Schematic Reference: Instrument Cluster Schematics .

Did you review the system operation and perform the necessary inspections?

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Go to Step 2

Go to Symptoms - Instrument Panel, Gages and Console

1. Disconnect the speedometer adapter.
2. Connect the J 33431-C Signal Generator and Instrument Panel Tester lead to the vehicle speed signal circuit at the speedometer adapter connector and the other lead to a good ground.
3. Turn the ignition ON, with the engine OFF.
4. Set the J 33431-C to generate a speedometer signal. The speedometer signal should briefly drive the speedometer to 55 mph (88.5 km/h).

Does the speedometer briefly indicate the correct speed?

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Go to Step 8

Go to Step 3

1. Turn the ignition OFF.
2. Disconnect the speedometer.
3. Turn the ignition ON, with the engine OFF.
4. Measure the voltage from the speedometer ignition voltage circuit to the speedometer ground circuit.

Does the voltage measure greater than the specified value?

9.0 V

Go to Step 4

Go to Step 5

Test the vehicle speed signal circuit for an open, for a short to ground or for a short to voltage. Refer to Circuit Testing and to Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

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Go to Step 11

Go to Step 7

Test the speedometer ignition voltage circuit for an open or for a high resistance. Refer to Circuit Testing and to Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

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Go to Step 11

Go to Step 6

Test the ground circuit of the speedometer for an open or for a high resistance. Refer to Circuit Testing and to Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

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Go to Step 11

Go to Step 7

Inspect for poor connections at the harness connector of the speedometer. Refer to Testing for Intermittent Conditions and Poor Connections and to Connector Repairs in Wiring Systems.

Did you find and correct the condition?

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Go to Step 11

Go to Step 9

Inspect for poor connections at the harness connector of the speedometer adapter. Refer to Testing for Intermittent Conditions and Poor Connections and to Connector Repairs in Wiring Systems.

Did you find and correct the condition?

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Go to Step 11

Go to Step 10

Replace the speedometer. Refer to Speedometer Replacement .

Did you complete the replacement?

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Go to Step 11

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Replace the speedometer adapter. Refer to Duramax® 7800 Medium Duty Diesel Engine Manual.

Did you complete the replacement?

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Go to Step 11

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Operate the system in order to verify the repair.

Did you correct the condition?

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System OK

Go to Step 2