FSMode

How to Use the L98 Field Service Mode

Field Servide Mode Theory
1. The Assembly Line Diagnostic Link (ALDL) is located under the left hush panel to the right of the
    steering wheel.
2. To put the Engine Control Module (ECM) into Field Service Mode use a paper clip to jumper pins A &
    B (upper right) of the ALDL.
3. When the ECM is in Field Service Mode the Service Engine Soon (SES) light is used to transmit
    information to the diagnostician.
4. The Field Service Mode has two forms:
    -Key On - Used to pull trouble codes
    -Engine Running - Used to display O2 sensor status

Pull Trouble Codes - Key On
1. With the ALDL pins A & B jumpered, turn the ignition key On but don't start the engine.
2. The SES will first flash a "12" as "on", "off", "on", "on" and repeat this sequence three times to
indicate the ECM is in Field Service Mode and trouble codes are to follow.
3. The ECM will next flash each trouble code three times followed by three "12"s, indicating that all
trouble codes have been sent.
4. Jot down each trouble code and use them to index into trouble shooting procedures for the engine.

Test the O2 Sensor - Engine Running
1. With the ALDL pins A & B jumpered, start the engine and allow several minutes for it to come to
   operating temperature.
2. Initially the SES will flash at about 2Hz indicating the ECM is still in Open Loop condition.
3. After several minutes the flashes should slow to about 1Hz indicating the ECM is in Closed Loop
    condition.
4. Raise the idle to around 2,500rpm and observe that the SES flashes settle back to 1Hz.
5. Note the duration of the Off and the On periods of the flashes:
    -Off period - indicates rich condition
    - On period - indicates lean condition
6. At normal and raised idle the SES should flash at 1Hz and the On/Off periods should be of the same
    duration; in this case all is well and the ECM has entered and is staying in closed loop with the
    air/fuel at stochiometric.
Diagnostics:
1. If the ECM fails to enter closed loop, check the coolant temperature Sensor and the O2 sensor.
2. If the ECM fails to stay in closed loop at raised idle, check the O2 sensor.
3. If the SES On/Off periods are not of equal duration, determine the cause of the rich or lean
    conditioin.

Back to Home

Test the ECM - Engine Running
1. Connect a 3,9000 Ohm 1/4 Watt resistor across ALDL pins A & B and start the engine.
2. The ECM should enter Limp Home mode where the it only uses throttle position, RPM and coolant
temperature to set injector pulse width.
3. This test can be used to test the ability of the ECM to enter Limp Home mode.

Test the IAC - Engine Running
1. Connect a 10,000 Ohm 1/4 Watt resistor acros ALDL pins A & B and start the engine.
2. The ECM will be forced into closed loop, ignoring the run timer, and force idle to 1,000RPM.
3. This test is useful in diagnosing an erratic idle problem.