Questions and Answers
Celebrity SES Light
Q. My vehicle has the world famous Service Engine Soon light on. The only problem I notice wrong with it is when it idles or I am stopped at a traffic light. The engine will rev up and down the entire time I am stopped. It doesn't stall out. I have heard it may be the speed sensor that is causing this problem.
I'd like your opinion before I shell out the hundred dollars for a new one.
I have a 1987 Chevrolet Celebrity 2.5 liter FI, automatic transmission with 112,000 miles.
Thanks
A. If the Service Engine Soon light is on, then the computer has detected a malfunction and it has stored a Diagnostic Trouble Code (DTC) related to it. I would check the codes before I spend any money on a part that may or may not fix the problem.
To extract a trouble code from the computer for diagnostic purposes, the Assembly Line Communication Link (ALDL) connector is used.
ALDL Connector (Located Underdash)The ALDL diagnostic connector is located in the passenger compartment. Terminal B of the connector is the test terminal and terminal A is the ground used for diagnostic display.
If the test terminal is grounded with the ignition "ON" and engine not running, the system will enter the diagnostic mode. In this mode, the computer will display a Code 12, three consecutive times, by flashing the "CHECK ENGINE", "SERVICE ENGINE SOON" or "MALFUNCTION INDICATOR LAMP" or light. A Code 12 consists of one flash, a short pause, then two flashes in rapid succession. After Code 12 is displayed, any stored trouble codes will be displayed by flashing the "CHECK ENGINE", "SERVICE ENGINE SOON" or "MALFUNCTION INDICATOR LAMP" light. All codes are displayed 3 times each then a code 12 will repeat. In the diagnostic mode, the computer will also energize all computer controlled relays and solenoids. After determining the trouble code(s), see trouble code description.
If you are not getting any DTC's at all, then the problem is most likely a defective EPROM. Replace the EPROM only after trying a known good PCM. If you get codes with the known good PCM, then you will need to replace the EPROM or PCM.
Additional Information provided courtesy of ALLDATA
