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Questions and Answers

Chevy Truck ABS Diagnosis

Q. Hello, I have a 1991 Chevy Silverado 1500 2WD Pickup. 350 V-8, AT with O.D. and cruise control. 700r4 Transmission. RWAL system. 101,000 miles.

My brake/anti-lock warning light comes on only when I go over 35 mph. I have a GM code reader and I was retrieving the ABS code of open or grounded circuit to the vehicle speed sensor so I took it to a mechanic and he put my truck on his computer and he said he retrieved a code of an open at the brake light switch. I replaced the switch and the warning light still comes on.

I got under my truck and disconnected the wire harness that goes to the speed sensor and noticed a little transmission fluid inside so I cleaned it and ohmed the sensor out and I got an open. Looking at the schematic I would think I should have read resistance. My question is should I have read resistance or will it show open because the vehicle rear wheels need to be moving? When retrieving ECM codes I do not get a code 24 I only get code 12 so do you think it is a bad VSS?

My brakes are good, fuses are good, I have plenty of brake fluid, the connections are good and secure to the sensor, the control unit and brake switch.


A. What you need to do is pull the codes for the ABS system. ABS Diagnostic Trouble Codes (DTC)'s can be displayed by jumping terminals "A" and "H" of the Data Link Connector and observing the flashing of the "BRAKE" warning light. This should only be done when the brake lamp is "ON".

The terminals must be jumped for about 20 seconds before the code will begin to flash. Count the number of short flashes starting from the long flash. Include the long flash as a count. Sometimes the first count sequence will be short. However, subsequent flashes will be accurate. If there is more than one failure, only the first recognized code will be retained and flashed.

Note: DTC's 6, 9, and 10 will only turn the "BRAKE" lamp on while the fault exists. These are soft codes faults and can only be read with the Tech 1 scan tool. Do not jump terminals "A" to "H" if the "BRAKE" lamp is not on. Jumping the terminals with the "BRAKE" lamp off will set a false DTC 9. This will turn the "BRAKE" lamp on and disable the antilock system.

To clear the Diagnostic Trouble Codes:

  1. Make sure the ignition switch is "OFF.
  2. Pull the Stop Lamp fuse.
  3. Wait five seconds.
  4. Reinstall the fuse.
  5. Verify no Diagnostic Trouble Code (DTC)s are stored.

The VSS can best be tested with an Digital Storage Oscilloscope (DSO). The VSS is a signal generator and puts out an A/C signal which the ABS module can read. You may be able to get a reading on an digital A/C volt meter. Hook up the volt meter to the sensor connector and spin the wheel, you should get a reading. If you do, then the VSS is probably good. Generally if the speedometer is working, then the VSS is working.

If you are getting a code 12 that indicates that the self-diagnosis system is operating properly. If you aren't getting a code 24, then that confirms the VSS and the circuit is okay. The ABS DTCs are:

DTCs 1, 11 and 12 ECM Malfunction
DTC 2 Open Isolation Valve or Malfunctioning ECU
DTC 3 Open Dump Valve or Malfunctioning ECU
DTC 4 Grounded Antilock Valve Reset Switch
DTC 5 Eccessive Actuations of the Dump Valve
DTC 6 Erratic Speed Signal
DTC 7 Shorted Isolation Valve or Faulty ECU
DTC 8 Shorted Dump Valve or Faulty ECU
DTC 9 Open Circuit to Speed Signal
DTC 10 Brake Lamp Switch Circuit
DTCs 13, 14 and 15 ECU Malfunction

Additional Information provided courtesy of ALLDATA

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