I HAVE CHANGED THE SPEED SENSOR AND EVERY THING WORKS FINE FOR ABOUT 30 MIN. THEN THE TRANSMISSION SEEMS TO INTO SAFE MODE. THE SERVICE LIGHT DOES NOT COME ON BUT THE ABS WARNING LIGHT DOES. ARE THESE PROBLEMS RELATED OR DO I HAVE TWO SEPRATE PROBLEMS? MY SUBURBAN IS A 4X4. THE WIREING DIAGRAM IN THE HAYNES MANUAL SHOWS THE WIRING FROM THE SPEED SENSOR GOING TO A SPEED SENSOR BUFFER BUT MAKES NO REFERANCE TO IT IN THE CHECK OR REPLACE SECTION OF THE MANUAL. IF THE PCM IS NOT GETTING VOLTAGE, COULD MY PROBLEM BE IN THIS BUFFER? IF SO WHERE IS IT LOCATED?
CODE 72 SPEED SENSOR on 1993 Chevrolet Suburban 1500
by chuckvanr in Sylmar, CA on October 18, 2009
ANSWER by Oakman on October 23, 2009
I am not sure where the speed sensor buffer is located on your 1993 Chevrolet Suburban but I can tell you how it works. The vehicle speed sensor (which is in the transfer case near the output shaft) sends its signal to the speed sensor buffer. The job of the buffer is to process the signal and send it to the dash (speedometer), cruise control, ABS, and engine control module. The loss of the vehicle speed signal will can cause the transmission to go into it's fail safe mode.One thing to look at is the operation of the speedometer when the problem occurs. If the speedometer continues to function correctly then you can be fairly certain that the speed sensor and wiring the dash are good. This would leave the fault with the buffer, the engine computer, or the wiring from the buffer to the computer. It sounds like the engine computer and the ABS module are both loosing their vehicle speed input. This could be caused buy a faulty buffer but it is not a common failure.