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Most likely you have an open wire in the ABS circuit, but see below for the pinpoint test procedure for each code, you will need to measure voltage using a DMM.
Circuit Description The PCM converts the data from the vehicle speed sensor to a 128k pulses/mile signal. The EBCM uses the vehicle speed signal from the PCM in order to calculate the rear wheel speed.Conditions for Running the DTC C0235 The ignition is ON.C0236 and C0237
Conditions for Setting the DTC C0235 The EBCM detects low voltage on the vehicle speed signal circuit for 500 milliseconds.C0236 The rear wheel speed signal is less than 6 km/h (4 mph) for 5 seconds, or 120 seconds in order to set multiple missing sensor signal DTCs.C0237 The EBCM detects an erratic rear wheel speed signal for 105 milliseconds.P1504 The PCM detects low voltage on the vehicle speed signal circuit for 45 seconds.Action Taken When the DTC Sets If equipped, the following actions occur:
Conditions for Clearing the DTC
Diagnostic Aids Thoroughly inspect connections or circuitry that may cause an intermittent malfunction. Test Description The numbers below refer to the step numbers on the diagnostic table.
Circuit Description The ABS relay supplies battery voltage to six valve solenoids. The EBCM microprocessor applies the grounds needed to activate each solenoid. The low side of each solenoid coil has a feedback circuit to the EBCM microprocessor. When a solenoid is commanded OFF, the feedback voltage is high. When a solenoid is commanded ON, the feedback voltage is low.Conditions for Running the DTC
Conditions for Setting the DTC The EBCM detects an internal malfunction.Action Taken When the DTC SetsC0265 If equipped, the following actions occur:
C0266 If equipped, the following actions occur:
Conditions for Clearing the DTC The conditions for setting the DTC are no longer present and you use the scan tool Clear DTCs function.Diagnostic Aids Important: Whenever the EBCM is replaced for DTC C0265 or C0266, the ABS pump motor and motor circuitry must be tested for the proper resistance. Refer to steps 7 and 8 in the diagnostic table below for testing procedures and resistance values.C0265 Thoroughly inspect connections and circuitry that may cause an intermittent malfunction. C0266 Replace the EBCM if DTC C0266 continues to set intermittently.Test Description The numbers below refer to the step numbers on the diagnostic table.
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