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CHART B-1, ODOMETER POWER/GROUND PROBLEM - ELDORADO & SEVILLE
Fig. 57: Chart B-1, Odometer Power/Ground Circuit Diagram (Eldorado & Seville)
Courtesy of GENERAL MOTORS CORP.
An incorrect odometer reading can be caused by problems relating to power and ground circuits not only in Instrument Panel Cluster (IPC), but those of other components within the computer network, the computer wake-up signals, the vacuum fluorescent power supply, the odometer mileage memory chip (EEPROM) contained in BCM, or by data link between computer components.

NOTE:Test numbers refer to test numbers on diagnostic chart.

If no codes are set in diagnostic memory, observe state of odometer reading with headlight switch and twilight sentinel switch in the OFF position. This is important because these signals can wake-up the BCM and otherwise cause a misdiagnosis of a malfunctioning primary circuit.
When odometer reads all zeros, actuating the interior courtesy light switch helps isolate cause of problem. This symptom is caused by a failure in computer system other than the IPC. Further isolation of this fault is diagnosed in CHARTS B-2, B-3 and B-4.
When odometer is blank or reads all 8's, actuating headlight switch helps isolate cause of problem. If odometer reads all zeros, problem is caused by faulty ISO ION-3 feed circuit to BCM (open or shorted to ground) or BCM itself. If odometer is blank or reads all 8's, problem is one of several others within computer network. It display is normal, the cause is due to an open or short to ground in circuit No. 588 (16-volt VF input to radio) or circuit No. 8 (incandescent bulb output from IPC).
Cycling the high beam or headlight dimmer switch helps further isolate cause of problem. If high beams cycle, problem is related to IPC itself IPC ground circuit, vacuum fluorescent power supply circuit or components related to these circuits. Further fault diagnosis is contained in CHART B-5.
If high beams did not cycle when dimmer switch was actuated, turning on courtesy lights switch helps further isolate system problem. If courtesy lights turn on, problem is related to IPC itself, the Central Power Supply (CPS) to IPC logic circuits ground, the 12-volt power circuits from CPS to IPC, or the CPS itself. Further problem isolation is contained in CHART B-6.
If courtesy lights did not turn on when switch was actuated, several other causes for problem can be isolated by following CHART B-7 and CHART B-8, depending on whether the lights-on chime sounds when test is run.
If the odometer reads ERROR, BCM EEPROM or internal IPC failure is indicated. Jumpering pin "M" of the ALDL to ground disables the data line and isolates the IPC from the BCM. With no serial data, the IPC should display "0" in the odometer following the crank cycle.