Monday, June 20, 2011

Saturn SL2 Does Not Start, MIL Light On?

This is the procedure for extracting trouble codes from your Saturn:

See Figure 1
To enter the self-diagnostic mode, either connect a scan tool to the ALDL or DLC. On 1991-95 models, use a jumper wire to connect terminals A and B of the ALDL. Turn the ignition switch ON , then the PCM will enter the diagnostic program and report trouble codes on the scan tool or by flashing the SERVICE ENGINE SOON light.
All codes are 2 digits between 11 and 99. Codes are displayed on the SERVICE ENGINE SOON light by flashing the light with short and long pauses to distinguish digits of one code from digits of another. The short pause is used between digits of the same code, long pauses are between different codes. For example, the Code 12 sequence will be: flash, pause, flash-flash, long pause.
  1. When the diagnostic mode is entered, Code 12 is displayed 3 times. This indicates that the internal diagnostic system is operating. If Code 12 is not displayed, the self-diagnostic program is not functioning properly. If only Code 12 is displayed, no system malfunctions have been stored.
  2. Any existing system fault codes are displayed in order from low to high, except for Code 11. Each code is displayed 3 times, followed by the next code, if any.
  3. On vehicles with an automatic transaxle and on which exist stored transaxle codes, Code 11 will be displayed last, then the SHIFT TO D2 (1991-1992 models) or HOT (1993-98 models) light will begin flashing transaxle codes or flags.
  4. When all engine and transaxle codes have been displayed, Code 12 will flash again. At this point, all output devices are driven, except the fuel pump, so these circuits can be checked.
  5. This procedure can be repeated as required by cycling the ignition switch OFF , then ON again with the ALDL terminals jumpered together. The code display will begin again as in Step 1.


Fig. Fig. 1: ALDL Pinout


Here are some tests for the MAP sensor:
See Figures 1, 2 and 3
  1. Unplug the MAP sensor connector and turn the ignition switch ON . Check for about 4.5-5 volts between connector terminal C (gray wire) and ground.
  2. Check for the same voltage between terminals C and A (black wire). This is a ground through the PCM.
  3. Reconnect the wiring, then install a Saturn diagnostic probe, or equivalent, on the green wire and connect either a voltmeter probe or a scan tool to the ALDL. Connect a vacuum gauge to the intake manifold.
  4. With the ignition switch ON and the engine not running, the return signal voltage should be close to the supply voltage in Step 1, if below a 1000 ft. (305 meters) altitude. If above this altitude, see the chart for the correct voltage.
  5. With the engine at idle, there should be at least 16 in. Hg (54 kPa) of manifold vacuum and 1-1.5 volts on the signal return wire. When the throttle is opened suddenly, the signal voltage should increase. It may only change momentarily.
  6. If the voltage at idle is the same as in Step 1, or if the voltage does not change when the throttle is moved suddenly, the sensor is faulty and must be replaced.


Fig. Fig. 1: With the ignition turned ON, measure the MAP sensor circuit for a reference voltage

Fig. Fig. 2: Schematic for the MAP sensor circuit



Fig. Fig. 3: MAP sensor return voltage altitude compensation chart (with the ignition ON and the engine not running)

1 comment:

  1. Hg (54 kPa) of manifold vacuum and 1-1.5 volts on the signal return wire. When the throttle is opened ... cleafvacuum.blogspot.com

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