnect the PCM connector located on the same side as the manufacture's logo.
  3. Turn ON the ignition.
  4. Connect a test lamp to a battery ground and probe the tachometer control circuit at the PCM harness connector.

Is the test lamp ON?

--

Go to step 5

Go to step 4

4
  1. Check the tachometer control circuit for an open.
  2. If a problem is found, repair as necessary.

Was a problem found and repaired?

--

Go to Step 7

Go to Step 5

5
  1. Check the tachometer control circuit terminal for a faulty connection at the PCM and repair as necessary.
  2. Did you find and repair a problem?

--

Go to Step 7

Go to Step 6

6

Important: Replacement PCM must be programmed. Refer to Powertrain Control Module Replacement .

Replace the PCM.

Is the action complete?

--

Go to Step 7

--

7
  1. Start the engine.
  2. Observe the tachometer.

Does the tachometer operate properly?

--

System OK

Go to Tachometer Inoperative in Instrument Panel, Guages and Console